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FOR MUSICAL 
INSTRUMENTS 


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TTmTI 

• If*™! 
























ELECTRONIC IGNITION 
KIT 


TOTAL ENERGY DISCHARGE electronic 

PEAK PERFORMANCE - higher output voltage under all 

IMPROVED ECONOMY-no loss of ignition performance 

between services. 

FIRES FOULED SPARK PLUGS no other system can better the 

ACCURATE TIMING - prevents contact wear and arcing 

SMOOTH PERFORMANCE- immune to contact bounce and 

PLUS 

SUPER POWER SPARK-3% times the energy of ordinary 

capacitive systems - 2Vi times the power of inductive systems. 

OPTIMUM SPARK DURATION 3 times the duration of ordinary 
capacitive systems - essential for use on modern cars with weak fuel 

BETTER STARTING- full spark power even with low 

battery. 

CORRECT SPARK POLARITY unlike most ordinary C.D. systems 
the correct output polarity is maintained to avoid increased stress on the 
H.T. system and operate all voltage triggered tachometers. 


TECHNICAL DETAILS 

The basic function of a spark ignition system is often lost among claims 
for longer 'burn times' and other marketing fantasies. It is only necessary 

5000 times the energy of the spark, to realise that the spark is only a 
trigger for the combustion. Once the fuel is ignited the spark is insignificant 
and has no effect on the rate of combustion. The essential function of the 
spark is to start that combustion as quickly as possible and that requires 


L E D. STATIC TIMING LIGHT for accurate setting of the engine's 
most important adjustment. 

LOW RADIO INTERFERENCE fully suppressed supply and absence 
DESIGNED IN RELIABILITY an inherently more reliable circuit 



AS REVIEWED IN 

ELECTRONICS TODAY INTERNATIONAL JUNE '81 ISSUE 
and EVERYDAY ELECTRONICS DECEMBER '81 ISSUE 
FITS ALL NEGATIVE EARTH VEHICLES, 

6 or 12 volt, with or without ballast 



OPERATES ALL VOLTAGE IMPULSE TACHOMETERS 

PRICE £2.95 P P ) q aPt ° 


STANDARD CAR KIT £14.85 
ASSEMBLED AND TESTED £24.95 


PLUS £1 


TWIN OUTPUT KIT £22.94 

For MOTOR CYCLES and CARS with twin ignition systems 

ASSEMBLED AND TESTED £34.70 


U K. P.&P. 


ELECTROIMIZE DESIGN Dept. C 

Goods normally despatched within 7 da 
Magnus Road. Wilnecote, 


E3 


TYPICAL SPECIFICATION 


SPARK POWER (PEAK) 

SPARK ENERGY 
(STORED ENERGY) 

SPARK DURATION 

OUTPUT VOLTAGE (LOAD50pF 

EQUIVALENT TO CLEAN PLUGS) 
OUTPUT VOLTAGE (LOAD 50pF + 500 
EQUIVALENT TO DIRTY PLUGS) 
VOLTAGE RISE TIME TO 20 KV 
(Load 50pF) 

TOTAL ENERGY DISCHARGE should n 
inductive systems or hybrid so called reacti 


TOTAL ORDINARY 
ENERGY CAPACITIVE 
DISCHARGE DISCHARGE 


36 mJ 10 mJ 

135 mJ 65 mJ 

500 pS 160 jjS 


25 jjS 30 jjS 










VOL. 11 NO. 9 SEPTEMBER 1982 



PROJECTS . . . THEORY . . . NEWS . . . 
COMMENT . . . POPULAR FEATURES . .. 



SCHOOLS 


Electronic Design Award 




A full report appears on page 590. 


_i, photographs and articles published in 

EVERYDAY ELECTRONICS is fully protected, 
and reproduction or imitations in whole or in part 
are expressly forbidden. 


PROJECTS 

SOUND SPLITTER by J. D. Rogers 560 

A multi-effects unit for the musician 

CB BATTERY CHARGER by A. Flind 568 

Constant current charger for NiCad cells 

SCREEN WASHER DELAY by G. L. Stoneman 576 

Single operation extended wash time 

MONTHLY PLANNER by A. P. Donleavy 581 

Electronic calendar with event memory 

TEMPERATURE INTERFACE FOR THE TRS80 599 

by 0. N. Bishop 
Construction and software 

CONTINUITY TESTER by J. Moulder 604 

Audible circuit checker 

SERIES 

TEACH-IN 82 by O. N. Bishop 571 

Part 12: Computing circuits 

A.C. MAINS by A. Kenyon 596 

Part 3: Phase relationships; Power factor 

FEATURES 

EDITORIAL 559 

The Young Generation; New Season Is A’Coming 

JACK PLUG AND FAMILY by Doug Baker 566 

Cartoon 

SHOP TALK by Dave Barrington 567 

Product news and component buying 

COUNTER INTELLIGENCE by Paul Young 570 

A retailer comments 

EVERYDAY NEWS 577, 590 

What’s happening in the world of electronics 

PLEASE TAKE NOTE 577 

Circuit Exchange—Invader Landing Game 

FOR YOUR ENTERTAINMENT by Barry Fox 578 

Resistance to Space, Junk Mail, Car Statics 

RADIO WORLD by Pat Hawker G3VA 580 

A Better Picture, The Sting 

CONSUMER ELECTRONICS SHOW by Barry Fox 588 

Report from America 

READERS LETTERS 589 

Your news and views 

SEDAC 592 

The Twelve winning designs 

SQUARE ONE 607 

Beginner's Page: Ohm’s law 


Our October issue will be published on Friday, 

September 17. See page 587 for details. Readers Services • Editorial and Advertisement Departments 559 


Everyday Electronics, September 1982 


553 













3 si 



Bradley 

Marshall Ltd 

OF EDGWARE ROAD 



Crimson Elektrik 


PROFESSIONAL AMPLIFIER MODULES 



































































VOL. 11 NO. 9 


SEPTEMBER 1982 



F. E. BENNETT 


JACQUELINE DOIDGE 

Editorial Office* 

KINGS REACH TOWER 
STAMFORD STREET 
LONDON SE1 9LS 
Phone: 01-281 8873 


Advertisement Manager 
R. SMITH 
Phone: 01-281 8671 

Representative 
R. WILLETT 
Phone: 01-261 6865 

Classified Supervisor 
B. BLAKE 
Phone: 01-261 5897 


Advertisement Offices 
KINGS REACH TOWER 
STAMFORD STREET 
LONDON SE1 9LS 


THE YOUNG GENERATION 

Electronics casts its spell at an early age and younger members of 
our society are readily attracted to this technology. Whilst at school 
many children become actively involved in experimenting and building 
projects to meet actual needs or perhaps just to satisfy inquisitive 
minds. 

If given but a modicum of encouragement by members of teaching staff, 
this youthful enthusiasm is likely to be self-sustained throughout the 
final and formative years at school. Even more fortunate are those 
children who have a teacher with a similar interest in electronics. With 
such a mentor to guide and advise them in these (usually extra¬ 
curricular) activities, young minds will develop their appetite for 
electronics in the best and most logical manner. 

Schools, themselves, can gain materially from the practical work of 
their electronically inclined pupils, a fact most clearly demonstrated 
by the Schools Electronic Design Award Competition (SEDAC), sponsored 
by Mullard Ltd and this magazine. The culmination of this competition 
was the presentation of prizes by the Parliamentary Under-Secretary 
of State, Department of Education and Science. The Minister praised 
the endeavours and inventiveness of the young entrants and the 
excellence of their working models: he was particularly pleased by the 
practical aspect of the competition and welcomed this encouragement 
of interest in electronics and computers amongst the young. The 
Minister voiced one regret: that was concerning the absence of girls 
amongst the finalists. We agree with him that this cannot be because 
of lack of interest in electronics among schoolgirls—our own evidence 
points to the contrary. So come on girls, don’t let the fellows get away 
with it at next year’s SEDAC. 

The success of our first national schools competition has been well 
noted. We are delighted to announce that Mullard Ltd have agreed 
to join us in sponsorship of a second Schools Competition, to be 
launched this autumn. Full details will appear in next months Everyday 
Electronics. 

NEW SEASON IS A’COMING 

October is always a notable month on account of the renewed activity 
amongst constructors after a summer lull. It is also the optimum time 
for newcomers to make a start in this stimulating and useful leisure 
pursuit. 

Next month we will give a pair of transistors with every copy and 
inside will be suitable designs for their use. For those hovering on the 
edge there’s encouragement to take the plunge in the first of a six-part 
series. Introducing Electronics. 

Finally, to something over which we have little control. Economic 
facts of life make it necessary for the price of Everyday Electronics 
to be increased to 80p as from next month. But it will be well worth 



Readers' Enquiries 

We cannot undertake to answer readers’ letters requesting modifications, 
designs or information on commercial equipment or subjects not published 
by us. All letters requiring a personal reply should be accompanied by a 
stamped self-addressed envelope. 

We cannot undertake to engage in discussions on the telephone. 
Component Supplies 

Readers should note that we do not supply electronic components for 
building the projects featured in EVERYDAY ELECTRONICS, but these 
requirements can be met by our advertisers. 

All reasonable precautions are taken to ensure that the advice and data 
given to readers are reliable. We cannot however guarantee it, and we 
cannot accept legal responsibility for it. Prices quoted are those current 


Back Issues 

Certain back issues of EVERYDAY ELECTRONICS are available 
worldwide price 80p inclusive of postage and packing per copy. Enquiries 
with remittance should be sent to Post Sales Department, IPC Magazines 
Ltd., Lavington House, 25 Lavington Street, London SE1 OPF. In the event 
of non-availability remittances will be returned. 

Binders to hold one volume (12 issues) are available from the above 
address for £4 60 inclusive of postage and packing worldwide. 

Please state which Volume. 

Subscriptions 

Annual subscription for delivery direct to any address in the UK: £11 -00. 
Overseas: £12-00. Cheques should be made payable to IPC Magazines 
Ltd., and sent to Room 2613, Kings Reach Tower, Stamford Street, 
London SE1 9LS. 


Everyday Electronics , September 1982 


559 














C ircuits to give a stereo simulation 
from a mono input have been 
published before but these are nor¬ 
mally for hi fi use and are not suitable 
for the musician. It might also be said 
that a “stereo” splitter would be of 
little value to the average musician 
with only one amplifier. In; this unit 
however, the stereo splitter circuitry 
is modified so as to be useful with 
even a small, single input amplifier. 

The unit combines an improved 
pseudo-stereo effect with two other 
useful functions, described later, to 
give a device which has a wide range 
of applications. 

The unit is simple to use and easy 
to construct, no test equipment is 
needed and no setting up or adjust¬ 
ments are required. The circuit con¬ 
tains only two i.c.s, both common 
types, there are no unusual com¬ 
ponents and it uses a standard size 
Veroboard and ready made case. 

To describe the unit itself and de¬ 
scribe its applications it is best to 
consider both of these together, and 
this is done later, under a separate 
heading for each of the three 
modes of operation. These modes are 


Front cover: courtesy Bert Kempster, Riverside 
Organ Studios Ltd, Crawley. 



selected by a small toggle switch on 
the side of the unit and 1 are: 

SWITCHER MODE, FREQUENCY SPLIT MODE 

and crossmix mode. Regardless of 
which of these modes is set, selecting 
normal on the footswitch will always 
give a “straight” signal at the main 
output. 

All effect/normal signal routing is 
performed by electronic (f.e.t.) 
switches, as this is superior to normal 
switching and does away with the 
need to use a d.p.d.t. footswitch. A 
wider choice of footswitch is made 
available since a s.p.s.t. or indeed 
any type of footswitch can be used, 
and yet always give smooth and 
reliable switching. This is because the 
footswitch itself only has to carry a 
d.c. command voltage while the 
actual audio signal switching takes 
place on the circuit board. 

CIRCUIT OPERATION 

The circuit diagram for the Sound 
Splitter is shown in Fig. 1. 

Although the circuit uses six op- 
amps, these are contained in just two 
packages, IC1 and IG2 one dual and 
one quad. ICla acts as a input buffer 
and a preamplifier. At its input is a 
network which adds a treble pre¬ 
emphasis. 


When footswitch SI is set to 
normal (contacts open) a negative 
voltage is fed to the gates of the two 
f.e.t.s. (TR1, TR2) and this switches 
them both off. IClb therefore has 
only Rll in its negative feedback 
loop and so it acts simply as a unity 
gain inverting amplifier stage. 

Its signal passes via R17 to the 
output (TR2 has no effect as it is off) 
where C9 and R19 form a treble cut 
(de-emphasis) filter to cancel out the 
effect of the pre-emphasis that was 
added at the input. This restores a 
virtually flat response overall and the 
net result is noise (hiss) reduction, 
since any noise arising from any 
source within the unit is reduced 1 by 
being attenuated at the last possible 
point, the output. The circuitry is low 
noise anyway, but this gives a further 
improvement. 


EFFECT SWITCHING 

Now consider the same parts of 
the circuit when effect is selected. 
Now SI is closed so the voltage sent 
to the f.e.t. gates is positive, which 
switches them both on. IClc and d 
form two bandpass filters and so will 
only pass certain section® of the audio 
spectrum, in this case centred 1 at 


560 


Everyday Electronics , September 1982 






approximately 300Hz and 3kHz. These 
bands are now added (via TR1, now 
on) to IClb where, being in antiphase 
to the main signal (via RIO), they 
cancel out parts of the response. The 
output of IClb therefore becomes 
notched. 

C7 is also brought into circuit when 
TR1 comes on, increasing the nega¬ 
tive feedback around IClb so re¬ 
ducing the gain, but only at high 
frequencies. The result is that fre¬ 
quency response above 7kHz is re¬ 
moved. Fig. 3a shows the final 
response. 

COMPENSATION 

Since three bands have now been 
removed from the response, a notice¬ 
able drop in overall sound level would 
be heard upon switching between 
NORMAL and EFFECT, so TR2 has been 
included to compensate for this by 
bringing R16 into circuit whenever 
effect is selected. This gives a 
general level boost and also reduces 
the treble de-emphasis at the output, 
so even though response around 3kHz 
and above 7kHz has been removed, 
the overall “brightness” is preserved. 
The final result is a unique change 
of timbre without any volume loss. 


The other, more important, reason 
for including TR2 is to ensure that 
when normal is selected the main 
output is isolated from the other 
channel crossmix network (CIO, Cll, 
R26). 

So much for the main signal' path, 
now consider the secondary channel, 
which consists of IC2a and b and 
associated components. Normally the 
signal from the preamp (via R20) 
plus an identical but inverted signal 
from IClb (via R21) are fed to IC2b 
and these cancel to give zero output. 

When effect is selected however, 
parts of the main signal are removed, 
as described earlier, and where this 
takes place the two inputs to IC2b 
can no longer cancel out and so there 
is now an output at these frequencies, 
that is around 300Hz, 3kHz and above 
7kHz. See Fig. 2b. 

This can be summarised as “what¬ 
ever is removed from the main chan¬ 
nel appears in the secondary chan¬ 
nel”. 

The secondary path continues to 
IC2a, which, with VR1 forms a centre 
zero volume and phase control. When 
VR1 is at zero resistance IC2a is an 
inverting amp; when VR1 is at full 
resistance IC2a is a non-inverting 



amp; when VR1 is at lOkll resistance 
then there is no output at all since 
IC2a is equally inverting and non¬ 
inverting. 

Finally, the signal is fed via CIO, 
Cll, R26, R27 to the secondary out¬ 
put jack SK3. R26, CIO, Cll only 
have effect when crossmix mode is 
selected. Cll is included so as not to 
lose, in crossmixing mode, the feature 
whereby a second amp does not have 
to handle low frequencies. It forms 
a highpass filter with R26 whenever 


Fig. 1. The complete circuit diagram of the Sound Splitter. 



Everyday Electronics, September 1982 


561 



























































crossmix mode is selected thus pre¬ 
venting bass signals from the main 
output getting into the secondary 
output. R27 is included to isolate the 
output of the op-amp from cable 
capacitance which could otherwise 
cause instability. 

R28 is placed across S2 to prevent 
switch clicks by equalising any small 
d.c. offsets that could otherwise ac¬ 
cumulate, but it is of sufficiently 
large value as to have negligible 
effect on the actual audio signals. 

SWITCHER MODE 

In switcher mode pressing the 
footswitch routes the signal altern¬ 
ately to the two outputs (main and 
secondary), switcher mode allows 
better use to be made of any amp 
with two or more inputs or channels, 
that is most amps. Each channel can 
be set up for a different sound and 
then selected remotely and instantly 
by the footswitch. It also allows any 


other effects unit (or combinations of 
effects units), placed in either the 
main or the secondary path(s) to be 
switched, see Fig. 3b and c. 

So as to allow improvement of 
other effects units, the level of the 
secondary output has been made 
variable, by means of VR1. Designers 
of effects circuits can only have an 
average input level in mind, so it is 
likely that any real instrument output 
is either too low (so you don’t get 
the best signal-to-noise ratio from 
the effect) or too high (so it over¬ 
loads). This control allows the “drive 
level” to any effect unit to be boosted 
(up to X3) or reduced compared to 
the instruments original level. To 
find the best level, turn the control 
until maximum output is obtained 
without distortion occurring. 

If you do not have two inputs on 
your amp, then the simple and in¬ 
expensive “Two-way Remixer” box 
(described later) can be used instead. 


Fig. 3. A few applications of the Sound Splitter from the simple to the more complex. 
Set the footswitch to EFFECT for the stated effect in each case. 



This accepts any two inputs and com¬ 
bines them into one output. 

SPLIT MODE 

When set to normal, there is a 
“straight” signal from the main 
output, but upon pressing the foot¬ 
switch certain bands of frequencies 
are removed from it, thus altering 
the response. 

To keep the overall volume con¬ 
stant a general boost is automatically 
applied, enough to make up for the 
removed band®. So, just by using the 
main output on its own, this mode 
provides a footswitchable tonal 
change effect, from normal to a 
lighter, coloured, sound not possible 
using normal tone controls. See 
Fig. 3a. 

The missing bands of frequencies 
are not lost, but are transferred to 
the previously “dead” secondary out¬ 
put. All of the audio frequency 
spectrum is therefore still passed but 
is split into two complementary out¬ 
puts, whatever is absent from one 
will be present in the other, see 
Fig. 2. 

Having generated these two out¬ 
puts, what to do with them is up to 
the user, but some tried suggestions 
are: 

(1) To a simple passive remixer 
(see Fig. 7). This then allows you pan 
between two entirely opposite tonal 
responses. The relative volumes of 
these can be balanced by using VR1 
to preset the level of the secondary 
output. 

(2) To two channels of the same 
amp. Each channel now handles 
different parts of the signal, so tone 
controls and any built-in effects (such 
as reverb or tremolo) can be applied 
to some frequencies and not to 
others, with some unusual results. 
The footswitch can be used to return 
to normal single channel operation 
of the amp at any time. 

(3) To two separate amps (or to 
two sides of a P.A. system). This 
produces a pseudo-stereo effect which 
gives a spatial enhancement or 
“spread” of the sound. 

Two useful features are that (i) it 
is possible to use a much smaller 
amp/speaker for the secondary chan¬ 
nel since its output signal consists 
only of certain bands of the audio 
spectrum, the lowest of which is 
centred around 300Hz, so there is no 
deep bass present which would re¬ 
quire a more substantial amp/speaker 
to handle it. And (ii) the second amp 
can be positioned any distance away 
and its volume conveniently “remote 
controlled” by VR1 on this unit. See 
Fig. 3f. 

(4) One output via effects unit(s), 
the other straight. Then as in (1), (2) 
or (3) above. 

(5) Both outputs via effects unit(s). 
Then as (1), (2) or (3) above. 


562 


Everyday Electronics, September 1982 





































































Prototype Sound Splitter and Two-Way Remixer, 



COMPONENT ASSEMBLY 

Construction is straightforward, but 
is most convenient if carried out ini a 
certain order. If you follow 1 the stage- 
by-stage instructions given here, and 
tick off each part as you proceed! it 
will ensure that everything gets done 
with the least possible effort and that 
nothing is forgotten. 

The majority of the components 
are mounted oni a piece of 0-1 inch 


With the latter two methods, novel 
variations oni existing effects are pos¬ 
sible by feeding the effect with 
certain frequencies onlyi and allowing 
the others to bypass it or even to go 
via another effects unit. Here then 
are hundreds of possibilities to be 
explored 1 , just a few examples are 
given in Figs. 3d, e and! g. 

All of the effects using the fre¬ 
quency split mode have been found 
to soundl better or worse depending 
on the relative phases of the final 
signals appearing at the speaker(s). 
There is no way of predicting 
whether a signal via any particular 
route will end up being inverted 
as this depends on many stages in 
the circuits within the amp(s) and 
any effects unit(s) used, as well as on 
which way the speakers are wired. 

To take all possible situations into 
account, the Sound Splitter secondary 
output has been given the facility 
whereby it can be of either phase 
relative to the other output. It is 
controlled by VR1, which has a centre 
zero arrangement. Volume increases 
towards either sidle of centre but the 
phase of the output is different de¬ 
pending on whether it is in the + ve 
or the — ve half of its rotation, as 
marked on the panel. 

CROSSMIX MODE 

In the crossmix mode there is nor¬ 
mally “straight” signal from the main 
output and upon pressing the foot- 
switch the frequency splitting as 
described above occurs, but in this 
mode the two channels are internally 
cross-mixed in such a way as to pro¬ 
duce new tonal colours. 

These are available from either one 
of the outputs and can be varied by 
using VR1 to alter the percentage of 
the mix coming from the secondary 
channel. In this mode then, VR1 
effectively becomes an unusual tone 
control. 


COMPONENTS'^ 


R16 1-5kn 


R4 22kO 


R17 


•7kn 


R5 


■5kO 


R6 82kO 
R7 5-6M1 
R8 330kfl 
R9 330kO 
RIO 47kfi 
R11 47k£! 

R12 1MH 
R13 lOkO 
R14 68kn 

All fwatt carbon film ±5% 

Capacitors 

Cl lOOnF polyester type C280 
C2 220pF polystyrene 
C3 10nF polyester type C280 
C4 10nF polyester type C280 
C5 22nF polyester type C280 
C6 22nF polyester type C280 
C7 470pF polystyrene 

Semiconductors 

TR1, 2 BF244 n-channel f.e.t. (2-off) 

IC1 LF347 quad j.f.e.t. op-amp 

IC2 LF353 dual j.f.e.t. op-amp 


R18 8-2kO 
R19 2-2kfl 
R20 6 - 8kO 
R21 6-8M2 
R22 lOkft 
R23 27k£7 
R24 10k£i 
R25 27kO 
R26 8-2kO 
R27 3300 
R28 1MO 


See 

*l3e 


C8 1/iF 25 V elect. 

C9 10nF polyester type C280 
CIO 150nF polyester type C280 
C11 150nF polyester type C280 
Cl2 10^F 10V tantalum 
C13 10/xF 10V tantalum 


lOOkQ carbon log. potentiometer 

s.p.s.t. footswitch 

s.p.d.t. centre-off miniature toggle 



SKI stereo jack socket with d.p.d.t. switched contacts (Tandy 274-277) 

SK2, SK3 standard jack socket with break contacts (2 off) 

B1,B2 PP3 9V battery (2 off) 

Shipboard: 0-1 inch matrix size 24 strips x 37 holes; aluminium case with lid 
size 125 x 100 x 40mm; control knob with index marker; P,P3 battery clips 
(2 pairs); Veromounts. 

Two-Way Remixer 

R1, R2 150kH JW carbon (2-off) 

VR1 47kil carbon linear potentiometer 

SKI, 3 standard jack socket (2-off) 

SK2 standard jack socket with break contacts 

Case: 100 x 50 x 25mm enamel finish aluminium diecast box (Maplin 
DCM5002). 


Everyday Electronics , September 1982 


563 














564 


Everyday Electronics, September 1982 




































































































Veroboard size 24 strips X 37 holes 
as shown in Fig. 4. Cut the board to 
size and then make all the necessary 
breaksi in the copper tracks using a 
spot face cutter or a small drill bit 
(about 3mm diameter). Drill the board 
fixing hole where shown in Fig. 4. The 
other three corners of the board 
should be supported by pieces of 
plastic foam stuck to the bottom of 
the case. In fact, separated Vero- 
mounts are ideal for this, just stick 
squares of either type of the velcro 
material where the three other 
corners of the board will rest. 

By having just one bolt point and 
three resilient mounts the board is 
well anchored and is insulated from 
the metal case but is not held too 
rigidly, so accidental knocks to the 
case are less likely to be transmitted 
through and damage the board. 


CIRCUIT BOARD 

Begin board construction by solder¬ 
ing in all of the flat-lying resistors, 
then the 220pF and 470pF capacitors. 

Solder in the six larger of the links. 
Use insulated wire for these so as to 
prevent any shorts should the links 
flex and touch other components. 

Solder in the two smaller links, 
using bare wire. 

Insert and solder in the two i.c. 
sockets, then the eight vertically 
mounted resistors and finally the rest 
of the capacitors. Next solder suffi¬ 
cient lengths of insulated stranded 
wire to the board to reach the 
appropriate case mounted com¬ 
ponents. 

Position and solder in place the 
two transistors and then insert the 
i.c.s into their sockets, carefully 
checking their orientation. 


CASE DETAILS 

Drill out holes in the case, for the 
footswitch, the potentiometer, the 
toggle switch and the three jack 
sockets, following the dimensions 
given in Fig. 6 (unless of course a 
different case is used). A small hole 
is also required in the bottom of the 
case to put a small bolt through to 
hold the circuit board in place. The 
two jack sockets at the lower end 
have been placed so as to act as 
battery retainers. A strip of plastic 
foam across the bottom can be added 
to keep the batteries firmly in place. 

Clean 1 the case before proceeding 
to letter it. Letraset or similar rub- 
down transfers will give a profes¬ 
sional finish. Either the panel design 
shown on the prototype can be fol¬ 
lowed or the constructor can make 
up a design to suit himself. 

Fix the lettering for protection with 
a clear varnish. Letraset 101 aerosol 
is most convenient and gives a tough 
enough finish. When the varnish has 


dried, mount the sockets, potentio¬ 
meter and two switches. Put four 

stick-on rubber feet on the case 

underside. See Fig. 5. 

Interwire just the case mounted 

components, as shown, using the 

usual multi-strand hook-up wire, 
neatly routing all leads around the 
bottom of the case out of the way. 
Next interwire the circuit board to 
the case mounted components, keep¬ 
ing the wiring neat as before. After 
checking all connections, bolt the 
board into the case. 

REMIXER BOX 

Apart from its intended use with 
the Sound Splitter, the remixer box 
is a generally useful unit to have 
around, and being purely passive it 
needs no batteries. 

The circuit for the Two-Way Re¬ 
mixer is shown in Fig. 7. The two 


input signals to A and B are “mixed” 
across VR2, the amount of each 
appearing at the output depending 
on the resistance between VR2 wiper 
and the input socket. With the wiper 
in its mid-position, the output con¬ 
tains equal amounts of signals at A 
and B. 

The two 150kft resistors R1 and R2 
ensure that there is always a d.c. 
reference path for any connection to 
the box, these resistors* are placed 
prior to the potentiometer VR2 where 
they do not cause unnecessary re¬ 
duction of the signals. 

Construction is very simple, any 
metal box can be used as long as it 
is big enough to contain three plastic 
jack sockets and a potentiometer, see 
Fig. 8. 

A Maplin type DCM5002 diecast 
box was used for the prototype. The 
jack sockets had to have their con- 



E very day Electronics, September 1982 


565 











































The completed Sound Splitter unit with lid removed showing 
batteries in position. 

Left. Close-up view of the Sound Splitter component board. 


tacts folded down flat to fit into this 
particular box and a few strips of 
insulation tape were stuck on the 
inside of the case lid to prevent the 
contacts shorting to the metal case. 

The earth contacts of the three 
sockets are wired together and, since 
they are all plastic types, the metal 
case must be earthed via the potentio¬ 
meter, by soldering an “earth lead” 
to the back of its case. 

When there is no jack inserted in 
B then the unit is still usable simply 
as a volume control for the A input, 
because the B input is automatically 
earthed by the normally closed con¬ 
tact on the socket itself. 

EARTHING CONSIDERATIONS 

Problems can occur whenever two 
separate signal paths are ini use from 
a common source, because of the 
formation of earth loops, which in¬ 
variably give rise to unacceptable 
mains hum. 

However, these situations are auto¬ 


matically prevented in the Splitter by 
means of a break contact on the main 
output jack SK2 which, when a plug 
is inserted, disconnects the earth to 
the secondary output jack. So, when¬ 
ever both outputs are in use, only 
the lead from the main output will 
be directly earthed (the other lead 
will still be earthed, but from the 
“other end” via the amp.). 

If only the secondary output is 
being used, then its earth will remain 
connected, thus all situations are 
catered for. 

CONCLUSION 

The unit will accept an input from 
any instrument or other audio source, 
and the input jack switches on the 
batteries when a plug is inserted. 
Battery drain is low (about 10mA) 
so two PP3 batteries should last a 
long time. 

The Sound Splitter is an effects 
unit in its own right, and of course 
can be used as such, however, its 


real versatility lies in the many in¬ 
teresting ways in which it can be 
used in conjunction with any other 
effects unit and the Two-Way Remixer. 
Used in this way it can help in the 
musicians constant search for differ¬ 
ent sounds by squeezing a few more 
variations out of existing types of 
effects. 

As mentioned earlier there are so 
many combinations possible, simple 
and complex, that it is pointless to 
try and list them here, it is really up 
to the user to discover uses to suit 
his own equipment, taste and style 
of playing. 

Finally, for anyone who wishes to 
experiment with further effects, a 
useful control to add is a 47kXl log. 
potentiometer connected between the 
“capacitor ends” of R5 and R7. This 
allows the peaks of both the band 
pass filters in the unit to be swept 
to different frequencies, eventually 
forming into a single peak when the 
potentiometer is at zero ohms. U 


JACK PLUG & FAMILY... 



566 


Everyday Electronics, September 1982 




































By Dave Barrington 

Mastering PCBs 

Making printed circuit boards is a highly 
skilled profession and an art. So any news 
of a product that makes it easier for the 
non-professional to produce boards of a 
high quality is most welcome. 

One such system is the new CM100 
Circuit Maker kit from Electrolube. Due to 
space considerations, it is not possible to 
evaluate the kit from a "hands-on” view¬ 
point but the problems of making p.c.b.s 
will be the subject that we will return to at 
a later date. 

The kit provides all the necessary hard¬ 
ware and chemicals to produce positive 
photographic film masters from published 
layout diagrams and a final definitive 
circuit board. 

The kit can be broken down into two 
groups or packs consisting of equipment 
and chemicals for making the film positive 
masters and one for making the final 
printed circuit board. Amongst the film 
processing equipment is a photoflood 
bulb used for activating the sensitised 
film when placed over the published 
master diagram. 

We like the idea of a special "jig" which 
can be used as an exposure frame for the 
photographic part of the process, as well 
as a component assembly frame. A foam- 
backed plate, which forms part of the 
frame, is ideal for holding components in 
place whilst any excess leads protruding 
on the underside of the board can be 
trimmed prior to soldering. It is also 
claimed that the foam backing is heat- 
resistant which allows the components to 
be held firmly during the soldering 
operation. 

It is obvious that a great deal of thought 
and attention to detail has gone into 
making the CM100 kit as extensive as 
possible. As well as containing six 
double-sided copper-clad fibreglass cir¬ 
cuit boards (no single-sidedI), plus such 
items as rubber gloves, retouching pen, 
photographic dishes, thermometer, et¬ 
chant and drills, there are workbench 
charts and an instruction manual. 

Selliryg for about £70, first impressions 
would seem to indicate that the kit is 
rather an expensive outlay, but when 
weighed against the cost of purchasing 
finished boards it appears to be a reason¬ 
able investment. 

For more details of prices and local 


stockists of the CM100 Circuit Marker 
contact Electrolube Ltd., at Dept EE, 
Blakes Road, Wargrave, Berks, RG108AW. 

Catalogues Received 

A new 21-page components catalogue 
has just been received from Rapid Elec¬ 
tronics. The Autumn '82 Catalogue 
includes a wider range of Linear devices, 
plus data sheets, an extended range of 
capacitors and p.c.b. mounting trans¬ 
formers. 

Copies of the Rapid Components 
Catalogue can be obtained by sending 
45p to Rapid Electronics, Hill Farm 
Industrial Estate, Boxted, Colchester, 
Essex, C04 5RD. The catalogue will be 
sent free to customers who place orders 
for goods totalling over £10. 

A Shortform catalogue just published 
by Keyswitch Varley contains abbreviated 
data on their range, of relays, solenoids, 
controllers, timers and switches. 

New products described in the 12-page 
catalogue include a range of 1-, 2-, 3- and 
4-pole reed relays, 30A power relays and 
a solid state relay. Recent additions are 
a range of DIP slide switches and a 
double wound solenoid with integral 
solid state switch. 

Copies of the Shortform catalogue are 
available from Keyswitch Varley Ltd., 
Dept EE, Tom Cribb Road, Thamesmead, 
London SE28 0BH. 



The CM100 Circuit Maker from Electrolube 

Booklet 

A 20-page pocket guide entitled "The 
100 Most Asked Questions and Answers" 
has just been released by the Ferguson 
Video Advisory Service to show the 
capability of the Videostar range. 

Aimed at both the customer and trade 
staff, the booklet covers all aspects of 
their video equipment and accessory 
range, plus a section devoted to cross 
compatibility with other makes. It also 
contains some general information on 
lighting, sound recording and connecting 
leads. 

Copies of the booklet are available free 
from Thorn EMI Ferguson Ltd., Dept EE, 
Cambridge House, Great Cambridge 
Road, Enfield, Middlesex, EN1 1UL. A 
stamped addressed envelope would be 
appreciated. 

Public Address Amplifier 

We have been informed that all semi¬ 
conductors used in the P.A. amplifier 
(May/Aug) can be supplied by Hart 
Electronic Kits, Pennyland Mill, Oswestry, 
Shropshire. 

CONSTRUCTIONAL PROJECTS 

Monthly Planner 

A source of supply for the 1-bit clock 
timer, IC1, which forms the heart of the 


Everyday Electronics, September 1982 


Monthly Planner project has proved the 
most difficult item to locate. 

The E 050-16 clock chip is not normally 
available in the UK but the author, Mr 
Donleavy has made special arrangements 
with the Swiss manufacturers to supply 
them to E.E. readers. 

The E 050-16 costs £6-50 and the crystal 
90p from A. P. Donleavy, 13 Wasdale 
Road, Liverpool 9. Add 20p postage and 
packing for all orders. 

The clock chip is available separately, 
but the crystal can only be supplied 
together with the E 050-16 device. H owever, 
other crystals can be used in the circuit. 

The rest of the semiconductor devices 
should be available from Ambit, Enfield, 
Electrovalue, Magenta and Watford Elec¬ 
tronics. 

The miniature keyboard push switches 
are now generally available and is left to 
individual choice on the type of switch 
used here. 


Continuity Tester 

Practically any of the low-voltage 
piezoelectiic transducers, available from 
most of our advertisers, should be suitable 
for the Continuity Tester. The device used 
in our model was the PB2720 (with case) 
obtainable from Ambit. 

The size of plastics case is not critical 
and any type may be used. 

Screen Washer Delay 

The relay used in the Screen Washer 
Delay is a low-profile encapsulated type 
with a 1000 ohm coil. This is a RS encap¬ 
sulated reed relay and is coded Blue, 
stock number 348-986. Any RS component 
supplier will be able to obtain this item. 

Note that the casing of capacitor Cl 
must be completely isolated from any 
metal when installed in a vehicle. This can 
be accomplished by wrapping in insulat¬ 
ing tape or rolled in a strip of polythene 
sheet. 


Temperature Interface for TRS-80 

A couple of special i.c.s are required 
for the TRS-80 Interface project. The 
LM334Z, adjustable current source used 
as a remote temperature sensor, is avail¬ 
able from Maplin Electronic Supplies; 
order No. WQ32K. The TL507C, single 
slope analogue to digital converter, may 
be obtained from most Tandy shops; 
stock.No. 276-1789. 

The printed circuit board has been 
designed to accommodate a specific 
RS transformer, stock No. 207-829. This 
transformer is available through any RS 
component dealer. Other suitably rated 
transformers may be fitted to the case and 
wired to the appropriate p.c.b. locations. 

The double-sided (20 + 20 way) wire- 
wrap edge connector may prove difficult 
to locate and may need to be cut from a 
larger version. One such item is the 2 x 
22-way strip from Watford Electronics 
which will allow alignment guides to be 
fitted. 


Sound Splitter 

There should be no component buying 
problems for the Sound Splitter or the 
add-on mixer unit. 

A suitable jack socket for SKI is the 
"stereo phone jack" from Tandy stores, 
stock number 274-277. 

On its own, a useful application for the 
Two-Way Remixer box is to couple two 
instruments to a single input amplifier. 


567 
























BY A. FUND 


S ince the long-awaited advent of 
legal CB radio, several types of 
40-Channel hand-held portable rigs 
have become available and are grow¬ 
ing increasingly popular. Most of 
these use HP7 type batteries, but 
with a current drain of 0-5A or more 
when transmitting, the cost of ordin¬ 
ary dry batteries quickly leads most 
users to invest in a set of recharge¬ 
able ni-cads. 

The problem of supplying a suit¬ 
able charger for these arises. Com¬ 
mercial chargers often cost as much 
as the batteries themselves and few 
incorporate any kind of automatic 
“full-charge” sensing feature. The 


charger described here is the author’s 
answer to this problem, at around 
half the cost of most units. 

It provides fully automatic, trouble- 
free performance and is simple to 
construct. It was designed to charge 
the complement of ten AA sized 
(HP7) ni-cads in the Harvard 410T 
rig, however, the Dixons’ Harrier 
WT2 40-channel handheld appears to 
be identical to the Harvard, and the 
Tandy Realistic 1001 also uses the 
same battery complement. Possibly 
other portables such as the DNT and 
Alba are similarly powered, so this 
charger design may prove useful to 
many readers. 


RECHARGEABLE NI-CADS 

A few facts about the ni-cad cells 
themselves may be of interest before 
continuing. Most readers will prob¬ 
ably be aware that these are avail¬ 
able as direct replacements for the 
comrtion sizes of 1-5V dry cell, and 
that they can be recharged up to 
1,000 times. 

In general they should be charged 
by a constant current, the value of 
which is normally quoted as being 
about a tenth of the cell’s capacity 
in ampere-hours (Ah). The AA size 
has a capacity of 0-5 Ah so the 
charge rate should be around 50mA. 
It is also stated that they cannot be 
damaged by long term continuous 
overcharging at this rate, however, 
it cannot be seen to do them much 
good either. 

Since the cell voltage rises quite 
steeply when the fully charged con¬ 
dition is reached, it is a fairly simple 
matter to detect this and reduce the 
charge rate accordingly. An advan¬ 
tage of this type of charger is that 
the battery can be connected at any 
time for “topping-up”, regardless of 
it’s initial state of charge. 

It is unwise to let batteries of 
ni-cad cells become completely ex¬ 
hausted as one cell will inevitably 
run down before the others, which 
then “reverse-charge” the flat cell 
and cause permanent damage to it. 
So topping-up whenever the equip¬ 
ment is not in use will greatly reduce 
the risk of this occurring. 


CIRCUIT DESCRIPTION 

Fig. 1 shows the complete circuit 
of the CB - Battery Charger. Trans¬ 
former Tl, bridge rectifier D1 to D4 


Fig. 1. Circuit diagram of the CB Battery Charger. 



568 


Everyday Electronics, September 1982 





































and capacitor Cl provide a smoothed 
d.c. supply of about 20V. The circuit 
has to charge the battery on a fixed 
current until a pre-set full-charge 
voltage is reached and then, maintain 
this voltage by reducing the current. 

The LM723 regulator i.c. was 
chosen for the circuit as -it contains 
a stable reference voltage source and 
an amplifier for comparing this 
reference with the battery voltage. 
The reference voltage of about TV 
appears at pin 6 and any noise 
present is decoupled by C2 before 
it is connected to the non-inverting 
amplifier input on pin 5. The inver¬ 
ting amplifier input, pin 4, is con¬ 
nected to the potential divider R4, 
R5 and VR1 placed across the cir¬ 
cuit’s output. 

The chip also has provision for cur¬ 
rent limiting, but the operation of 
this is not really sharp enough for 
the present purpose so a constant 
current generator circuit based on 
TR1 and TR3 has been incorporated. 

The action of this configuration is 
quite simple. A silicon transistor 
begins to conduct when it’s base-to- 
emitter voltage exceeds about 0-6V. 
The bias current supplied by TR2 to 
TR3 causes TR3 to conduct until the 
voltage across R6 reaches 0-6V, at 
which point TR1 starts to conduct 
away surplus bias current to pro¬ 
gressively limit the conduction of 
TR3. Thus the current through TR3 
can be calculated from Ohm’s law as 
being 0-6V/R6, and it will remain 
virtually constant regardless of load 
and voltage conditions. 


So provided the voltage fed back 
from the output potential divider is 
lower than the reference voltage, the 
output of IC1 will be fully positive, 
causing TR2 to supply about 10mA 
of bias to the constant current 
generator. This bias passes through 
the l.e.d. D2 which indicates that 
charging is in. progress. 

Once the feedback voltage reaches 
the reference value however, the 
drive to TR2, and hence the bias is 
rapidly reduced, so the output cur¬ 
rent drops to a value just sufficient 
to maintain the output voltage at the 
desired level and the l.e.cL virtually 
extinguishes, indicating that charging 
is complete. 



CIRCUIT BOARD 

Construction, using the simple 
p.c.b. shown in Fig. 2 is quite straight¬ 
forward. This diagram also shows the 
layout of all components. It’s a sen¬ 
sible precaution to mount the trans¬ 
former Tl, D1-D4 and Cl first, and 
then to carefully apply power and 
check that the voltage across Cl is 
around 20 to 25V d.c. Remember that 


COMPONENTS 


Resistors 
R1 22kO 
R2 lOkfl 
R3 6800 
R4, 5 18kO (2 off) 

R6 120 
Atl JW carbon ±5% • Page 567 

Capacitors 
Cl 470//F 63 V elect. 

C2 1/zF 35V tantalum 
C3 lOOOpF ceramic 
C4 1/zF 63V elect 

Semiconductors 
D1 -4 W005 50V, 1A rectifier 
D5 TIL209 miniature red l.e.d. 
D6 1N4001 silicon 
TR1 BC214L silicon pnp 
TR2 BC184L silicon npn 
TR3 BD136 silicon pnp 
IC1 LM723 adjustable voltage 
regulator 

Miscellaneous 

Tl Miniature mains transfor¬ 
mer, 9V-0-9V secondary 
VR1 lOkO miniature horizontal 
preset 

PL1 Plug to suit host CB equip¬ 
ment (the prototype uses a 
2-5mm jack plug) 

Single sided glass fibre p.c.b., 116 
x 36mm; plastic case, 120 x 65 x 
40mm (type BIM 2004); mains 
lead; twin cored lead; 7/0-2mm 
wire; l.e.d. mounting clip; mount- 
hardware for Tl (4 BA). 





Everyday Electronics, September 1982 


569 




























some parts of the p.c.b. will be live 
at 240V mains whilst testing, so take 
adequate precautions when doing this. 

The transformer is an inexpensive 
9-0-9V miniature type, the centre-tap 
of which is unused! so that it provides 
an 18V output. It is fastened to the 
board with a couple of 4BA screws. 

After assembling the rest of the 
circuit, check with an ammeter that 
the output of the unit into a resis¬ 
tance of 100 ohms is around 50 to 
60mA. Setting the output voltage 
requires a little more care. 

The voltage of a ni-cad cell when 
approaching the fully charged con¬ 
dition is about 1-45V, so the unit 
should be set to supply this value 
multiplied by the number of cells 
in the battery. The author’s CB rig 
is the Harvard model 410T, which 
contains ten cells but also incor¬ 
porates an internal silicon diode in 
series with the batteries in it’s 
charging circuit, so an extra 0-6V 
has to be added to compensate for 


this, bringing the total to 14-5V4- 
0-6V=15-2V. 

A check of the circuit diagram of 
your rig will show whether it contains 
a similar internal diode. Note that 
the voltage setting must be made 
with the charger supplying some 
current, so a resistance of lkilohm 
should be placed across the output 
before carefully adjusting VR1 for 
the correct output voltage. 

The finished board is designed to 
slip into the moulded mounting slots 
provided in the specified case, and 
on the author’s prototype, the cable 
strain relief clamps are simply a few 
turns of insulating tape around the 
input and output leads. A small hole 
is drilled for the l.e.d. which is then 
secured by means of a mounting clip. 

FAST CHARGER 

Experimenters might like to note 
that this circuit can be adapted quite 
simply for other output voltages and 
currents. A further interesting ap- 



By PAUL YOUNG 


Out and About 

Recently it occurred to me that it might 
be of interest to look in on component and 
equipment suppliers, giving a short history 
and other points concerning their busi¬ 
nesses. 

There can be few if any of our readers 
who are not acquainted with the firm of 
J. Bull. I have no doubt, that many readers 
have in the past called at his shop in 
Croydon to purchase goods. Unfortunate¬ 
ly, a large increase in rent forced him to 
move to Haywards Heath and store the 
greater part of his stock on his farm a few 
miles away. It was with expectation and 
excitement that I drove down to his home 
near Brighton to learn his story, which is 
an interesting one. 

Jesse Bull served with the Fleet Air Arm 
during World War II and started up his 
business in 1946 with his gratuity. The 
bulk of his business was and still is Mail 
Order, and originally he specialised mainly 
in "Kits". His first Kit was a television set 
made mainly from ex-RAF radar parts 
and used a 5-inch green tube. It proved to 
be very popular because commercially 
made sets were in short supply. 

The advent of the transistor gave a real 
boost to the sales and the company sold 
over 50,000 transistor radio kits. Older 
readers may remember, “The Good 
Companion" “The Pocket Four” and the 
"Spiderless Three". 

The importation of cheap Japanese sets 
gradually put paid to this market. Although 


Jesse still sells the more popular Kits, the 
emphasis has shifted more to bargain lines 
which are regularly displayed in his 
adverts. In addition he issues a monthly 
bargain list I was particularly taken with 
his offer of 10 kilo (22 lb) of components 
for £14-50 including postage and packing, 
this seemed to be an offer that cannot be 
refused. 

When we had finished our discussion, 
Jesse took me around the sheds that 
house his stock. Each shed is 100 yards 
long and about 15 yards wide, and is 
packed with every conceivable piece of 
electronic equipment you can imagine. 
You name it, I am sure Jesse Bull must 
have it. 

After we had examined two of the sheds, 
he smiled at me and said, "Do you want 
to see any more?" I had to reply honestly, 
“As interesting as they are, I will take the 
rest as read I” 

Computer Doctor 

Turning again to computers, in this 
country we don't seem to use them as 
frequently in diagnostic medicine as they 
do in Russia. Over there, due to the 
shortage of Doctors, 85 per cent of whom 
are women, the computer is often sub¬ 
stituted for the G.P. and hard as it is to 
believe they claim a better success rate 
than the real G.P.s. 

While I was trying to evaluate how this 
would work out in practice, my resident 
Imp, who can never be entirely suppres- 


plication would lie in the construction 
of a “fast” charger. The ten-hour 
charge rate usually quoted for ni-cads 
is the maximum at which they will 
withstand indefinite overcharging; if 
provision is made to reduce the 
supply current when the charge is 
complete they can be charged in far 
shorter times, down in fact to as 
little as fifteen minutes. 

A charge rate of 1A, completing 
the charge in 50 minutes is appar¬ 
ently quite feasible for the AA size. 
Note that if they are overcharged 
at these higher currents gas will 
form and they will vent, thereby 
losing electrolyte. The “button” types 
of cell have no provision for pressure 
venting and under similar conditions 
may explode, so don’t try it with 
these! 

However, with an up-rated trans¬ 
former and some heatsinking for TR3 
this design could be modified into an 
efficient high speed charging system. 


sed, paints the picture for me. "Can't you 
imagine it?" he says, “This poor chap 
goes into the surgery with an ingrowing 
toenail, sits down at the computer and 
presses all the relevant buttons. 

"Unfortunately unknown to him, this 
machine is malfunctioning due to a couple 
of dry joints on one of the circuits and 
having put him out cold with a smart tap 
on his ‘noggin’ with a Black Jack, pro¬ 
ceeds to operate. When the poor man 
regains consciousness he finds either he 
has had a vasectomy or is minus an earll" 

While reluctantly agreeing with my Imp, 
I have an answer, which is this, that even 
real flesh and blood doctors make similar 
mistakes, usually by getting their patients 
mixed upl 


Ship’s Doctor 

I must confess this method of treating 
patients, is rather like the method used in 
small ships. On a ship of less than 5,000 
tons, a Doctor need not be carried. This 
chore being carried out by the Captain or 
Mate. 

To assist them they have a Board of 
Trade Medicine Chest, which contains 
all the medicants likely to be required on 
the voyage. These are numbered and there 
is a book of instructions. 

If, for example, a sick sailor complains 
of headaches, the book would instruct 
the Locum to give him one tablet number 6. 
On returning to port, the skipper would 
report to the port Medical Officer, who 
would question him about any sickness 
on the voyage. 

I must conclude with the story of the 
skipper reporting to the Medical Officer, 
which goes as follows: M.O. "Well 
Captain, did you have a good voyage?" 
Captain. "Splendid, thank you.” M.O. 
“Any medical problems?” Captain, “Not 
really, I did have one of the crew who had 
stomach pains and I had run out of pill 
number 12.” 

M.O. “Oh! What did you do?” Captain, 
"OhI it was quite simple really, I gave him 
one number 7 and one number 511" 

I am confident that computerised 
medicine will do better than that. 


570 


Everyday Electronics, September 1982 









BASIC ELECTRONIC THEORY 
WITH EXPERIMENTS 

COMPUTING CIRCUITS 


In this final instalment of the series we 
1 bring our study of electronics up to date. 
We see how some of the simple circuits we 
have already studied can be put together to 
make more elaborate systems capable of 
performing calculations and logic. 

The term “computer” is taken to refer to 
the digital computer nowadays, since most 
of the computers we use are of this type. 
Analogue computers operate in a different 
way and, although they have been replaced 
by the digital computer as a calculating or 
logical tool, they still have important 
applications. 

They are usually based on the oper¬ 
ational amplifier (see Part 7, EE April 1982). 
Indeed it was the need for high-performance 
amplifiers for use in analogue computers 
which gave rise to the 741 and other op- 
amps which are still so widely used today. 
Before we see how op-amps are used for 
calculating, we must study one more 
amplifying circuit. 


INVERTING AMPLIFIER 

The circuit configuration in Fig. 12.1 is 
different from the other op-amp circuits 
we have studied in that the input voltage is 
fed to the inverting input (—) of the amp¬ 


lifier. Consequently, an increase of V It! 
produces a decrease of K 0 U t, and the 
other way about. 

Since the output signal is fed back 
through R24 to the negative input, this 
amplifier has negative feedback. The 
amplifier is stable only when both of its 
inputs are at the same voltage. Since the 
non-inverting input is at OV, the amplifier 
can be stable only when the inverting input 
too is at OV. 

We can think of R23 and R24 as a 
potential divider (Fig. 12.2). For any given 
value of Km, the amplifier adjusts V 0 vt 
until the voltage at the inverting input (—) 
is exactly OV. In effect, if F TN is a positive 
voltage, a current flows from the input of 
VR1, through R23, R24, and into the 
output of IC1. If this current is /, and 
knowing that the p.d.s. are as shown in 
Fig. 12.2, we can calculate that: 



R23 R24 


The negative sign is needed because K IN 
and K 0 ut have opposite signs. Rearranging 
this equation we get: 




Fig. 12.1. Using an op-amp as an inverting amplifier. Offset null compensation is not 
essential and is omitted. You could connect up this circuit on Minilab and test its action. 
For positive input voltages from VR1, reverse the connections to the meter. 


With the values given in Fig. 12.1, the 
amplification is x(— 10). Note that the 
amplification depends only on the ratio 
between R23 and R24. It does not depend 
upon the gain of the op-amp circuit, or 
variations in the manufacture of the op- 
amp, or the temperature in which it is 
operating. 

If we use two high-precision resistors 
with high temperature stability, amplifica¬ 
tion is precisely determined and is stable. 
This is essential if we are to use the circuit 
for computing. 

Note that the calculation above does not 
include any current flowing into or out of 
the inputs of the op-amps. A small current 
(about lOOnA for the 741) flows to the base 
of each of the input transistors. These 
currents should be equal, otherwise an 
offset p.d. will appear between the inputs, 
causing errors in operation. 

In Fig. 12.3, R26 is roughly equal to R23, 
R24 and, R25 in parallel. These are effec¬ 
tively in parallel since one end of each is at 
OV under stable conditions. Since the 
current through R26 is only lOOnA, the p.d. 
across it is only 0-0034V. We can consider 
the input voltage to be almost OV, and the 
description above still holds good. 



Fig. 12.2. The resistors of the inverting 
amplifier re-drawn as a potential divider. 
The current flowing to the (—) input can 
be ignored as it is very small. 


Everyday Electronics, September 1982 


571 
































EXPERIMENT 12.1 

Op-amp adder 

The circuit in Fig. 12.3 is recognisable 
as an inverting amplifier, but it has two 
inputs, via R23 and R25. Since R23, R24 
and R25 all have the same value, the gain 
of the amplifier is 1. We will be using only 
positive inputs, so outputs will be negative 
and the meter connections are reversed 
accordingly. 

R27/R28 and R29/R30 are potential 
dividers used for providing known input 
voltage: 

V A = (12 x 270/(150 4- 270) - 6)V =1-7V 
y B = (12 x 270/(180 + 270) - 6)V =1-2V 
The exact voltages at A and B depend upon 
the actual values of the resistors; use the 
meter to check that they are near to the 
expected values. Then reconnect the meter 
to the output of the op-amp. 

Connect flying lead X to point A. With 
a gain of 1 the output should be — F IN , so 
the meter should read 1-7V. Connect X 
to B\ the meter should read 1-2V. Input Y 
is identical to input X, so you should 
obtain the same pair of results by using 
lead Y instead and connecting it to A and 



Fig. 12.5. Potentials and currents in the 
adder when inputs are 1-2V and 1-7V. 


Now connect lead X to A and lead Y to B. 
Does the meter read 2-9V? The op-amp 
acts to bring its.in verting input to 0V. The 
currents which are flowing through R24 
and R25 (Fig. 12.5) combine and flow 
through R26. Thus the output voltage must 
be -100000 x 29 x 10-* = -2-9V, 
which appears on the meter as 2-9V. The 
currents are added, so the output voltage is 
the sum of the two input voltages. 


Answers to Part II 

11.1. Its voltage is easily trans¬ 
formed. 

11.2. So that current is relatively 
small and little power is lost 
from heating the cables. 

11.3. 0V. 

11.4. +339V. 

11.5. 114V. 

11.6. 6-25mA. 

11.7. 1.11 A, r.m.s. 

11.8. 8-4V. 

11.9. 10-6V. 

11.10. It increases in amplitude. 


EXPERIMENT 12.1 




Make the connections needed to find the 
sum of 1-2 and 1 -2, and read the result. 
Now find the sum of 1-7 and 1-7. You 
could experiment with other resistors in the 
potential divider to get different sets of 
input voltages and add them. If R29 is 
changed to 330fi, V m is —0-6V. Now you 
can find the sum of 1-7 and -0-6, in 
other words, subtract 0-6 from 1-7. Read 
the meter to find the answer to (I -7 — 0-6). 


EXPERIMENT 12.2 

An op-amp differentiator 

The mathematical operation of differen¬ 
tiation is a way of calculating a rate of 
change. In Part 9 (EE June 1982) it was 
explained that the current flowing into or 
out of a capacitor depends on the rate of 
change of voltage. In this circuit (Fig. 12.6) 
we change the voltage on one side of a 
capacitor. The current flowing out of the 
other side goes to an op-amp. 


The layout for this Experiment is shown 
in Fig. 12.7. To begin with, the capacitor is 
charged to +6V. Now press and hold SI to 
discharge C9 through R23. Note the 
maximum value reached by the needle of 
the meter. It kicks up, showing that voltage 
is falling, but quickly returns to 0V as the 
capacitor discharges and the rate of fall of 
F lfi decreases to zero (see Fig. 9.5). 

Release SI: the meter kicks down, as the 
voltage increases. 

Now replace R23 by a wire link, so that 
C9 may be discharged more rapidly. Press 
and hold SI. The rate of change of V a is 
much greater now, and the needle moves 
much further up the scale. 

This is only a simple demonstration, but 
it shows how an op-amp can be used to 
calculate rate of change of voltage. Such a 
circuit could be used in real-time computing, 
for example, when we want to measure a 
rate of change of a quantity such as velocity, 
to calculate acceleration. 


572 


Everyday Electronics, September 1982 





































EXPERIMENT 12.2 




Fig. 12.7. The layout of the components on the Verobloc for the circuit in Fig. 12.6. 


QUESTION TIME 

12.1. Which of the quantities in this 
list is not an analogue quan¬ 
tity: velocity of a car, engine 
temperature, the number of 
wheels on the car, the amount 
of fuel in the tank. 

12.2. If R24 in Fig. 12.1 is replaced 
by a 1 -2Mn resistor, what will 
be the gain of the circuit? 

12.3. If the circuit of Fig. 12.1 has 
/in = —20mV, what is its 
output? 

12.4. If a circuit is set up as in Fig. 
12.2, but with three inputs, and 
the input voltages are 1-2V, 
0-4V and —1-1V, what is the 
output voltage? 

12.5. If in Fig. 12.6, C9 was con¬ 
nected to +3V instead of 
+ 6V, and SI was NOT 
pressed, what would be the 
reading on the meter? 


Suppose that the counter has 8 flip-flops 
in series (Part 4, EE January 1982), then the 
greatest binary number the counter can 
register is 1111 1111, or 255 in decimal. The 
velocity may range smoothly from 0 to 100 
metres per second, but the count can take 
only 256 distinct values in the range 0000 
0000 to 1111 1111. We can have a count 
of 134, or one of 135, but we can not have 
anything in between. Instead of having a 
single meter to indicate the quantity, we 
need several lamps or other devices, one to 
indicate the state of each digit. 

The great advantage of representing 
values in digital form is that it is so much 
easier to deal with them at high speed with 
relatively simple and reliable circuits. We 
have already seen in Part 4, how the two 
kinds of binary digit are represented by OV 
and +5V respectively, and how gates can 
be built which will perform logical opera¬ 
tions. Now we will see how these gates can 
be used for performing calculations. 


ANALOGUE AND DIGITAL 

It is implied in the previous sentence that 
we can represent velocity by a voltage. 
Both are analogue quantities in that they 
can vary smoothly over a given range and 
be represented by the position of a pointer 
on a scale. 

The velocity of a vehicle over the range 
0 to 100 metres per second can be repre¬ 
sented in an electronic speedometer by 
voltages in the range 0Vto+ 5V(Fig. 12.8a). 
Then a voltage of 3-524V, represents a 
velocity of 70-48 metres per second. For 
each possible value in the range of 
velocities there is a corresponding voltage. 
As the velocity changes and hence the 
voltage changes, an op-amp differentiator 
could compute acceleration in metres per 
second per second. 

We can also represent velocity as a binary 
digital number, as in Fig. 12.8b. The voltage 
is fed to a voltage-controlled oscillator, and 
the number of pulses per second is counted. 



Fig. 12.8. Analogue and digital quantities, (a) a wholly analogue system using an analogue 
computer (b) analogue input converted to digital form. The digital output can be fed to 
a digital computer to calculate other quantities (for example, acceleration). 


Everyday Electronics, September 1982 








































EXPERIMENT 12.3 

A Half-Adder 


The circuit in Fig. 12.9 (Verobloc layout 
in Fig. 12.10) includes a logic gate which 
we have not used before, an exclusive-or 
gate. The arrangement of the four gates 
within the 4070 i.c. is just the same as that 
of the 4011 (see Fig. 4.10), but their action 
is different. The exclusive-or gate has two 
inputs. Its output is high when either one 
input or the other input, (but not both), 
inputs is high. Table 12.1 is its truth table, 
and Table 12.2 reminds us of the truth 
table for nand gates. 

The half-adder circuit has two inputs, 
which are normally held low by R23 and 
R24. When the buttons are pressed, the 
corresponding inputs are made high. The 
state of the outputs is shown by the l.e.d.s; 
a “0” is represented by the l.e.d. being off, 
a “1” by it being on. 

Try the combinations of inputs listed in 
Table 12.3, by pressing the corresponding 
buttons and note the state of the outputs. 
Use the truth tables to work out how the 
circuit operates. 

We think of this circuit as having two 
inputs, representing two numbers which 
are to be added together. Each number has 
only 1 digit, so the numbers to be added can 
be either 0 or 1. If you press neither button, 
this is equivalent to adding 0 + 0. Their 
sum is indicated by the state of D3 (column 
ZS in the table). You will have found that 
0 + 0 = 0,0 + 1 = 1, and l +0 = 1. 

When we sum I and 1 in binary arith¬ 
metic, the result is 10 (= 2 in decimal). In 


Table-12.1: EXCLUSIVE-OR. 


Inputs 

A B 

Output 

Z 

0 0 

0 

0 1 


1 0 


1 1 

0 


Table 12.2: NAND. 


Inputs 

A B 

Output 

Z 

0 0 

0 1 

1 0 

1 

1 

0 


Table 3: Results of testing Half- 
Adder. 


A(S1 l ) nPUt S(S1) 

Outputs 

ZC(D2) ZS(D3) 

0 0 

0 1 

1 0 



EXPERIMENT 12.3 




Fig. 12.10. The layout of the components on the Verobloc for the circuit in Fig. 12.9. 
Unused inputs are connected to 0V or +6V so that the i.c.s operate correctly. 


effect we say “1 plus I gives 0, carry I". The 
1 is carried over into tne next column on the 
left, and we write down the answer, 10. In 
this circuit the carry digit is represented by 
D2 (column ZC). 


FULL ADDER 

The half-adder can perform only the 
most basic of summing operations. It sums 
two 1-digit numbers and produces a 1-digit 
answer, with a carry digit. 

When we add numbers on paper we sum 
one column at a time and take the carry 
digit over to be added in the next column to 
the left. Similarly, we could have several 
half-adders, each dealing with the corres¬ 
ponding digits from two multi-digit num¬ 
bers, and passing the carry digit to the next 
half-adder to the left. This gives a full- 
adder, which can consist of enough half¬ 
adders to allow it to sum two numbers of 
any given number of digits. 


EXPERIMENT 12.4 

Full Adder 

Fig. 12.11 shows a 2-digit full adder. It 
can add a number A'A (where + is the 
least significant digit), to a number B'B, 
giving the sum as a 2-digit number Z'Z, 
with a third carry digit, ZC. 

It is clear from Fig. 12.11 that this 
circuit consists of two half-adders. Digits A 
and B go to the first half-adder (IC3a, 
lC4a and IC4b), and digits A' and B 1 go to 
the second (IC3b, IC4c and IC4d). The 
carry digit from the first half-adder is 
added to the sum digit of the second half¬ 
adder by the exclusive-or gate (IC3c), just 
as we add in the carry digit in ordinary 
arithmetic. 

The suggested Verobloc layout for Expt. 
12.4 is shown in Fig. 12.12. 

The A 1 and B 1 digits are given the value 
“1” by pushing the flying leads A 1 and B l 
into sockets A20 and A22. 


574 


Everyday Electronics, September 1982 

















































EXPERIMENT 12.4 




Fig. 12.12. The layout of the components on the Verobloc for the circuit of 
Fig. 12.11. 


As an example of how to use the circuit, 
try the addition, 1 + 3. A 1 A is 01, B l B is 11 
(the binary equivalent of decimal 3). A 1 is 
to be 0 so lead A 1 can be left loose; B 1 is to 
be 1, so push lead B' into socket ,422. Then 
press SI and S2 to make both A and B 
represent 1. You should find that D3 and 
D2 do not light, but D1 does. This indicates 
the sum 100 (equivalent to decimal 4). 


Answers to Part 12 

12.1. Number of wheels. 

12.2. 120 times. 

12.3. +2-4V. 

12.4. (1 -2 + 0-4 -1-1)V = 0-5V. 

12.5. C9 is charged to + 3V so km 
is not changing. Its rate of 
change is zero, so k 0 cT is zero. 


Try adding other pairs of numbers, such 
as 1 + 2, 2 + 2, 2 + 3, and 3 + 3. 

The readout can be made much clearer 
by using the Minilab display module. 
Remove the connections to D1, D2 and D3. 
Now make these connections to the 
display module: 

Digit Z —from J24 to display o 
Digit Z 1 —from B22 to display b 
Digit ZC—from B9 to display c 
Connect display d and L inputs to the 0V 
rail (strip M of the Verobloc). Switch on 
the module (S8), and you can now read the 
answers directly in decimal; the display 
module does the binary-to decimal de¬ 
coding for you. Run through all the sixteen 
different sums that the adder can do, from 
0 + 0 to 3 + 3, and check that it gives the 
right answers. 


OTHER CALCULATIONS 

Readers who are proficient in binary 
arithmetic will know that binary subtraction 
can be performed by a routine which 
essentially consists of addition. We can 
multiply two numbers together by a series 
of additions. 

For example, to find 3 x 5, we add three 
fives together. First we add 5 + 5 = 10, then 
we add a third 5 to that sum to get 15- All 
we need is a full adder, and a way of 
counting how many times the addition has 
been performed. This is just what happens 
in a microprocessor when it has been 
programmed to multiply. 

Division can be done by repeated sub¬ 
traction. To divide 15 by 5 we subtract 5 
from 15, leaving 10. We continue subtract¬ 
ing 5 from the remainder until the result is 
zero. Zero is reached on the third sub¬ 
traction, so (15 + 5) = 3. This is how logic 
circuits are able to perform the four basic 
operations of arthmetic. 


MICROPROCESSOR 

To perform other mathematical opera¬ 
tions we simply have to arrange for a 
sequence of such operations to be per¬ 
formed. A microprocessor, the heart of the 
microcomputer, can be programmed to 
perform such sequences and thus can be 
made to carry out all kinds of mathematical 
operation. 

The circuits which are used in a computer 
are basically very simple ones, concerned 
with elementary logical operations such as 
nand and nor, and the simple addition of 
two binary digits. The reason that com¬ 
puters seem to have almost human powers 
is that they can be programmed to perform 
a sequence of tens of thousands of such 
simple operations in a single second without 
making mistakes. 

It is hoped that readers who have 
followed this series will have gained some 
insight into the inner workings of many of 
the electrical appliances such as thermo¬ 
stats, radio sets, and amplifiers, which are 
so common in our homes today. It is also 
hoped that this final part of the series will 
have helped take some of the mystery out 
of the mighty microcomputer. 


Everyday Electronics, September 1982 































































BY G.L.STONEMAN 


W ith all the rain, sleet and snow 
that seems to deluge our cars 
every year, windscreens tend to be¬ 
come soiled very quickly, leading to 
a reduction in the driver’s visibility. 

To overcome this problem, the 
driver must regularly operate the 
windscreen washers. This results in 
the driver removing his hand from 
the steering wheel and interrupting 
concentration! on the road ahead. 

This article describes the construc¬ 
tion of a simple but effective device 
that operates the electric screen 
washers for a period up to about 
10 seconds or so after a single press 
of a switch, thereby ensuring that 
one hand does not have to leave the 
steering wheel for more than a 
second or two allowing the driver to 
concentrate more on driving. 

The circuit is designed for opera 
tion in cars with a negative earth 
but details are also provided for 
modifications to allow it to be fitted 
and used on positive earth vehicles. 

CIRCUIT DESCRIPTION 

The complete circuit diagram of 
the Screen Washer Delay is shown 


in Fig. 1. It can be seen that only 
a few components are required. 

When the washer switch is pressed, 
Cl is immediately charged up to I2V 
and the relay contacts close. When SI 
is released, Cl slowly discharges 
through the coil of RLA. During this 
time, the relay contacts RLA1 stay 
closed and supply power to the wind¬ 
screen washer motor, resulting in a 
stream of water to the screen. 

As Cl discharges through the relay 
coil, the current through the relay 
decreases and eventually a point is 
reached where this current is in¬ 
sufficient to operate the relay. The 
contacts then open resulting in the 
pump motor turning off. VR1 is used 
to set the operating time of the pump 
motor. 

COMPONENTS 

The relay used on the prototype 
was a low-profile p.c.b. encapsulated 
reed type with a 1,000 ohm 9-12V 
coil. This is colour coded blue. If a 
different type of relay is used, one 
that has two pairs of contacts, com¬ 
bined wash and wipe with one-shot 
operation could be achieved. 


Capacitor Cl should be one with 
axial leads (one from each end) rated 
16V or more. The can should be com¬ 
pletely insulated before mounting 
using p.v.c. tape or tubing. 


CIRCUIT BOARD 

Construction is straightforward 
with the components being mounted 
on a piece of 0-1 inch pitch strip- 
board measuring 13 strips by 24 holes. 

Regin by drilling the two 6BA 
clearance holes in the board! for 
mounting purposes. Use the 4-way 
terminal block as a template for this 
using the two outermost holes. The 
board is to be secured using the ter¬ 
minal block fixings through the case. 



Fig. 1. The complete circuit diagram of the Screen Washer Delay 



COMPONENTS 


R1 100D iW carbon ±5% 

Cl 1,000/iF 16V elect, axial leads 
VR1 470 ohm miniature horizontal preset 

RLA encapsulated reed type relay, 1,000 
ohm coil, 9 to 12V operating voltage 

FS1 5A with in-line fuseholder 

Stripboard: 0-1 inch matrix, 13 strips x 
24 holes; 5A screw terminal block, 4-way; 
6BA fixings: 25mm long screw, nuts 
(2 off), 5mm long spacer; plastics case, 
Vero 202-21025K; rubber grommet; 
5A auto connecting wire. 



Everyday Electronics , September 1982 


























Also use the terminal block as a 
template for drilling the holes in the 
case. 

Make the necessary breaks in the 
copper tracks using a spot-face cutter 
or small drill bit. These are necessary 
to isolate the fixings from the com¬ 
ponents soldered to the tracks. 

Position and solder the components 
and flying leads to the board accord¬ 
ing to the layout in Fig. 2. A suitable 
case for this project is the General 
Purpose Plastic Box from Vero, size 
72 X 50 X 25mm. Drill a hole of size 
suitable for a grommet to be fitted 
to carry the wires from the board to 
the terminal block fitted to the case 
top. 

Fit the board and terminal block to 
the case and wire the flying leads 
to TB1. Fig. 3 shows the suggested 
way of mounting the board using a 


Fig. 2. Layout of the components 
on the stripboard, breaks to be 
made on the underside and wiring 
details tothe existing car electrics. 


Fig. 3. A suggested method of 
mounting the component board 
in the case. 


lock-nut arrangement. This is neces¬ 
sary as the unit will be subjected to 
vibration in the car. 


INSTALLATION 

Wiring to the existing car electrics 
is also shown in Fig. 2. Do not use 
single-core wiring for this. Use 5A 
wire intended for use on a car, 
readily available from car spares 
shops. Fig. 2 shows wiring for a 
negative earth system. For positive 


TERMINAL BLOCK |p|» 

PLASTIC CASE ^— I gL-> 

1 i 

1 fel 

i 

Y “HI 

V 

l(«»—/ 

)i 


earth cars, simply reverse the capa¬ 
citor, Cl, and the connections to the 
pump motor. In either case make sure 
that a 5A fuse is fitted, connected to 
the “live when on” terminal on the 
ignition switch, (or a suitable terminal 
in the fuse box). 

The unit should be mounted in a 
dry place away from exhaust fumes 
and extreme temperatures. It could 
be fitted to a suitable position inside 
the car, on the parcel shelf perhaps 
using self adhesive foam pads. H 


NEWS 


GIRL GRANTS 

Only two per cent of engineering technicians in 
British industry are female. To encourage more girls 
into technician employment the Engineering Industry 
Training Board is offering 250 grants to firms willing 
to recruit girls over and above their normal planned 
technician intake. 

Each grant is worth £6,000 and the scheme starts 
in September 1982. 

Fibrevision 

Eighteen lucky families in Milton Keynes are having 
a free trial of British Telecom’s Fibrevision, the “wired 
city” of the future. Through optical cable they get five 
TV channels, pay-TV, Prestel and FM radio selected 
by hand-held infra-red control. 


Sir Michael Edwardes, of 
British Leyland fame, is to 
be a part-time director of 
Project Mercury, private 
industries answer to British 
Telecom. 


Intelpost, the Royal Mail’s 
high-speed public facsimile 
transmission service, is now 
extended to cover the whole 
of Holland. 


SPEAKING BOOK 

Following on from TTs 
educational “Speak and 
Spell” for children, the com¬ 
pany has developed a logical 
follow-on in the form of a 
talking book. The text has 
a barcode which is scanned 
manually with a lightpen by 
the child to produce the 
spoken sound. 

It will be available in the 
USA by Christmas and in 
the UK next year. 


PLEASE TAKE NOTE 

CIRCUIT EXCHANGE- 
INVADER LANDING GAME 

(August 1982) 

The circuit diagram incorrectly 
shows the cathodes (k) of the l.e.d.s 
D3 to D12 connected to the positive 
supply rail (B1 + ). This link must 
be removed, leaving these l.e.d.s 
connected to the negative supply 
via the 470 ohm resistor, R8 (as 
shown). 


Everyday Electronics, September 1982 


577 




















































By BARRY FOX 


Resistance to Space 

Everything electronic and mechanical 
goes wrong in the end. And it isn’t just 
domestic equipment that fails. This is why 
satellites, which are beyond the reach of a 
repair engineer, must be made to the 
highest possible standard of reliability. 
But Meteosat-1, the European weather 
satellite which was launched in November 
1977, failed almost exactly two years later. 
The power supply just shut down. So a 
replacement, Meteosat-2 had to be built 
and launched in June 1981. So what went 
wrong with Meteosat-1 ? 

The answer is to be found in the April 
1982 issue of the journal of the Institution 
of Electronic and Radio Engineers, albeit 
in very obscure wording. After the failure 
a team of aerospace experts met at the 
European Space Agency Research Centre 
in Darmstadt, West Germany, and built a 
breadboard replica of the circuit that had 
failed in space. With this circuit they then 
tried to simulate the fault. 

To quote the IERE journal "It appeared 
that a digital circuit, which was designed 
to be triggered only in the case of over¬ 
current, was able to oscillate depending 
on the value of a resistor. Technological 
studies showed in fact that a degradation 
mode specific to this resistor caused its 
resistance value to be equal to this critical 
value". 

When you translate this technical "gob- 
bledegook” into plain words, it means that 
a single resistor in the power supply 
changed value as it aged. This tripped a 
safety circuit breaker like a fuse, in the 
satellite power supply. 

Thankfully Meteosat-2 has different re¬ 
sistors. It also has relays which can by¬ 
pass the protection devices if they go 
haywire. 

So far Meteosat-2 has been working 
without problems. But when you get down 
to the nitty gritty, the stark truth is that a 
meteorological satellite which costs tens 
of millions of pounds to put into orbit 
failed because a single resistor developed 
a fault. 


Domestic Facsimiles 

There’s a lot of talk these days about 
document transmission systems, for use 


in homes and small offices. Already of 
course large offices have facsimile equip¬ 
ment which can transmit pictures over the 
telephone line. 

This is possible, despite the small band¬ 
width of a phone line, because the trans¬ 
mitting machine scans the source picture 
slowly, to produce a slow or low frequency 
stream of information. This is sent down 
the phone line to control a receiver which 
prints out a copy picture at equally slow 
rate, for instance onto heat sensitive 
paper. 

There are obvious advantages in having 
a facsimile machine in the home, hooked 
up to a telephone. Where it’s impossible 
to describe something by spoken word, 
you can send pictures or graphs down the 
line. Where there are strings of facts and 
numbers to be communicated, it's safer to 
send them as a written page. 


Car Statics 

A tip on static. Modem cars have nylon 
or similar man-made fibre upholstery. This 
wears well but can generate very high 
voltages of static electricity when rubbed, 
for instance when the driver or passenger 
slides across a seat. Then when you get out 
of the car and put a key in the door to lock 
it, you get a very unpleasant jolt as a spark 
jumps across. 

Car owners try all kinds of cures. Often 
you’ll see a length of chain or conductive 
fibre trailing from the rear bumper. This is 
intended to keep the car at earth potential. 
But static is an unpredictable beast and 
you’ll often still get just the same belt from 
an earthed car. 

The reason is that your body picks up 
the high voltage charge as you get out of 
the car and separate yourself from the nylon 
upholstery. This leaves two oppositely 


Domestic facsimile reception could 
eventually replace the postman. Trans¬ 
mission and delivery take only ias long as 
the machines take to scan a page. You 
don’t even have to be. at home to receive 
the transmission. If a message comes in 
while you are out, it's there waiting for you 
on your return, like a spoken message on 
an answering machine. 

Japanese Line 

Needless to say the Japanese are al¬ 
ready excited about domestic facsimile 
systems. The Japanese Post Office, 
Nippon Telegraph and Telephone (NTT) 
is hard selling Minifax. Even by the end of 
last year NTT had installed nearly 4,000 
Minifax units in Japanese homes, for a 
connection fee of around £12 and a 
monthly rental of £8. 

The NTT sales campaign was based on 
the idea of using Minifax instead of tele¬ 
phone answering machines. But now the 
practical problems have started to emerge. 

Junk Mail 

The Minifax receiver canhot distinguish 
between messages which the subscriber 
wants to receive, and those which are un¬ 
welcome. So obscene callers can send 
lewd pictures and text down the phone line 
to unsuspecting victims. 

Perhaps even worse, advertising firms 
can deluge subscribers with unwanted 
material. This isn’t just inconvenient to the 
recipient, it is also very expensive. The 
subscriber has to pay for all the paper used 
by his Minifax machine to receive whatever 
comes down the line. 

At the moment junk mail sent through 
the post doesn't cost the recipient any¬ 
thing. You can even get your own back on 
anyone who sends you too much junk 
mail, by simply returning it to sender with¬ 
out a postage stamp. The originator of the 
junk mail then has to pay twice the normal 
postage and very soon strikes your name 
off their mailing list. But the owner of a 
Minifax has no choice but to pay for all 
junk mail. 


charged objects, the car and your body or 
your highly charged body and an earthed 
car. Either way you’ll get a shock when you 
touch the car again—unless the weather is 
damp so that the charge leaks away 
naturally, very quickly. 


The trick 

The trick for dry weather, which took me 
an infuriatingly long time to learn, is to get 
out of the car in a special way. As you lift 
off the car seat you make sure you are 
holding the car metal work with a wide firm 
grip. So as you leave the car seat any static 
equalises or discharges through the wide 
contact area of your hand. 

Usually you won’t feel a thing. And you 
won’t get a shock when you touch the car 
door with a key because there’s no longer 
any charge to jump across. 


578 


Everyday Electronics, September 1982 














ELECTRONIC 

HOGSIEStFRIR 


Alexandra Pavilion November 18 - 211982 


Launched in response to demand for a 
major national show of the highest 
quality, EVERYDAY ELECTRONICS, PRACTICAL 
ELECTRONICS and PRACTICAL WIRELESS 
are presenting the biggest and best event 
ever to be staged for the electronic 
hobbies enthusiast. 

If your company is involved in the 
manufacture of electronic components, 
equipment, ancillary systems for 


electronic projects, home computers, 
amateur radio, citizens band, video 
games, musical instruments-you should 
be there! 

For further information about exhibiting 
contact: The Exhibition Manager, 
Electronic Hobbies Fair, ipc Exhibitions 
Ltd., Surrey House, 1 Throwley Way, Sutton, 
Surrey SMI 4QQ. Tel: 01-643 8040. Extn. 4873. 



L 


Send to: The Exhibition Manager, Electronic Hobbles Fair, ipc Exhibitions Ltd., Surrey House, i Throwley way, Sutton 


.Surrey SMi 4QQ. | 


Everyday Electronics, September 1982 


579 
















RADIO WORLD 


By Pat Hawker, G3VA 


A Better Picture? 

N EW and developing technology makes 
interesting reading and journalists 
not unnaturally tend to play up the im¬ 
portance of each and every advance. On 
the other hand, a considerable number of 
last year’s “exciting new breakthroughs” 
gradually sink unsung out of sight. "In 
five years time everybody will be ..." is an 
attractive cliche since the writer is usually 
well aware that in five years time nobody, 
except possibly himself, will remember 
that over-confident forecast and hold it 
against him. 

The number of financially successful 
inventors (or nowadays the more usual 
research and development teams) is 
relatively small: “Pioneerin' don’t pay” 
was a classic belief of Andrew Carnegie. 
But the consumer electronics industry 
keeps on hoping: video discs, digital 
audio, large-screen projection television, 
direct-broadcast satellites, the electron¬ 
ically wired city, high definition television 
and of course home computers are all 
seen as growth areas. 

However, the industry does display 
some worries about all-digital audio that 
has to com pete with high-quality analogue 
audio, a branch of the industry that is 
currently feeling the effects of the long 
industrial recession. Similarly with video 
discs that have arrived on the scene rather 
later than expected and without the record 
as well as playback facility of the video 
cassette recorders which have proved one 
of the few really popular new products 
of recent years. 

Surprisingly VCRs have been in greater 
demand in Europe than in either the USA 
or Japan. 

Cinema Quality 

What about high-definition television 
(HDTV)? Japanese, American and Irish 
broadcasting organisations (NHK, CBS 
and RTE) recently co-operated in demon¬ 
strating the remarkable NHK 1125-line 
system with wide-screen (1-85 :1 aspect 
ratio) and separation of the chrominance 
and luminance components. 

All who saw the demonstration agree 
that the system provides a superb picture, 
virtually the equivalent of good cinema 
film. You can sit very close to the screen 
without being worried by the line structure. 
Several forms of display device, including 
a widescreen picture tube and high- 
resolution projection systems, have been 
successfully developed. 

But there are problems. The 27MHz of 
basic video bandwidth cannot be accom¬ 
modated in our broadcast bands, not even 
the European 12GHz satellite band al¬ 
though CBS wants to try using two 
adjacent DBS channels in the United 
States, when that country begins direct- 
broadcast satellite services. 

To obtain real benefit it needs a large 
screen. We still seem some way off from 
a high-resolution, large-screen display 
system within reach of many viewers. 


HDTV does offer the possibility of all¬ 
video cinema presentations or for the 
dubbing of electronic video on to film. The 
idea of using 1000 line television to make 
cinema films is an old one. 

In 1951-52 a British company, High 
Definition Films, had a black-and-white 
1000-plus line system and showed that 
with such a system the very high cost of 
film making could be reduced. But I do 
not think they ever overcame the problem 
that film-makers, particularly the produc¬ 
tion teams, much prefer the techniques 
used in film (short sequences, single 
camera, post-production editing) to those 
of the large multi-camera electronic studio. 
Paradoxically, electronic production can 
today closely resemble that of film— 
though this then tends to put the cost of 
video up because of the high cost of 
machine-time for intensive editing. 

The old question “how good is good 
enough?” may well be asked, and certainly 
I suspect it will be some time before many 
viewers in their own homes will be Watch¬ 
ing pictures of the quality shown in 
Killarney. Though all credit to the Jap¬ 
anese engineers who began work on the 
system in 1970 and have since made 
remarkable progress. 


Solar Storms 

The month of June witnessed some 
severe solar flares and other disturbances 
that had the effect of upsetting h.f. 
propagation conditions. These included 


The Sting 

Did: they fall or were they pushed? 
That seems to be the question that 
can be resolved only in the American 
courts. 

I refer to the astonishing “Japscam” 
operation in which major Japanese 
electronics firms admit they parted 
with $648,000 for secret information 
that they thought would allow them 
to market computers plug-compatible 
to those of the giant IBM firm. The 
money was paid to a Silicon Valley 
consultancy firm Glenmar. But Glen- 
mar happened to be a “front” for an 
FBI operation “Pengem” (Penetrating 
the Grey Electronics Market). 

The Japanese firms claim they had 
no knowledge that the secret data 
was “stolen”. Indeed, since it was 
material fed by IBM to the FBI for 
this operation it was arguably not 
“unlawfully” obtained by Glenmar— 
and indeed this type of sting opera¬ 
tion does appear to have more than 
an element of “agent provocateur” 
about it. 

The Americans are clearly worried 
about the continued flow of informa¬ 
tion and high-technology to East 
Europe, and competitive “know how” 


at least one almost total "blackout" during 
which it is possible to spin the dials of a 
powerful shortwave receiver and yet hear 
no signals except those within ground- 
wave distances. 

It is a strange experience to find usually 
crowded frequencies devoid of all activity. 
Total blackouts occur only during day¬ 
light and often seem most severe between 
about 5 and 10MHz. 


More complaints 

The Home Office report on the investi¬ 
gation by British Telecom engineers of 
complaints about interference to tele¬ 
vision and radio reception for 1981 shows 
very substantial increases both in com¬ 
plaints received (70,452, up 96-85 per cent) 
and completed investigations (60,571 up 
47-42 per cent). Much (but not all) of the 
increase is due to interference from the 
27MHz amplitude-modulated CB rigs 
before the issue of licences for 27MHz f.m. 

It is also clear that much of this inter¬ 
ference was not due to spurious or 
harmonic radiation from the CB trans¬ 
mitters but reflects the vulnerability of so 
much domestic electronic equipment to 
strong local signals, in other words poor 
"electromagnetic compatibility” (emc). 
Domestic equipment is much less affected 
by f.m. signals but the report does further 
dent the original Home Office case for 
advocating 934MHz for CB on the grounds 
that lower frequencies would cause 
interference due to harmonics. 

The investigators found 14,359 cases 
where complaints were due to CB rigs, 
considerably more than the usual worst 
offender—the thermostats in central heat¬ 
ing systems etc. which accounted in 1981 
for 8,318 complaints. A large proportion 
of the CB interference was found to be 
due to direct breakthrough into the audio 
stages of solidstate domestic equipment 
and a lot could be prevented if manu¬ 
facturers added some bypass capacitors 
and ferrite-bead chokes. 


also to Japan. FBI have recently been 
briefing American elecronics firms 
about techniques of industrial espion¬ 
age that are far more sophisticated 
than the usual stories of bugging the 
boardroom. 

In the UK, one gains the impres¬ 
sion that inter-firm com,petition does 
include a certain amount of trickery. 
For example there appear to be firms 
that advertise non-existent jobs and 
then pump applicants about what 
their present firms are up to. But 
in-depth acquisition of design data is 
probably a good deal less common— 
partly I suspect because so many 
British firms and engineers are firmly 
convinced that if an idea is “not in¬ 
vented here” (NIH) it must be worth¬ 
less! 

Perhaps the most surprising feature 
of the incident is the lavish amount 
that the Japanese firms admit 
paying. By any standard, £350,000 is 
a fantastic sum to pay for information 
received, whether acquired legally or 
illegally, and shows the scale on 
which the computer industry now 
operates. 


Everyday Electronics, September 1982 











1 


o 

Doctor 

m i 


• 

Dentist PP 



: 

Bridge Write 

DATE 

Birthday 




Start holidays 

Monthly 

Planner 

• 

End holidays 




Cinema 




2 

Set O j 

v - 

3 


BY A. P. DONLEAVY 


T HIS project describes the con¬ 
struction of a calendar, which 
displays the date and a particular 
event which is to occur on that day, 
for example, a visit to the dentist or 
perhaps a birthday. There are nine 
possible events, one of which can be 
selected for a particular day. 

The design uses two simple 164-bit 
ram memories to store the events 
for the month. These memories are 
easily programmed using d.i.l. 
switches and the information for any 
particular day can be changed at any 
time. 


SYSTEM OPERATION 

Fig. I gives a block diagram of the 
system illustrating the various func¬ 
tions. The crystal controlled dock 
produces one negative going pulse 
every 24 hours. This is fed to a 
dedmal counter which displays the 
date information and also to a binary 
counter. 

The outputs from the binary 
counter are connected to the address 
inputs of the memory. Each binary 
number from the counter represents 
a day of the month. The information 
in the memory store for that day is 
then displayed in the event display. 

Thus there are then two counters 
which operate in parallel, the dedmal 
counter for the date and the binary 
for tiie memory address. The binary 
counter resets itself and the dedmal 
counter on the 32nd pulse. 


CIRCUIT DESCRIPTION 

The rircuit is shown in Fig. 2. IC1 
an EO50-16, provides the clock 
pulses. A brief description of the 
function of this i.c. may be of inter¬ 
est. It is made by MEM, part of a 
Swiss watch making concern, and is 
primarily intended for industrial 
timekeeping uses. The chip uses 
32,768Hz crystal and an infernal 
dividing rircuit to provide negative 
going pulses at intervals of seconds, 
minutes, hours and days at pins 12, 
11, 10 and 7 respectively. 

The crystal XTAL1 acts as the time 
base for IC1. For correct functioning 
there should be a 1-5V difference 
between pins 16 and 1 of IC1 and this 
is provided by D1 and Rl, using the 
forward volt drop of the l.e.d. 

The daily output pulses from pin 
7, IC1, are fed via R2 to input pins 


9 of IC3 and 2 of IC4a. IC2 and 3 are 
both cmos 40110 counter/latch and 
drivers for a seven segment display. 

These two i.c.s count the daily 
pulses and display the date. When 
the count of IC2 goes from 9 to 0, a 
pulse appears at the carry output, 
pin 10, which is connected to the 
clock up input of IC3. The clock 
down facility of these ix.s is not used, 
nor the latch and toggle facili¬ 
ties. Hence, pins 7, 6 and 4 are tied 
to earth. 

The two MAN3740 displays are 
common cathode types with their 
cathodes connected to the collector of 
TR1 for a reason which will be ex¬ 
plained later. 

BINARY COUNTERS 

IC4, is a 4520 cmos dual four-bit 
binary counter. The two counters are 
connected together to form an eight- 
bit binary counter and this is done 
by connecting the Q4 output (most 
significant bit) of one counter, pin 6, 
to the enable input of the other 
counter. The enable and clock 
inputs of this i.c. can be reversed 
causing it to increment on a differ¬ 
ent polarity pulse transition. 

Since one count is required for 
each day of the month, a maximum 
of 31 counts are required, so that 
the counter is arranged to reset 
itself at a count of 32. This repre¬ 
sents a binary output of 00100000, 
the 1 being on pin 12. The reset 
inputs of IC4 are 7, 15, and for the 
decimal counters, IC2, IC3 it is pin 
5. 

IC7 and IC6 are two cmos 64-bit 
random access memories (ram) with 
a 16 x 4 arrangement. That is 16 loca¬ 
tions with a 4-bit word. The two mem¬ 
ories together provide 32 locations, 
of which a maximum of 31 will be 
used for the days of the month. 

The information stored in a loca¬ 
tion is essentially a number from 
0 to 9 written in b.c.d. (binary coded 
dedmal). The outputs from the 
memories are from pins 5, 7, 9, 11, 
and are fed to the inputs of IC8, a 
cmos b.c.d. to dedmal decoder, the 
74042. Thus the four-bit word 
written in the selected memory 
location is displayed directly as a 



581 


Everyday Electronics, September 1982 











































number 0 to 9 by the l.e.d.’s con¬ 
nected to the outputs of IC8. The 
outputs of IC8 are normally high, 
and go low when selected. 

As previously stated, each mem¬ 
ory has 16 locations numbered from 
!0000 to 1111. So for example, on the 
third day of the month the binary 


counter will be at 0010 since the first 
location is at zero (0000) and not at 
1 (0001). The corresponding 

address (location) inputs (pins 13, 
14, 15 and 1) of the two memories 
are connected to each other, as are 
the data inputs (pins 4, 6, 10 and 
12) and outputs (5, 7, 9 and 11). 


Memory enable, me, inputs are 
arranged so that if IC6, for example, 
is at logic 1, then IC7 is at logic zero 
and vice versa. This is achieved with 
the arrangement of the four nand 
gates of IC5. When the me pin is at 
1, the outputs of the i.c. have a high 
impedence; and give out no informa- 


Fig. 2. Monthly Planner circuit diagram. Note that SX need not be an actual switch as this is only required to ground pin 5 of IC1 at midnight 



582 


Everyday Electronics, September 1982 


























































































































tion (having feri-state or three-state 
oultputs). Also the inputs will not 
accept new information. Hence the 
i.c. is ineffective in the circuit. 

Between the counts of 0 and 15, 
pin 11 of IC4 is at 0, so IC6 we is at 
0 and IC7 me is at 1. Thus IC6 is the 
active memory. Between the counts 


Fig. 3. Power supply 
circuit. Note that 
some 9V adaptors 
drop below 9V under 
load so an extra 
diode in series with 
D13 may be neces¬ 
sary to prevent B1 
discharging. 


on the first day of the month. S3 to S6 input data to the memory. 


V D D 





7 7 7 7 




ov 


of 16 and 31, pin II of IC4 is at 1, 
thus IC6 me is at 1 and the IC7 me 
is alt 0. IC7 is now the active mem¬ 
ory. 

To read information from the 
memory, the write enable we input 
must be at 1. A read cycle is accom¬ 
plished by causing a 1 to 0 transition 
of the me pin while the we remains 
high. This happens as follows. The 
output of IC1 is a negative pulse of 
about 32/is. The falling edge of this 
pulse increments the binary counter 
(IC4), thus establishing the address. 
This pulse is also applied to pins 8 
and 1 of IC5 via D2, causing the me 
pin of whichever memory l.c is 
active to go high. 

The rising edge of the time pulse 
will then cause the me of the active 
chip to go low again, thus fulfilling 
the requirements for the information 
in the memory to be transferred to 
the outputs. This sequence’ of events 
happens automatically as the timing 
pulses arrive from IC1, since the we 
(pin 3) are held high by R5, as S2 
is normally open. 

MEMORY INPUT 

To write information into the chip 
the we input must be low, and the 
me input must see a 1 to 0 transition. 
To do this, when SI is pressed the 
counters will increment one count 
every second until the required date 
is reached. On pressing S2, the we 
input goes low, and the me input also 
sees a 1 to 0 transition from the nega¬ 
tive pulse transmitted via C2 and 
R5. D2 is included to stop this write 
pulse from being transmitted to the 
inputs of IC3 and IC4 and increment¬ 
ing the counters. 

On receiving this 1 to 0 transition, 
the data at the inputs of the i.c. is 
written into the memory at the selec¬ 
ted location. The data inputs are pins 
4, 6. 10 and 12, and in this design 
represent the range from the LSB 
(least significant bit) to the MSB 
(most significant bit) in that order. 
Pins 4, 6, 10 and 12 are tied) toV DD 
by R9 R8 R7 and R6 respectively. 

By switching in any of S3 to S6, 
the information can be set. For 
example, if S3 and S4 are closed, the 
information presented to the data 


Everyday Electronics, September 1982 


583 





































































inputs is 0011 (3). However, the 
information at the outputs is the 
complement of the inputs. So if 0011 
is set ait the input, the outputs would 
give 1100. 

This difficulty is easily overcome 
merely by relabelling the switch posi¬ 
tions 1 instead of 0, and 0 instead of 
1. Having set the switches to the 
desired information, pressing S2 will 
write the information into the 
memory. 

POWER SUPPLY 

Almost all the current in the cir¬ 
cuit is consumed by the displays and 
the l.e.d.s. The unit is therefore in¬ 
tended to be powered by a 9v calcu¬ 
lator adaptor, which allows a perma¬ 
nent display of the date and event. 
However IC1 is only specified to a 
maximum of 5 • 5V. So a 5V regulator, 
IC9, is used to provide the Y DD 
voltage. 

However, should for any reason 
the power supply be cut off, then all 
the stored information will be lost. 
To avoid this, a back up battery is 
used. The circuit in Fig. 3 shows the 


arrangement. When the power is 
coming from the adaptor and is cut, 
the base current supply to TR1, and 
also the supply to D3 to D12, is out. 
Hence, the displays are disconnected, 
but the back up battery supplies cur¬ 
rent to the i.c.s and the information is 
retained. 

The current supply in this case is 
about 2mA so a PP3 battery can 
supply enough current for many 
hours. Diodes D13 and D14 stop one 
supply feeding current into another. 



Fig. 4. Optional battery only power supply. 
The display is enabled with S7. 


COMPONENTS'^ 


Resistors 

R1, 2 18kfi (2 off) 

R3 10kO 

All JW carbon ±5% 


C3 


4-7pF sub-miniature ceramic plate 
0-0047(uF polyester, axial lead 
47y«F 16V tantalum bead 



Semiconductors 

D1 Miniature (3mm) red l.e.d. 

D2, 13, 14 1N4001 silicon rectifier (3 off) 

D3-11 Miniature (3mm) l.e.d. colours to suit (9 off) 

D12 TIL221 0-2in green l.e.d. 

TR1 BC108 silicon npn 

IC1 E050-16 1-bit clock timer 

IC2, 3 40110B CMOS decade up/down counter/driver (2 off) 

IC4 4520B CMOS dual binary up/down counter 

IC5 401 IB CMOS quad 2-input NAND gate 

IC6, 7 40114B (74C89) 64-bit random access memory (2 off) 

IC8 74C42 CMOS b.c.d. to decimal decoder 

IC9 78L05 5V, 100mA regulator 

XT AL1 32,768Hz miniature tubular quartz crystal 

XI, 2 MAN3740 (TIL313, DL304 or HP5082) 0-3in high, 7-segment, 

common cathode display (2 off) 


See 

Shop 

Talk 

page 567 


Miscellaneous 

SI, 2 Push-to-make miniature keyboard switch (2 off) 

S3-6 4-s.p.s.t. 8-pin d.i.l. switch 

S7* Push-to-make miniature push button 

SKI P.C.B. mounting power socket to suit plug on 9V mains adaptor 

B1 9V PP3 batter^ 

B2-4* 1-5V size AA batteries (3 off) 

0-1 in stripboard, 62 holes by 39 strips; single sided copper clad Paxolin or 
s.r.b.p. sheet, 175 x 125mm (front piece); wooden picture frame, internal 
dimensions 175 x 125mm; red plastic display filter, 65 x 35mm; 9V mains 
adaptor; PP3 battery clip; 3mm l.e.d. mounting clip (9 off); 16 pin d.i.l. holder 
(7 off); 14 pin d.i.l. holder (3 off); 1/0-6mm sleeved wire for board links; 7/0-2mm 
wire for interconnections; 6BA or M2-5 spacers, 13mm long (4 off); 6BA or 
M2-5 screws, 6mm long (8 off); 6BA or M2-5 nuts (3 off); 6BA or M2-5 nylon 
screw, 6mm long. 

* Components marked thus are only required for battery only operation. 


If the 9V adaptor voltage drops 
below 9V on load!, a 1N4001 diode 
must be added in series with D13, 
effectively reducing the voltage at 
which the back-up battery is brought 

Therefore, the output of the adap¬ 
tor must be measured, on load, to 
determine if this diode is required. 

For flexibility in positioning, the 
supply circuit shown in Fig. 4 can be 
used. Using 3 1-5V cells, the display 
will only light when the additional 
switch, S7, is pressed. With the low 
consumption the battery life, should 
be over a year. 


COMPONENTS 

D1 may be any red l.e.d. but a 3mm 
is advised for size consideration. Use 
good quality l.e.d.s for D8 to Dll 
since the current drive from the 
output of IC8 is not high. Any colour 
or size may be chosen, the prototype 
used a 3mm with a mixture of colours. 
D12 should be a green l.e.d. since the 
circuit is employing the forward 
voltage drop to prevent the diodes 
from passing current into the outputs, 
which are off. 

The seven segment displays may 
be any one of the following: TIL313, 
DL304, HP5082 as they are pin for 
pin equivalents of the MAN3740. 

The two 40114 memories given in 
the components list are pin for pin 
equivalents for 74C89 i.c.s. It would 
be possible to use a 74LS42 (ttl) for 
IC8, but the overall current would 
increase by about 8mA which would 
slightly reduoe the back-up battery 
life in the situation where the mains 
adaptor is frequently disconnected. 

Switches SI and S2 are push-to- 
make circuit board mounted types, 
designed for making up keyboards. 


CIRCUIT BOARD 

All components except for D3 to 
Dll are mounted on a piece of 01 
inch matrix stripboard, 62 holes x 39 
strips. Fig. 5 shows the layout. This 
diagram also Shows the breaks made 
in the copper tracks. 

Use of i.c. sockets is recommended 
since the removal of i.c.s is often 
necessary when debugging circuits. 

There are many wiring connections 
to be made, and many breaks to be 
made in the tracks, so some construc¬ 
tional errors may occur, so take care. 

The 8-pin d.i.l. switch is mounted 
on the copper side of the board, using 
small pieces of wire soldered to the 
tracks and soldered to the d.i.l. tags 
on the other ride of the board. Dist 
carded leads are useful for this. 

Solder the capacitors, transistors, 
and diodes as dose to the board as 
possible so that they do not stand 
proud of the mounted ICs. Use vero- 
pdns for making the connections 
from the board to l.e.d.s D3 to Dll. 


584 


Everyday Electronics, September 1982 












































































































































FRAME 

Owing to the nature of the project, 
the unit Should ideally be housed in 
something more attractive than the 
usual plastic box. To this end, the 
prototype was housed in a commer¬ 
cially available photograph frame of 
internal dimensions 125 x 175mm, 
the glass is replaced hy a piece of 
p.c.b. of the same size. Use metal 
polish to polish the copper to a bright 
finish. Copper board with a paxolin 
base, besides being much cheaper, is 
also better than glass fibre for this 
purpose, since the woven texture of 
glass fibre tends to come through the 
copper. 

Drill the holes for the diodes D3 
to Dll, the holes for SI and S2, and 
cut out a rectangle 43 x 25mm for 
the display window. Repolish the 
board if necessary. Also drill the 
pillar support holes using the p.c.b. 
as a template and finally spray a 
clear varnish on the board to stop the 
copper from tarnishing. The use of 
copper hoard as a frontpiece can be 
made to look very effective. 

On the prototype model, two white 
panels of self-adhesive p.v.c. sheet 
were stuck in the position shown in 
the accompanying photographs. On 
this, the functions of each event 
l.e.d. can be written in felt pen 
(although we used Letraset on the 
model) and wiped off should the 
event change. 

Another possibility, if the picture 
frame system is used, is to use a 
card over the frontpiece with suit¬ 
able cutouts for the displays and 
l.e.djs, and to replace this every 
month with a fresh set of events 
written in. 

The strip board is held to the 
frontpiece using four 0-5in long 
6BA spacers. The switches SI and S2 
should just protrude through the 
holes drilled in Ithe frontpiece. Use a 
red plastic filter to cover the window 
for the display The l.e.d.s are held 
in place on the frontpiece using 
bezel dips. 

The baok up battery can be tied to 
the stripboard using wire threaded 
through the holes. 

SETTING UP 

No calibration is required since the 
timing depends on the crystal fre- 
auency and any minor trimming of 
this crystal would be beyond the 
means of most constructors. 

So assuming the unit is debugged 
and working, the first thing to be 
done is to synchronise the binary and 
decimal counters. Do this by pressing 
SI (set) and waiting until the date 
display resets to 1. This may take a 
long time depending on the random 
number in the binary counter when 
first switched on. A quicker way is 
to momentarily short pirn 5 of IC2 
or IC3 to V D d. The display will zero. 


Note the date, when automatically 
reset by the binary counter, will 
reset itself to 01 and not 00, for the 
following reason. The hinary counter 
increments on the leading edge of 
the pulse so on the 32nd pulse both 
counters are reset by the leading 
edge, a process which takes less than 
l/'S, and 32/ts later the trailing edge 
of the timing pulse increments the 
date (decimal) counter. The differ¬ 
ence in count will always be the 
same and therefore is of no conse¬ 
quence for the working of the unit. 


Table 1. Event input codes. Note switch 
open = 0 and switch closed = 1. 


EVENT 

BINARY CODE 

S6 

S5 

S4 

S3 

No event 


0 

0 

0 

0 

Doctor 

(D3) 

0 

0 

0 


Dentist 

(D4) 

0 

0 


0 

Bridge 

(D5) 

0 

0 



Birthday 

(D6) 

0 

1 

0 

0 

Start holiday 

(D7) 

0 


0 

1 

End holiday 

(D8) 

0 


1 

0 

Cinema 

(D9) 

0 

1 



Aux. 2 

(D10) 


0 

0 

0 

Aux. 3 

(Dll) 

1 

0 

0 

1 


The unit can now be programmed. 
When the programming switches S3 
to S6 are open then the information 
at the programming inputs are 1111, 
and 0000 when all these switches are 
closed. However, because the infor¬ 
mation of the output is the complc 
ment of that of the input, it will be 
necessary to label the open position 
as an “0” and closed position as “1” 
S3, S4, S5 and S6 represent the LSB 
to the MSB respectively. 

To aid the setting up procedure, 
a small plate was screwed to the 
back of the stripboard and labelled 
with the event codes and the “1” and 
“0” positions of switches S3 to S6. 


EVENT INPUT 

To write the events for each day 
of the month into the memory, the 
procedure is as follows : 

Suppose that, for example, the 
bridge dub meet on the first day of 
the month, then ensure that the date 
reads 01 and set up the code for 
bridge on switches S3 to S6. From 
Table 1, that code is 0011 (switches 
S3 and S4 dosed and S5 and S6 
open) and when set, press S2 
(white) and the information is put 
into the memory. 

Proceed to day 02 by pressing SI 
(set) and enter the code for the 
event planned for this day. If nothing 
is planned, the memory must be told 
by writing in the No event code, 0000 
(all switches open) and pressing S2 
(write). 

Continue through all the days in 
the month, inserting the relevant 
event code for each with S3 to S6. 

Remember that no event must be 
entered on all days on which nothing 
is scheduled. It is not necessary to 
use all nine event codes and events 
can be duplicated on different days 
should, for example, the dentist need 
be visited twice in the same month. 

To make the timer send out its 
daily pulse at midnight to change 
the date, it will be necessary to stay 
up once until midnight and momen¬ 
tarily short pin 5 of IC1 to OV (“sx” 
on fig. 2). This resets all the internal 
connters. This operation will need to 
be performed again if the power from 
both the supplies it cut. 

The information written into the 
memory can be read non destructively 
any amount of times merely by pres¬ 
sing SI until the required date is 
reached. Finally, when the monthly 
information has been written, the 
correct date is set with SI. H 


586 


Everyday Electronics, September 1982 


















INTRODUCING 

ELECTRONICS 


An easy treatment of the basic theory from d.c. circuits through to analogue anch digital systems. Illustrated with simple 
experiments using standard components connected up with 2-amp terminal blocks. The solderless technique employed 
makes this the ideal series for newcomers. 


Prize Winning Project 

COMPUTER 
EXPANSION SYSTEM 

A motherboard for the ZX81 Computer containing 4K bytes of static RAM. Plugs into the back of the ZX81 to provide unique 
address decoding signals for a further eleven IK blocks, and these with buffered address and control lines and data bus are 
available at three parallel single-sided sockets for user defined expansion. Equipped with RESET and assisted +5V supply. 


OPTICAL TACHOMETER 
p\\J \5 CAR LIGHTS ALERT 

SIMPLE SHORTWAVE RADIO 


Everyday Electronics, September 1982 


587 













Consumer Electronics 
-Show- 


I n America, interest in hi fi and 
• audio is still declining; the video 
cassette market is stabilising; video¬ 
discs are not selling as well as ex¬ 
pected and the video games market is 
booming so fast it couldJ soon produce 
a glut of outmoded games cart¬ 
ridges on the market and commer¬ 
cial casualties among firms selling 
them. These are the clear signs 
which emerge from the 1982 Summer 
Consumer Electronics Show, held in 
Chicago, in June. 

CES is now acknowledged as the 
largest, and most important, exhibi¬ 
tion of its kind in the world. It is 
also the most influential. Around 
70,000 wholesalers, retailers and 
journalists from all round the world 
congregate for four days to look at 
new products from over a thousand 
companies. What they see, think, 
order for their shops and write about 
in print can have a decisive effect on 
what the public buys over the next 
year. 

HI-FI AND COMPUTERS 

In America, as in Europe, the audio 
and hi fi trade is suffering badly both 
from the recession and widespread 
interest in video. The large manu¬ 
facturers hope that things will pick 


up again when the Compact digital 
audio disc is launched. But lack of 
interest amongst American record 
companies has put back the digital 
disc launch until at least this time 
next year, or six months after the 
launch scheduled for Europe and 
Japan. 

Home computer sales are clearly 
on the rise and it was good to see 
Britain’s own Sinclair system attrac¬ 
ting much attention on the Timex 
stand. It sells in the USA for just 
under one hundred dollars. 

VIDEO MARKET 

The video cassette market in 
America has now stabilised, with a 
split of around two-to-one between 
VHS and Beta formats. Plans to 
launch the European V2000 format 
have now been shelved 

JVC has now shown a miniature 
VHS portable recorder, called VHS-C. 
This is smaller and lighter than any 
other portable recorder using half 
inch tape, because it uses a scaled 
down version of the standard VHS 
cassette. The tape can be replayed 
either in the portable, or in a stan¬ 
dard domestic VHS recorder using a 
dummy VHS cassette in which the 
mini cassette is temporarily housed. 


Stereo VHS recorders, with Dolby 
B noise reduction, are now available, 
along with pre-recorded stereo VHS 
software. So far there has not been 
a stereo Beta machine on sale in the 
USA, although they are available in 
Japan. Sound quality is always a 
problem with stereo on a domestic 
video format, because the slow run¬ 
ning tape has only a narrow audio 
track, and splitting it in half for 
stereo degrades the already poor 
signal-to-noise ratio. 

Sony has now developed a system 
for encoding stereo sound inside the 
video waveform. Although the com¬ 
pany refuses to explain how this is 
done, it seems that the waveform 
coding is in addition to a conventional 
linear track in mono or stereo. This 
preserves compatibility with existing 
Beta machines and tapes. 

LASERS AND DISCS 

Laservision players and videodiscs 
have been on sale in the USA since 
December 1978 and the cheaper RCA 
Selectavision disc system has been 
sold since March 1981. Both formats 
were on show at Chicago, with much 
puff and publicity. But the retail sales 
of both formats have been dis¬ 
appointing. This has caused JVC and 


Everyday Electronics, September 1982 












Thora-EMI to pull the plug on VHD, 
the third videodisc system which was 
scheduled for launch around the 
world this summer. No firm date for 
a VHD launch has been set. 

The US trade breathed an audible 
sigh of relief at the delay on VHD 
because shop keepers already have 
difficulty in explaining to customers 
the difference between the two in¬ 
compatible disc formats. They do not 
welcome the added confusion of a 
third, and again, incompatible format. 

VIDEO GAMES 

The video games market is boom¬ 
ing, largely because several com¬ 
panies are now making games cart- 
tridges for consoles sold by other 
companies. It is, for instance, now 
possible to buy a games cartridge 


for an Atari video game from nearly 
a dozen different software companies. 

Even Mattel, makers of the rival In- 
tellivision system, now offer Atari 
cartridges. But to confuse the issue 
even further Atari has now announced 
the launch of another games console 
which is incompatible with the Atari 
units and cartridges already on the 
market. 

As most of the games available are 
either banal, boring or pander to our 
baser violent instincts, the video 
games explosion is not altogether 
welcome. Also, because of com¬ 
petition, ever-more complex games 
appear every day. So some manufac¬ 
turers may soon go out of business 
and suffer the same fate as the video 
graphic blips which their game 
players must strive to eliminate. 


TELEVISION 

The USA has not yet agreed on a 
standard for the transmission of 
stereo sound with TV. Three different 
multiplex systems have been pro¬ 
posed, none the same as that being 
recommended as the European stan¬ 
dard. There is also no agreement on 
a.m. stereo, with six competitive sys¬ 
tems trying to win support on the open 
marketplace. There is no agreement 
yet on direct broadcasting by satellite. 
But many video buffs now own three 
metre wide dishes which enable them 
to tap into the satellite links between 
cable stations. In this way they can 
watch pay TV programmes free of 
charge—at least until the stations 
start electronic scrambling. 



LETTERS 


Everyone needs a Woolies 

Dear Mr. Fox, 

I am writing to you to defend the vicious 
attack you made on Woolworth's 
employees in your article For Your 
Entertainment on MFP (Music for 
Pleasure) records in the July issue of 
Everyday Electronics. 

First of all I don't understand what you 
mean by "shops like Woolworths”. 
Firstly you should have wiitten "shops 
like Woolworth", and secondly there is no 
other shop like Woolworth. The reason 
for this is that Woolworth is a general 
store stocking a lot of different items, and 
not specialising in any fields whatsoever. 

When you say "shops like Wool- 
worths" I suppose you mean shops such 
as Boots and Tesco whose names are 
mentioned at the end of many adverts 
with Woolworth in the “and other leading 
stores" category. Well these stores 
specialise in toiletries and food respec¬ 
tively. Other "general stores” are usually 
very big, for example, Bentalls, 
Chiesmans, like their pricesl So you see 
there is no other shop like Woolworth. 
Anyway I digress. 

As for the "assistants often know next 
to nothing about what's available", firstly 
it's very difficult to keep track of what 


albums are sold as the stock is very large, 
and there is usually more than one helper 
in the record department, so that one 
rarely knows what has and has not been 
sold, and thus what is and what is not in 
stock. After all the assistants are only 
human and not computers as most 
customers would want them to be. 

Secondly, with regard to actual MFP 
records, do you know how difficult it is 
to stock them? Obviously not if what you 
write is anything to go by. I can tell you 
that Woolworth only sells them as a 
service and not as a profit making item. 
This must be true for all the stores that 
sell MFP records. This therefore is bound 
to get the albums neglected and put to 
the back “out of the way”. Also the 
suppliers of these albums don't go out 
of their way to help the retailers. All they 
do is give the retailer an MFP turnstile 
rack and a batch of records to fill it up 
with and that's it. 

Only when one particular album has 
sold an obvious greater amount than the 
others can a retailer order a set of that 
particular record. That is a very rare 
occurrence and more often than not what 
happens is that when the stocks in the 
MFP rack have become low, the suppliers 
are informed and another batch of assorted 
records is sent up. You'd be very lucky 
to get one of the more popular albumsl 

So you see it’s very difficult for a retailer 
to keep track of what’s in the rack. This 
explains both the knowing "next to 
nothing" and why other specialist shops 
do not stock MFP records: they’re just 
too much bother I 


I have worked on and off for Woolworth 
for two and a half years now, and am fed 
up with people "slanging them off" at 
every opportunity. It is a good general 
store with the items sold being of good 
quality. Any item which is not will auto¬ 
matically be exchanged since the after¬ 
sales service is on the whole very efficient. 
Most important is the fact that the prices 
are kept as low as possible. 

I hope you publish this letter so that 
the readers of your article will be able to 
hear both sides of the story. It's rotten to 
take the poor service of the suppliers of 
MFP records out on Woolworth, it's not 
Woolworth's faultl 

Martin Gosling, 
Chessington, 
Surrey. 

I am quite happy for the editor to publish 
your letter in its entirety. 

/ am sorry you think I made a vicious 
attack on Woolworth. / can only say I have 
watched what / regard to be the decline of 
British Woolworth with disappointment. 

Two stores near my home in North 
London have recently closed and I can't say 
/ am surprised. Prices were high compared 
to other shops. The range of goods was 
very poor. 

In America the story is very different. 
Frankly / would not be surprised if Wool- 
worth finally go out of business in Britain, 
although I would be sorry because we need 
a general store of this type. 

I note your comments on MFP with 
interest. 

Barry Fox. 


Everyday Electronics, September 1982 










Everyday. News 

TOP MARKS FOR SCHOOLS 


Minister applauds budding engineers at SEDAC prizegiving ceremony 


T he first Schools Electronic Design Award Com¬ 
petition (SEDAC), sponsored jointly by Mullard 
Ltd, and Everyday Electronics, reached a splendid 
and exciting climax on June 29 when the final 
judging and presentation of prizes took place in 
Mullard House, London. 

The finalists—12 in all—came from schools all over the 
country. Prom Durham, Middlesex, Dorset, Surrey, 
Leicester, West Midlands, Lancashire, Cornwall and 
Yorkshire. It said something for the determination of the 
contestants that, in spite of a crippling rail strike, there 
was a 100 per cent turn out. 

Opening the prizegiving ceremony, Ivor Cohen, managing 
director of Mullard Ltd., stressed the importance of tech¬ 
nology to the future of the country and praised the 
enthusiasm and hard work that schools had obviously put 
into their projects. 

Pat Barnes, managing director of the Youth & Practical 
Group, IPC Magazines Ltd., endorsing Mr. Cohen’s remarks, 
said: “If SEDAC has helped concentrate attention on the 
importance of encouraging an interest in technology among 
our youngsters—all of whom are growing up in an age of 
increasing technological development—the efforts of all those 
concerned with organising the competition have been amply 
rewarded.” 

The prizewinners, who received their awards from William 
Shelton, MP, Parliamentary Under-Secretary of State, Depart¬ 
ment of Education & Science, Were as follows: 

1st: Peel Sixth Form College, Bury, Lancs. £150 
The SEDAC Trophy and components to the value of £100. 
2nd: Burscough Priory High School, Burscough, Lancs. 

£100 and components to the value of £100. 

3rd: Mellow Lane Comprehensive School, Hayes, Middlesex. 

£50 and components to the value of £100. 

The runners-up were Belmont Comprehensive School, 
Durham; Queen Elizabeth’s School, Wimborne, Dorset; St. 
Peter’s & Merrow Grange Comprehensive School, Guildford, 
Surrey; Lutterworth Grammar School, Leics; High Park 
School, Stourbridge, W. Midlands; Richard Lander School, 
Truro, Cornwall; Aireborough Grammar School, Leeds, 
Yorks; Tettenhall College, Wolverhampton, W. Midlands; and 
Hollins High School, Accrington, Lancs. All received com¬ 
ponents to the value of £50. 


The proud winners of the 1982 SEDAC Trophy, Anthony Hudson 
and Martin Lysejko of Peel Sixth Form College show off the trophy 
to the Minister William Shelton, MP. Ivor Cohen of Mullard and 
Pat Barnes of IPC Magazines Ltd. 

The top prizewinners’ project was a ZX81 Microcomputer 
Expansion System. Burscough Priory High School’s entry was 
an Oscilloscope Companion. And a Velocity Measurer won 
third prize for Mellow Lane Comprehensive. 

In his address the Minister congratulated all concerned 
and said he was impressed especially by the practical aspect 
of the competition—for in addition to preparing a written 
paper the contestants had to produce a working model for 
final assessment. Mr Shelton emphasised the government 
concern that involvement in technical subjects such as 
electronics be fully encouraged amongst our school children. 

Registering some surprise that there were no girls amongst 
the finalists, Mr. Shelton hoped the fair sex would demon¬ 
strate their interest in technology by coming forth in 
strength in next year’s competition. 

Winding up the ceremony, Mr aBrnes made the announce¬ 
ment that (he competition would be run again in 1983. 


590 


Everyday Electronics , September 1982 












... from the World of Electronics 



SEDAC TROPHY 

A specially designed trophy was awarded to the winner of the 
Schools Electronic Design Award Competition. 

As a fitting symbol of contemporary electronics this trophy is in 
the form of a high grade glass fibre printed circuit board. The 
copper tracks are gold plated. 

The p.c.b. is mounted on a polished wooden base and this carries 
a small brass plate engraved with the name of the school and 
title of the winning project. 


JUDGING PANEL 

Scrutiny of Papers (Stage I) and evaluation of working 
models (Stage 2) was performed by a panel of four judges 
representing the two sponsoring parties. The panel members 

Gerald Crowther Head of Application Laboratory, 
Mullard Limited. 

John Warren Technical Manager, Consumer Elec¬ 
tronics Division, Mullard Ltd. 

Fred Bennett Editor, Everyday Electronics. 

Brian Terrell Assistant Editor, Everyday Elec¬ 

tronics. 

The judges' task was difficult and exacting. After long and 
careful deliberation—with the objects of the Competition 
always in mind—the declared results were agreed 
unanimously. 

The judges wish to record their admiration for the generally 
high standard throughout, both in the presentation of 
written work and in the practical realisation of the designs. 
All students participating deserve the highest commendation. 
Those who, in several instances, worked entirely ajone merit 
additional praise. All did honour to their schools, a fact that 
will surely be recognised by these establishments. Their 
efforts provide a model for members of other schools to 
emulate in future contests. 


(far left) Simon Rainey of Burscough High describes the merits of 
his Oscilloscope Companion project to the Minister. This won 
Simon second prize in the competition. 

(centre) Balijinder Dhanda, Michael Finnemore and Michael Stallery, 
from Mellow Lane Comprehensive, look anxiously as judges inspect 
their Velocity Measurer. This project was eventually placed third. 

(below) Putting the Minister to the test on his Digital Readout 
Ergometer is Mark Vaughan of Richard Lander School. 





A VISITOR’S VIEW 


On June 29, the 12 finalists 
for the 1982 Schools Elec¬ 
tronic Design Award Compe¬ 
tition all gathered together 
at Mullard House in London 
to meet the judges and the 
press to demonstrate and 
discuss the end result of 
the past few months labours. 
As the final decision as to 
who was the overall winner 
of SEDAC 82 was not to be 
made until the judging panel 
had seen all the finalists 
involved and chatted to the 
pupils, the atmosphere was 
one of tense excitement. 

And indeed it was a close 
run event, for the quality of 
the entries was very high. 

It is interesting to note 
that out of the 12 final¬ 
ists, no fewer than five had 
utilised computers in their 
entries (four of these the 
ubiquitous Sinclair ZX81!) 
thus illustrating the mating 
of these two branches of 
technology—electronics and 
computing. 

But perhaps the most 
impbrtant factor to emerge 
wasl the sheer usefulness of 
all the entries; all had been 
designed with a specific 
application in mind. Ranging 
from the Bee Hive Tem¬ 
perature Meter from Aire- 
borough Grammar School 
which evolved from a liaison 
between the school’s bee¬ 
keeping club and electronics 
club, to ' the sophisticated 
ZX81 Microcomputer Ex¬ 
pansion System from Peel 
Sixthform College. 

This was the eventual 
winner and consists of an 
ingenious system to extend 
the application of the ZX81 
to enable it to be used as a 
teaching aid. By means of a 
motherboard plugged into 
the back of the Sinclair 
unit, additional daughter¬ 
boards are incorporated into 
the 'system, each with a 
specific function or task. The 
example on show at the 
judging ceremony analysed 
the classic physics experi¬ 
ment of determining the 
acceleration due to gravity 
and displayed the result 
graphically on the screen. 

This project was developed 
by two sixthformers, in what 
proved to be a most success¬ 
ful team of one hardware 
man and one software man. 

The second prize, a solo 


effort from Burscough High 
School, went to the Oscillo¬ 
scope Companion. This equip¬ 
ment will also perform a 
very valuable function 
within the school, for it is, as 
its name suggests, a unit to 
expand the facilities avail¬ 
able on a simple single beam 
oscilloscope. 

The third prize winner also 
has its application rooted in 
the school’s physics labora¬ 
tory, for Mellow Lane Com¬ 
prehensive School’s Velocity 
Measurer supersedes the old 
“ticker tape timer” in per¬ 
forming velocity measure¬ 
ments even on objects as 
diverse as parachutes and 
rolling footballs! 

However, it was not neces¬ 
sary to be at “the state of 
the art” to be considered as 
a winner. Provided that the 
idea was good and the exe¬ 
cution of that idea practical, 
the entries did not have to 
be too sophisticated circuit- 
wise. This was illustrated by 
some of the runners up and 
in particular St Peters and 
Merrow Grange Comprehen¬ 
sive School with their Logic 
Demonstrator, a console type 
unit to teach the principles 
of Boolean algebra. 

Although the complete list 
of all the other runners up 
is too long to describe in 
detail, another one does 
deserve a mention as it 
proved quite popular with 
those attending the prize¬ 
giving, and that is The Elec¬ 
tronic Pressure Gauge from 
The Hollins County High 
School. 

This project measures lung 
pressure and displays the 
result on a graph of pressure 
vs. time and also calculates 
and records maximum level 
attained. Quite entertaining 
when invited to blow into a 
piece of plastic pipe and 
have the results appear 
before your eyes! 

The overall impression was 
one of competence and confi¬ 
dence amongst the pupils in 
an event thoroughly enjoyed 
by all those who took part. 
Which . brings us back to 
the purpose of SEDAC; if 
these are the electronics 
engineers of tomorrow then 
the future certainly looks 
good. Congratulations to 
students and staff alike. 


Everyday Electronics, September 1982 


591 








Velocity Measurer 


T he Velocity Measurer can be used in 
the laboratory with greater ease 
and accuracy than with conventional 
methods of taking velocity measure¬ 
ments, such as with the electro¬ 
mechanical ticker timer. It is capable 
of measuring small changes in velocity 
at selectable sampling rates, and to store 
this data in a semiconductor memory. 
After the experiment, the data may be 
read out in single steps to allow a 
velocity-time graph- for any moving 
object to be plotted. 

The project uses ultrasonics for 
determining velocity of the object 
based on the Doppler effect. The unit 
emits a constant frequency 40kHz 
sound wave. This reaches the object 
and is reflected back to an ultrasonic 
transducer mounted on the unit. The 
moving object causes the reflected 
sound waves to apparently increase in 
frequency in proportion to its velocity. 



The circuitry computes the difference in 
transmitted and reflected frequencies 
to calculate the speed of the object. 

Sixteen spot velocity measurements 
are made during the motion of the 
object on release of the START switch, 
5 per second, 10 per second or 50 per 
second depending on the setting of the 
Speed Selector Control Switch. 


Outputs exist on the unit (1) to allow 
connection to a proprietary memory 
bank to store the results of many 
experiments which is able to feed a 
chart reader to automatically produce 
velocity-time graphs; ( 2 ) for connection 
to an oscilloscope to display velocity 
directly. 


ZX8I Expansion System 


P lugging this expansion system into 
the back of a ZX81 computer imme¬ 
diately adds another 4K bytes to the 
existing IK bytes of RAM on board. 

The system has its own regulator 
which helps share the load with the 
ZX81 resident regulator, and three 37 
pin expansion sockets. _ 

Signal lines AO to A9, RD WR and 0 
from the ZX81 bus are buffered. High 
order address decoding generates 16 
enable lines, with the lower 5 lines 
enabling RAM and the upper 11 lines 
reaching the expansion sockets. 

RAM consists of 8 X 2,114 static 
RAMS (each 4 bits X 1,024) configured 
to realise 8 bits X 4,096. It is kept low 
in the memory map to ensure con¬ 


tinuous RAM for Basic. 

The expansion socket bus reaches the 
3 sockets wired in parallel and consists 
of the following. 0V. +5V . Afl-A9 

(buffer ed). D4-D7, 0 WR, RD, RESET, 
WAIT and 11 enable lines labelled E0- 


Peel Sixth Form College 



E10. The latter are active low and e 
able blocks of 1,024 memory locations. 
These can be used to enable RAM, 
EPROM, I/O and any other device 
which is capable of supporting a bus. 

The particular value of this piece of 
equipment (Motherboard) is its ability 
to allow inexpensive ZX81 computers to 
be used for acquiring data from in¬ 
progress experiments and display the 
-d results on a t.v. screen. Addi¬ 


tional hardware (daughter boards plug¬ 
ged into the sockets) and software are 
needed to accomplish this. 

Some ideas in mind for daughter 
boards and software are: measurement 
and graphical display of radioactive 
decoy; a spectrum analyser; an I/O 
board for digital control applications; 
a serial I/O port for communication to 
RS232 devices such as printers/ 
terminals. 


Coulomb Meter 


Queen Elizabeth 


I n the school science laboratory, experi¬ 

ments are often carried out which re¬ 
quire the calculation of the electric 
charge which has flowed in the circuit 
during the experiment If the current is 
constant this is easily done using an 
ammeter and a timepiece. In some elec¬ 
trolysis experiments the current can 
change considerably during the process. 
This makes continuous monitoring 
necessary with regular adjustments to 
maintain constant current. 

A more accurate, reliable and easier 
to use system was sought and resulted 
in the design and development of the 
Coulomb Meter. 

The meter is able to read up to 1,000 
coulombs and has a sensitivity from 
1/j C/sec to 10— 4 C/sec in five switched 
ranges. The large l.e.d. display which 
indicates charge flow is capable of being 
set in a count-up or count-down mode 


i-L 

5 I-p--1 

u 


{> 


to/from a preset value. When the preset 
value is reached, the unit switches off 
power to the experimental apparatus. 

Experimental current flow passes 
through the range select/amplifier cir¬ 
cuitry to produce a potential difference 
which is applied to the control inputs 
of a voltage-to-frequency stage (v.c.o.). 


MsTslol 

The output frequency is thus propor¬ 
tional to the current flowing. 

The high frequency oscillator output 
reaches divider circuitry to produce a 
10Hz display counter input for 1A ex¬ 
perimental current to 0-1C. 

Presettable inputs to the counter are 
set by means of b.c.d. thumbwheel 
switches. When the set value is reached, 
a control signal is produced that 
switches off experimental current flow. 


594 


Everyday Electronics, September 1982 
































































... from the World of Electronics 



SEDAC TROPHY 

A specially designed trophy was awarded to the winner of the 
Schools Electronic Design Award Competition. 

As a fitting symbol of contemporary electronics this trophy is in 
the form of a high grade glass fibre printed circuit board. The 
copper tracks are gold plated. 


1 


The p.c.b. is mounted on a polished wooden base and this carries 
a small brass plate engraved with the name of the school and 
title of the winning project. 


JUDGING PANEL 

Scrutiny of Papers (Stage I) and evaluation of working 
models (Stage 2) was performed by a panel of four judges 
representing the two sponsoring parties. The panel members 

Gerald Crowther Head of Application Laboratory, 
Mullard Limited. 

John Warren Technical Manager, Consumer Elec¬ 
tronics Division, Mullard Ltd. 

Fred Bennett Editor, Everyday Electronics. 

Brian Terrell Assistant Editor, Everyday Elec¬ 

tronics. 

The judges' task was difficult and exacting. After long and 
careful deliberation—with the objects of the Competition 
always in mind—the declared results were agreed 
unanimously. 

The judges wish to record their admiration for the generally 
high standard throughout, both in the presentation Of 
written work and in the practical realisation of the designs. 
All students participating deserve the highest commendation. 
Those who, in several instances, worked entirely done merit 
additional praise. All did honour to their schools, a fact that 
will surely be recognised by these establishments. Their 
efforts provide a model for members of other schools to 
emulate in future contests. 


(far left) Simon Rainey of Burscough High describee the merits of 
his Oscilloscope Companion project to the Minister. This won 
Simon second prize in the competition. 


(centre) Balijinder Dhanda, Michael Finnemore and Michael Stallery, 
from Mellow Lane Comprehensive, look anxiously as judges inspect 
their Velocity Measurer. This project was eventually placed third. 


(below) Putting the Minister to the test on his Digital Readout 
Ergometer is Mark Vaughan of Richard Lander School. 



A VISITOR’S VIEW 


On June 29, the 12 finalists 
for the 1982 Schools Elec¬ 
tronic Design Award Compe¬ 
tition all gathered together 
at Mullard House in London 
to meet the judges and the 
press to demonstrate and 
discuss the end result of 
the past few months labours. 
As the final decision as to 
who was the overall winner 
of SEDAC 82 was not to be 
made until the judging panel 
had seen all the finalists 
involved and chatted to the 
pupils, the atmosphere was 
one of tense excitement. 

And indeed it was a close 
run event, for the quality of 
the entries was very high. 

It is interesting to note 
that out of the 12 final¬ 
ists, no fewer than five had 
utilised computers in their 
entries I four of these the 
ubiquitous Sinclair ZX8III 
thus illustrating the mating 
of these two branches of 
technology—electronics and 
computing. 

But perhaps the most 
important factor to emerge 
u>as| the sheer usefulness of 
all the entries; all had been 
designed with a specific 
application in mind. Ranging 
froth the Bee Hive Tem¬ 
perature Meter from Aire- 
borough Grammar School 
which evolved from a liaison 
between the school’s bee¬ 
keeping club and electronics 
club, to the sophisticated 
ZX81 Microcomputer Ex¬ 
pansion System from Peel 
Sixthform College. 

This was the eventual 
winner and consists of an 
ingenious system to extend 
the application of the ZX81 
to enable it to be used as a 
teaching aid. By means of a 
motherboard plugged into 
the back of the Sinclair 
unit, additional daughter¬ 
boards are incorporated into 
the System, each with a 
specific function or task. The 
example on show at the 
judging ceremony analysed 
the classic physics experi¬ 
ment of determining the 
acceleration due to gravity 
and displayed the result 
graphically on the screen. 

This project was developed 
by two sixthformers, in what 
proved to be a most success¬ 
ful team of one hardware 
man and one software man. 

The second prize, a solo 


effort from Burscough High 
School, went to the Oscillo¬ 
scope Companion. This equip¬ 
ment will also perform a 
very valuable function 
within the school, for it is, as 
its name suggests, a unit to 
expand the facilities avail¬ 
able on a simple single beam 
oscilloscope. 

The third prize winner also 
has its application rooted in 
the school’s physics labora¬ 
tory, for Mellow Lane Com¬ 
prehensive School’s Velocity 
Measurer supersedes the old 
“ticker tape timer” in per¬ 
forming velocity measure¬ 
ments even on objects as 
diverse as parachutes and 
rolling footballs! 

However, it was not neces¬ 
sary to be at “the state of 
the art” to be considered as 
a winner. Provided that the 
idea was good and the exe¬ 
cution of that idea practical, 
the entries did not have to 
be too sophisticated circuit- 
wise. This was illustrated by 
some of the runners up and 
in particular St Peters and 
Merrow Grange Comprehen¬ 
sive School with their Logic 
Demonstrator, a console type 
unit to teach the principles 
of Boolean algebra. 

Although the complete list 
of all the other runners up 
is too long to describe in 
detail, another one does 
deserve a mention as it 
proved quite popular with 
those attending the prize¬ 
giving, and that is The Elec¬ 
tronic Pressure Gauge from 
The Hollins County High 
School. 

This project measures lung 
pressure and displays the 
result on a graph of pressure 
vs. time and also calculates 
and records maximum level 
attained. Quite entertaining 
when invited to blow into a 
piece of plastic pipe and 
have the results appear 
before your eyes'. 

The overall impression was 
one of competence and confi¬ 
dence amongst the pupils in 
an event thoroughly enjoyed 
by all those who took part. 
Which brings us back to 
the purpose of SEDAC; if 
these are the electronics 
engineers of tomorrow then 
the future certainly looks 
good. Congratulations to 
students and staff alike. 


Everyday Electronics, September 1982 


591 








THE TWELVE 

WINNING DESIGNS 


Bee Hive Temperature Meter Aireborough Grammar 


Mr. B. Thorp (teacher), Andrew 
Jonathan Green (12), David Aliev 
Andrew Green (13) 


Smith (12), 
well (13), 



T HE beekeeping club at the school 
wanted to follow movement of the 
bee cluster in a hive during the winter 
months, and analyse the movement 
with respect to external factors and 
conditions. It was decided to use a tem¬ 
perature sensor to determine the 
cluster position from the body heat of 
the bees. The information was gathered 
from ten sensors at known positions in 
the hive. 

A digital voltmeter was used in the 
project in a novel way to allow the dis¬ 
play reading to be interpreted directly 
as degrees Celsius. 

Ten precision temperature sensors 
are used to monitor the temperature at 
the top of each frame in the hive. These 
sensors operate in a similar manner to 
a Zener diode, and have a breakdown 


voltage directly proportional to the 
absolute temperature (°K>. The output 
voltage obeys a linear relationship with 
temperature and is equal to 10mV/°K. 
On board calibration circuitry provides 
an accuracy of 1°K. 


The sensor voltage feeds one input of 
the digital voltmeter. The other input 
is maintained as 2-73V by means of a 
precision voltage reference i.c. Thus the 
reading on the display is in degrees 
Celsius (0°C=273°K>. 


Computer Control Interface Belmont Comprehensive 


Mr. S. Duncan (teacher), David 
David Williamson (14) 


(H), 


A computer interface unit allows a 
computer access to the “real 
world”. This project has been designed 
to provide such access for the very 
popular ZX81 Personal Computer. It 
plugs into the ZX81 bus and is con¬ 
structed to show how software may be 
used to control external devices, in this 
case two filament lamps. The lamps 
may be replaced by other electronic 
devices such as relays, thyristors, 
motors, electric valves to control com¬ 
plex circuitry and machinery. 

The control information reaches the 
various extrnal devices along the com¬ 
puter data bus (8-bits wide) and is 
routed to the appropriate device by de¬ 
coding the information appearing on 
the address lines at this time. 



The existing software commands the 
computer to repeatedly send data to a 
particular port number, OC. The port 
number appears on the lower half of 
the address bus at this time and is de¬ 



coded to enable the gates connecting 
the lamps to the data bus via suitable 
transistor buffers. The data consists of 
8 separate bits, either at logic 1 or 
logic 0 to represent transistor on and 
off states respectively. The data is 
modified for each successive "out” com¬ 
mand so the transistors are constantly 
being turned on and off which results 
in the lamps flashing on and off. 


Oscilloscope Companion 


T his inexpensive unit will convert a 
single beam ’scope with ext. sync, 
input to a dual beam version, expand 
the Y-amp volts/cm in both directions, 
allow current measurements to be 
taken from the screen display, display 
peak levels for easy reading from 
screen and has a time base delay. A 
precision rectifier/buffer is incor¬ 
porated to allow a voltmeter to be con¬ 
nected for more accurate measurements 
of voltage and current. 

The unit plugs directly into the 
oscilloscope Y-amp input set to lV/cm 
and signals to 'be investigated fed to 
left and/or right channel inputs on the 
unit. By means of a number of switches, 
the input signals may be routed through 
to any Of the Companion’s function 

Input impedance is in the order of 


10,000 megohms which makes special 
measurements possible such as those 
associated with bio-activity and pH 
values. Input voltages from lmV to 50V 
in 15 ranges may be measured. 

The Beam Splitter facility permits 
two waveforms to be displayed simul¬ 


taneously, one above the other readily 
allowing frequency and phase shift 
comparisons to be made. Either wave¬ 
form may be used for synchronisation. 

The Peak Voltage Detector detects 
the highest positive input voltage and 
remembers it for about 10 seconds. 



592 


Everyday Electronics, September 1982 






















































































G.E.M.N.I.F. 


High Park 


Mr. A. J. Merrett (teacher), David Wilkes (16), 
Andrew Williams (16) 


T his project was devised to overcome 
the inaccuracies associated with the 
ticker tape method of determining the 
velocity and equations of motion of a 
moving object using a ZX81 computer 
for data processing. 

Velocity measurement experiments 
conducted in the school laboratory 
usually employ a “standard” three- 
wheel trolley and this has been retained 
in this project with no external con¬ 
nections. Ultrasonic soundwaves are 
used to transmit the data obtained on 
the moving trolley to an u/s receiver 
on the mains unit. This data is fed to 
the ZX81, processed and returned to the 
main unit display circuitry after com¬ 
pletion of the experimental “run”. 

One of the trolley wheels is drilled 
with two small holes diametrically 
opposite. These act as windows to open 
or close an optical link, an l.e.d. light 



light activated switch on the other. As 
the trolley moves, the light activated 
switch operates and turns the ultrasonic 
oscillator on. The on-to-off rate of the 
ultrasonic transmitter is therefore pro¬ 
portional to the velocity of the trolley. 
The transmitter on and off periods are 


converted to logic l’s and 0’s respec¬ 
tively. The software instructs the ZX81 
to periodically read in this data to com¬ 
pute and store the trolley velocity. At 
the end of the “run” after a set num¬ 
ber of readings, the ZX81 outputs the 
calculated velocity to the display and 
TV screen in chronological order. 






Electronic Pressure Gauge Hollins County High 


Mr. J. S. Hagan (teacher), Darryl Grimshaw (16), 
Michael Howarth (16) 



T his project was designed and de¬ 
veloped for use in the school science 
laboratories where lung pressure was 
required to be measured. It replaces the 
previous difficult to control water mano¬ 
meter method. It also has other appli¬ 
cations in the laboratory where pres¬ 
sure measurements are to be taken, 
and with suitable software could be 
employed as a fluid flowmeter with 
results displayed on a TV screen. 

A ZX81 Microcomputer is interfaced 
to the Gauge hardware which reads in 
data processed by the on-board circuitry 
to plot a graph of pressure vs time, and 
calculate and display the maximum 
pressure level attained. 

The pressure produced by a person 
blowing down the plastic tube feeding 
the flowmeter sensor causes a paddle 
in the sensor to rotate at a speed pro¬ 


portional to the applied pressure. In¬ 
ternal Hall effect switches in the sensor 
produce a train of pulses at its output. 
The pulse rate is thus proportional to 
the applied pressure. 

These pulses are fed to a synchroniser 
circuit and then reach an eight-bit 


counter. The software previously loaded 
into the ZX81 from cassette tape 
periodically reads the data in the 
counters, and then causes them to be 
reset for the next count sample while 
using the read information to plot the 
graph and compute maximum value. 


Digital Results Storage System Lutterworth Grammar Mr/ H. Rigby (teacher), Richard Moulds (18) 



Everyday Electronics, September 1982 593 


















































T he Velocity Measurer can be used in 
the laboratory with greater ease 
and accuracy than with conventional 
methods of taking velocity measure¬ 
ments, such as with the electro¬ 
mechanical ticker timer. It is capable 
of measuring small changes in velocity 
at selectable sampling rates, and to store 
this data in a semiconductor memory. 
After the experiment, the data may be 
read out in single steps to allow a 
velocity-time graph for any moving 
object to be plotted. 

The project uses ultrasonics for 
determining velocity of the object 
based on the Doppler effect. The unit 
emits a constant frequency 40kHz 
sound wave. This reaches the object 
and is reflected back to an ultrasonic 
transducer mounted on the unit. The 
moving object causes the reflected 
sound waves to apparently increase in 
frequency in proportion to its velocity. 


The circuitry computes the difference in 
transmitted and reflected frequencies 
to calculate the speed of the object. 

Sixteen spot velocity measurements 
are made during the motion of the 
object on release of the START switch, 
5 per second, 10 per second or 50 per 
second depending on the setting of the 
Speed Selector Control Switch. 


Outputs exist on the unit (1) to allow 
connection to a proprietary memory 
bank to store the results of many 
experiments which is able to feed a 
chart reader to automatically produce 
velocity-time graphs; ( 2 ) for connection 
to an oscilloscope to display velocity 
directly. 


ZX8I Expansion System 


Peel Sixth Form College 


P lugging this expansion system into 
the back of a ZX81 computer imme¬ 
diately adds another 4K bytes to the 
existing IK bytes of RAM on board. 

The system has its own regulator 
which helps share the load with the 
ZX81 resident regulator, and three 37 

pin expansion sockets._ 

Signal lines AO to A9, RD WR and 0 
from the ZX81 bus are buffered. High 
order address decoding generates 16 
enable lines, with the lower 5 lines 
enabling RAM and the upper 11 lines 
reaching the expansion sockets. 

RAM consists of 8 X 2,114 static 
RAMS (each 4 bits X 1,024) configured 
to realise 8 bits X 4,096. It is kept low 
in the memory map to ensure con¬ 
tinuous RAM for Basic. 

The expansion socket bus reaches the 
3 sockets wired in parallel and consists 
of the following. 0V. 4-5V . A6-A9 

(buffer ed). DfrD7, 0 WR, RD, RESET, 
WAIT and 11 enable lines labelled E0- 


^ Mr ' H - 

fat £J7 JB Anthon 


Jill 




E10. The latter are active low and en¬ 
able blocks of 1,024 memory locations. 
These can be used to enable RAM, 
EPROM, I/O and any other device 
which is capable of supporting a bus. 

The particular value of this piece of 
equipment (Motherboard) is its ability 
to allow inexpensive ZX81 computers to 
be used for acquiring data from in¬ 
progress experiments and display the 
processed results on a t.v. screen. Addi¬ 


tional hardware (daughter boards plug¬ 
ged into the sockets) and software are 
needed to accomplish this. 

Some ideas in mind for daughter 
boards and software are: measurement 
and graphical display of radioactive 
decoy; a spectrum analyser; an I/O 
board for digital control applications; 
a serial I/O port for communication to 
RS232 devices such as printers/ 
terminals. 


Coulomb Meter 


I n the school science laboratory, experi¬ 

ments are often carried out which re¬ 
quire the calculation of the electric 
charge which has flowed in the circuit 
during the experiment. If the current is 
constant, this is easily done using an 
ammeter and a timepiece. In some elec¬ 
trolysis experiments the current can 
change considerably during the process. 
This makes continuous monitoring 
necessary with regular adjustments to 
maintain constant current. 

A more accurate, reliable and easier 
to use system was sought and resulted 
in the design and development of the 
Coulomb Meter. 

The meter is able to read up to 1,000 
coulombs and has a sensitivity from 
1/x C/sec to 10— 4 C/sec in five switched 
ranges. The large l.e.d. display which 
indicates charge flow is capable of being 
set in a count-up or count-down mode 


IQp—^ 

Sri 




to/from a preset value. When the preset 
value is reached, the unit switches off 
power to the experimental apparatus. 

Experimental current flow passes 
through the range select/amplifier cir¬ 
cuitry to produce a potential difference 
which is applied to the control inputs 
of a voltage-to-frequency stage (v.c.o.). 


BPI 

The output frequency is thus propor¬ 
tional to the current flowing. 

The high frequency oscillator output 
reaches divider circuitry to produce a 
10Hz display counter input for 1A ex¬ 
perimental current to 0-1C. 

Presettable inputs to the counter are 
set by means of b.c.d. thumbwheel 
switches. When the set value is reached, 
a control signal is produced that 
switches off experimental current flow. 


594 


Everyday Electronics, September 1982 




























































Digital Ergometer Readout Richard Lander 


Mr. B. Thomas (teacher), Mark Vaughan (16) 


A n ergometer is a piece of machinery 
designed to allow the work input 
for a particular task to be determined. 
The machine to be used with this 
project consists of a wheel attached 
to a set of pedals by means of a 
chain. A piece of rope is hung over 
the wheel rim held taught by mass at 
one end. As the wheel is turned via the 
pedals, a force due to friction is trans¬ 
ferred to the rope to lift the mass. 

This force multiplied by the revs/sec 
of the wheel enables the applied work 
to be determined. 

It was originally intended that the 
work done would be displayed on a 7- 
segment read-out. However, difficulties 
were encountered in the realisation of 
this and the circuitry modified to pro¬ 
duce two analogue signal read-outs on 
scaled meters. 

The force due to friction was deter- 



Logic Demonstrator 


St Peters and Merrow Grange Sister M. Clement (teacher), Jonathan Page (14) 


n n r- 




:e:£=d— 




T he Logic Demonstrator is intended 
for the student and others wishing 
to gain hands-on experience in investi¬ 
gating the functions of various logic 
gates and to assist one in understand¬ 
ing the basic concepts of Boolean 
algebra. It will also provide a useful 
“breadboard” for checking out project 
designs using NAND, AND, NOR, OR, 
EXNOR, EXOR and NOT gates. Logic 
circuits may be quickly “assembled” 
using only linked-plugs (wire with a 
plug at each end) before committing 
the design to permanent form. 

A useful feature is the logical expres¬ 
sion, circuit symbols and truth table 
displays for the six basic gates. 

An array of 42 logic gates (six of 
each of the seven types mentioned 
above) is available to the user. Each 
input and output is accessible to the 


user via sockets on the top panel. A 
set of jumper leads allow these to be 
easily interconnected as required. Two 
pulse generator outputs are provided 
in the design at similar sockets for feed¬ 
ing clock driven logic circuits, as are 


banks of logic l’s (+5V) and logic 0’s 
(OV). The generators may be switched 
to be in phase or in antiphase. Outputs 
are monitored on a seven-segment dis¬ 
play arranged to read 1 for logic high 
and 0 for logic low. 


Analogue to Digital Converter Tettenhall College 


Mr. G. M. Feather (teacher), Lee Chapman (16) 
Simon Monk (16), John Adlington (16) 



T his project illustrates how a micro¬ 
computer may, with suitable inter¬ 
face techniques, access, process and dis¬ 
play analogue data from measurement 
transducers obtained during the course 
of experiments. Many existing labora¬ 
tory experiments and equipment for 
measuring physical parameters provide 
an analogue output such as electronic 
thermometers, pH meters, sound level 
meters, radiation detectors and so on. 

This piece of equipment will process 
analogue signals and convert them into 
digital data (8-bits wide) which can be 
recognised and used by the micro¬ 
computer. Although designed for use 
through the PET User Port, it could be 
easily adapted for use with other com¬ 
puters with suitable software. 

Most of the electronics is contained 
in a single integrated circuit package 
known as an analogue-to-digital con¬ 
verter. An analogue signal (within a 
specific voltage range) is inputted to 
the converter i.c. analogue input and is 


clocked into the chip to provide a 
digital representation of the input volt¬ 
age level in the form of an 8-bit word 
(eight binary bits). The number of 
samples taken is dependent on the 
clock rate which has been selected to 
give a conversion rate of 28,000/sec. The 
8-bit wide information reaches the 
microcomputer data bus when enabled 
by a command from the software. 


On board scaling circuitry is included 
in the design to allow input voltage 
signals of up to 10V peak to be pro¬ 
cessed. Two input modules were pro¬ 
vided for demonstration: (1) a direct 
input module with 100 ohm switohable 
shunt. This simulates a load provided 
by a 1mA, 100 ohm meter and (2) a 
temperature module for displaying the 
temperature of the remote probe. 


Everyday Electronics, September 1982 


595 















































In order to explain the differences 
' between true power and apparent 
power in the examples discussed last 
month, it is necessary to examine the 
relationship between the alternating 
current waveform and the alternating 
voltage waveform in a reactive 
circuit. 


PHASE SHIFT 

In a purely resistive a.c. system, 
the current through a resistor rises 
and falls together with the voltage 
across it and) as such, is said to be 
in phase with it. 

However, in a capacitive a.c. sys¬ 
tem, the current through a capacitor 
“leads” the voltage across it by a 
quarter of a cycle. That is, when the 
voltage waveform is at zero, the 
current waveform is ait its peak, so it 
is said to have a phase difference of a 
quarter of a cycle (or 90 degrees, as 
one complete cycle is equal to 360 
degrees). 

In the third case, the inductive a.c. 
system, the current through an induc¬ 
tor “lags” behind the voltage across 
it by a quarter of a cycle. Again, the 
phase shift is 90 degrees but this 
time in the opposite direction. 

The waveform diagrams in Fig. 3.1 
illustrate these three cases and also 
show the conventional method of 
representing the phase relationship 
by means of a phasor diagram. The 
phasor diagram shows both the volt¬ 
age and current as vector quantities 
(a line with both magnitude and 
direction), which are imagined to re¬ 
volve anticlockwise about a fixed 
point. 0. 


By convention, the current phasor 
is taken as the reference phase and 
is drawn horizontally. 

A useful mnemonic to help re¬ 
member the phase relationship in 
reactive circuits is the word CIVIL, 
to be read as follows: 

In a capacitor (C), the current (I) 
leads the voltage (V), but the voltage 
(V) leads the current (I) in an induc¬ 
tor (L). 


POWER IN A.C. CIRCUITS 

If a 24 ohm resistor is connected 
across the 240V, 50Hz supply, the 
current through it would be 10A. 
Now, as the voltage and current are 
in phase in a purely resistive system, 
the power consumed by the resistor 
is equal to VXI, that is, 240V X10A 
= 2,400W. It follows that if the circuit 
was left on for one hour, the elec¬ 
tricity meter would register 2 -4 units 
used (as one unit equals on kilowatt- 
hour). 

If the 76mH inductor is connected 
in parallel with this resistor, the cur¬ 
rent through each branch of the cir¬ 
cuit would be measured as 10A 
(remember that the inductor also has 
an impedance of 24 ohms at 50Hz) 
but the current drawn from the sup¬ 
ply would be 14-14A. This is shown 
in Fig. 3.2. 

Furthermore, if the circuit was 
again left on for one hour, the elec¬ 
tricity meter would still only register 
2-4 units, indicating that 2,400W (the 
true power) was being used. The 
apparent power is 14• MAX240V= 
3.394W. 

The phasor diagram in Fig 3.2 
shows why the current flowing from 
the supply is 14 14A. Note that I R is 
in phase with V s (the supply voltage) 
whereas I L is lagging by 90 degrees, 
so that the resultant current (/ z ) is 
the vector sum of these two, that is 
14-14A. 

This resultant current can also be 
calculated by the application of 
Pythagoras’ theorem, which states 
that for a right angled triangle, the 
square of the hypotenuse equals the 



Fig. 3.1. Waveform diagrams showing the phase relationship between the voltage 
and current in a resistive circuit (top), a capacitive circuit (centre) and an inductive 
circuit (bottom). The phasor diagrams (right) illustrate this phase relationship. 


596 


Everyday Electronics, September 1982 
















































sum of the squares of the other two 
sides. So the r esult ant current, 
(fz)= n/V +Il 2 = V200= 14- 14A. 

POWER FACTOR 

So far, it has been established that 
the product of the supply voltage and 
the measured current is the apparent 
power (equal to 3,394W) and that 
registered on the electricity meter is 
the true power (equal to 2,400W). 
The ratio of true power to apparent 
power is known as the power factor. 

So for the circuit shown in Fig. 3.2, 
the power factor is 2,400/3,394= 
0-707 (note that the power factor 
cannot exceed) one). 

From the phasor diagram in Fig. 
3.2, it can be seen that the angle 0 
(greek symbol phi) is the angle at 
which the resultant current lags be¬ 
hind the applied voltage, and in this 
case it is 45 degrees. The cosine 
(abbreviated to cos) of this angle is 
equal to the ratio of the in phase 
current to the resultant current which 
equals 10/14-14=0-707. 

This shows that the cosine of the 
phase angle (cos <j>) equals the power 
factor. 

The lagging (out of phase) current 
component in a reactive system does 
no work and therefore does not regis¬ 
ter on the elecricity meter. For this 
reason it is known as the wattless 
component of the current. It is the 
in phase current which actually does 
the work in registering the true 
power. 

Therefore, in order to calculate the 
in phase current, multiply the resul¬ 
tant current (l z ) by the power factor 
(cos </>), remembering that, in a prac¬ 


QUESTION TIME 

3.1. The information plate on a piece 
of electrical equipment reads as 
follows: 240V—1 ph—15A—cos 
0=0-8. 

How many units will the equip¬ 
ment use per hour? 

3.2. What are the in phase and watt¬ 
less components of the equip¬ 
ment in 3-1? 


PART 2 ANSWERS 

2.1. (a) Jf c = 


1 =20 


2 x 3 142 
(b) / c = 


0-00004 

=800 


2 x 3,142 x 50 x 0 00004 
therefore current, /= V/X c = 
24OV/8O0=3A 

2.2. An inductive reactance is caus¬ 
ing the current to flow giving 
rise to apparent power. How¬ 
ever, as no true power is being 
consumed, the disc on the elec¬ 
tricity meter would be station¬ 
ary. 



tical circuit, the resultant current is 
the easiest to measure with an 
ammeter. In our example, (r = /z X 
cos </> = 14- 14A X cos 45° = 14 • 14 X 
0-707 = 10A. 

Similarly, the true power is the 
product of the apparent power and 
the power factor=240V X 14 • 14A X 
0-707 = 2,400W. This is the figure 
originally registered on the kWh ser¬ 
vice meter. 

POWER FACTOR CORRECTION 

Industrial electrical equipment and 
transformers contain coils and wind¬ 
ings and will therefore have induc¬ 
tance properties and lagging, out of 
phase current components. This type 
of equipment will also carry wattless 
current, sometimes called reactive 
volt-amperes. 

Take for example, a 500kVA trans¬ 
former supplying a works having 
many electric motors to drive pro¬ 
duction machinery, the total power 
factor of which adds up to 0-76. This 
transformer on full load at unity 
power factor could supply 500kW, 
but because the power factor is 0-76, 
the true power on full load is only 
500XO-76=380kW. 

If the power factor could be im¬ 
proved to say, 0-92 at 380kW, the 
transformer loading would be reduced 
to only 413kVA, which would leave 
an additional 87kVA to supply an 
extra load. 

In order to improve the power 
factor, it is necessary to reduce the 
phase angle, (0), the net result of this 
being that cos 0 (the power factor) 
will increase. This is achieved by 
adding a capacitor in parallel with 
the inductive load. Fig. 3.3 shows this 
done to the original example. 

As the 132-6/*F capacitor has an 
impedance of 24 ohms at 50Hz, the 
current through it (J c ) will also be 
10A. However, the current drawn 
from the supply is reduced to 10A 
from the 14-14A in- the example 
shown in Fig. 3.2. 

So the capacitive current has can¬ 
celled the effect of the inductive 


current and) this can be illustrated! by 
removing the resistive element from 
the circuit. The supply current will 
drop to zero but the current through 
both the reactive branches remains 
at 10A each. 

PARALLEL RESONANCE 

This effect is known as parallel 
resonance and occurs when the in¬ 
ductive reactance equals the capa¬ 
citive reactance. The impedance of a 
parallel resonant circuit is infinity at 
its resonant frequency. The formula 
for calculating the frequency at 
which a parallel LC circuit resonates 
(the same formula, incidently, as 
used in radio transmission and recep¬ 
tion) is: 

1 

frequency (f)= - 

2* /LC 

where L=inductance in henries 
C=capacitance in farads 
*=3 -142 (pi) 

The phasor diagram shown in Fig. 
3.3 gives the phase relationship be¬ 
tween the respective current com¬ 
ponents in the circuit. Note that l c 
leads the supply voltage (Vg) by 90 
degrees and is therefore acting in 
the opposite direction to Z L meaning 
the resultant current will be equal in 
magnetude and direction to f R . 

So the true power and the apparent 
power are equal) in this case giving a 
power factor of one. This is verified 
by observing that the resultant 
current is neither leading nor lagging, 
therefore angle 0 = zero and cos 
0 = 1 . 

The power factor can therefore be 
improved by adding capacitance in 
parallel with the inductive load. In 
reality it is not practical to increase 
the power factor to more than 0-92 
since when the equipment is switched 
off (assuming a unity power factor), 
the high voltages induced upon the 
change from parallel to series reson¬ 
ance could damage the windings of 
the electrical equipment. 

To be continued 


Everyday Electronics , September 1982 


597 






























598 
















































TEIY1PER4TURE 

INTERFACE 

FOR THE TfflIDY TRS-80 



I AST month dealt with the circuit 
*“ for this Interface describing in 
detail how it works. In this second and 
final part, details are given for the 
assembly and testing, together with 
the necessary software to run the 
system. 

PRINTED CIRCUIT BOARD 

The components are to be as¬ 
sembled on a single-sided p.c.b. The 
full size pattern to be etched is shown 
in Fig. 5. 

The layout of the components on 
the board, and wiring to case mounted 
components are shown in Fig. 6. 
First of all drill the four fixing holes 
in each corner. The specified plastic 
snap-fixing stand-off pillars used' in 
the prototype require a hole diameter 
in the board of 3 -2mm. The hole size 
to fit these to the case needs to be 
5mm diameter. Use the p.c.b. as a 
template when drilling the case and 
then enlarge case holes to 5mm. 

Begin assembly by soldering in all 
the link wires, 22 s.w.g. tinned copper 
wire was used on the prototype. Next 
fit and solder the i.c. sockets and re¬ 
sistors and voltage regulator followed 
by the capacitors and bridge rectifier. 


Everyday Electronics, September 1982 


Do not fit T1 or insert the i.c.s. yet. 

The reason for the suggested order 
of assembly will become apparent as 


you proceed. 

Attach sufficient lengths of stran¬ 
ded insulated wire to reach the case 
mounted components, including a suit¬ 
able length of 3-core mains cable. 

20-way ribbon cable fitted with an 
edge connector attached is required 
to connect to the TRS-80. Separate 
one end of the 30cm long ribbon 
cable into 20 separate wires, each 
about 4cm long. Strip about 5mm 
from each end and tin. Solder these 
ends into the board in a regular or¬ 
der, see photographs. T1 may now be 
soldered in place. 

If you contemplate expanding the 
interface sometime, it may be a good 
idea to carry out the preliminary 
wiring for this now. The remaining 
seven decoded! address lines, Q1 to Q7 
from IC9, data lines D1 to D7, RD, 
WR, +5V and 0V may be picked up 
from the underside of the p.c.b. and 
connected to an 18-pin d.i.l. socket for 
example mounted on the case. This 
allows connection to other interfaces 
using similar circuitry to IC3a/fC4a 
in the main unit. 

Prepare the case to suit the case 
mounted components, Dl, SI, SKI 
and a 13mm diameter hole for mains 
cable bush and fit these, with excep¬ 
tion of the bush, in place. 

With the plastic stand-off pillars in 
position in the case, the board can 
then be aligned with these pillars and 
firmly pushed onto them to hold the 
board secure. Feed the mains cable 
through its entry hole in the case (do 
not fit the strain relief bush yet). 
Wire the board to the case mounted 
components, with sleeving fitted over 
Dl leads and SI tags. 

EDGE CONNECTOR 

The connections to the computer 
are by way of a 20-way ribbon cable 


599 


Fig. 5. P.C.B. pattern viewed from the copper track side. 




Fvs 

1 

.e)\ 

i*S 

IK! 

+ 

j! 

Yl 

: 

o-v 0—0 

MB 

( 

-f 

9 1 

;ir 

, 







































Fig, 8. Wiring and connection details to the strip- 
board holding the 22 + 22 way edge connector. The 
labelling of the wires assumes that two 10-way 
ribbon cables have been used, called 1 and 2. 



itt ? 

Fig. 7. The TRS-80 expansion outlet viewed from the rear. Only the 



600 


Everyday Electronics, September 1982 
























































































The stripboard/edge connector fully wired. The two outermost connector positions should 
be fitted with inserts to act as alignment guides. 


(or two 10-way ribbons). The cable is 
already connected at the p.c.b. end 
and now requires a 20 + 20 way edge 
connector to be fitted to the other. 
Fig. 7 shows the arrangement of the 
signals viewed from the rear of the 
TRS-80 Level I and Level II. The top 
set of “fingers” are labelled as odd 
numbers, 1 to 39, with 1 at top left. 
The lower finger set is labelled using 
the even numbers between 2 and 40, 
2 being immediately below 1. The 
circuit diagram has been labelled 
accordingly. 

The TRS-80 needs a 20 + 20 way 
socket but this is not easy to obtain. 
You can use a more readily available 
22 + 22 type and not use the posi¬ 
tions at either end. The end pins 
should be removed and small pieces 
of s.r.b.p. slotted in their place. These 
will act as position guides since the 
TRS-80 does not have a polarising 
slot. This arrangement was adopted 
on the prototype. Mark the top of the 
socket with a label or paint to avoid 
the risk of plugging it on the TRS-80 
board upside down. (If you are using 
a different computer, you will need to 
obtain a diagram of the edge connec¬ 
tor, plug or socket and make connec¬ 
tions accordingly.) 

The colours shown in Figs. 6 and 8 
are those found on most kinds of rib¬ 
bon cable. If you keep to this system 
of colours, it will help to reduce wir¬ 
ing errors. For convenience in wiring 
up, the edge connector and ribbon 
cable are joined on a short length of 
stripboard. The socket is mounted on 
the copper track side and the wires 
brought in from the plain side to the 
appropriate track. This can be seen 
in Fig. 8 and the photographs. Take 
special care that the connections are 
made to the correct locations. The 
wires ends should be stripped and 
tinned before soldering to the strip- 
board. Check out each connection 
thoroughly after you have finished 
soldering the wires in place, using a 
magnifying glass to discover any sol¬ 
der bridges that may have occurred. 

It is worth while to use a multi¬ 
meter to test for short circuits be¬ 
tween each line and adjacent lines, 
and eliminate these before going any 
further._ 

The WR line is not used in this 
project, but since there is one spare 
wire in the cable and WR could be 
used for any user designed interface 
projects, it might as well be wired 
in now. 

THERMOMETER PROBE 

The construction of the probe is 
shown in Fig 9. A twin screened 
cable terminated in a 3-pin din 
plug connect the probe to the rest of 
the circuit via SKI. The cable can 
be several metres or even tens of 
metres long. In the prototype, the 
lead was about 1 metre long. There 
. was evidence that unshielded lead 


Everyday Electronics , September 1982 


picked up electromagnetic interfer¬ 
ence which, by causing short voltage 
spikes across R3, resulted in spurious 
triggering of the converter, IC2. It 
was found that this could; be reduced 
by wiring C6 to the output of IC2. 
For this reason it is strongly recom¬ 
mended that shielded cable is used 
for the probe. However, do not run 
the lead close to sources of electro¬ 
magnetic interference. The magnetic 
field from the coil of the tube 
of a video monitor can give rise 
to serious interference. For the same 
reason, keep the leads and the probe 
well away from TV sets, loudspeak¬ 
ers, and electric bells or buzzers. 

Slip a short length of sleeving over 
one wire before soldering it to the 
V+ wire of the i.c. An overlap joint 
is sufficient. It is advisable to use a 
heat shunt while soldering. Slide the 
sleeving over the joint. Next twist 
the wires of a 270ft resistor to the R 
and V— wires of the i.c. Make sure 


that the resistor wires are not short- 
circuiting the wires of the i.c. Solder 
the resistor in place. Although a 
0-25W resistor can be used, a smaller 
(0 125W) resistor is to be preferred. 
Now slip a short length of sleeving 
on to the lead. Solder the wire to 
the V— lead, and slide the 
sleeving over the joint. Finally, slide 
a piece of wide-bore sleeving or 5mm 
plastic tube over the whole assembly 
except for the body of the i.c. Note 
that the screen is not connected at 
the sensor end of the cable. 

Strip the free end of the cable and 
separate and trim the three conduc¬ 
tors. Make sure that these do not 
touch each other when soldered to 
PL1 as shown in Fig. 9. Sleeving may 
be required over the connections. 

TESTING 

Hefore plugging the i.c.s. into their 
sockets, the power supply section 
should be checked. First check for 


This assembly needs to be encased 



601 




continuity between all the OV line 
connections. 

With one lead from an ohmeter 
connected to the mains earth lead, 
work through the board using the 
circuit diagram to check that all the 
connections to the OV rail are made. 
Check the + 5V likewise with one 
ohmeter lead connected to the +ve 
end of R4. 

If these checks prove satisfactory, 
the unit may be plugged into the 
mains and switched on. A spot check 
across the power supply rails using 
a voltmeter set to 10V or more should 
give a reading of 5V. If so, switch off 
and insert the i.c.s. 

The decoding may now be tested 
by connecting the appropriate inputs 
of IC7 and IC9 to the OV line. This is 
easier done at the edge connector 
using short lengths of wire tem¬ 
porarily soldered to the appropriate 
wire-wrap pins. 

Unconnected inputs of ttl i.c.s. act 
as if they were high, so this in effect 
provides the address 60,000. The out¬ 
put A from pin 15 of IC9 should be 
low. If you make any one or more of 
the inputs to IC6 low, or disconnect 
any one or more of the inputs to 
IC3 or IC5, A should! go high. Switch 
off. 

Plug the edge-connector on to the 
computer board, taking care that the 
contacts on the connector are exactly 
aligned with the contacts on the 
board. Switch on the computer. If you 
fail to obtain the usual display 
(MEMORY SIZE?, on TRS-80) switch 
off. It is likely that one or more of 
the cable connections is wrong, or 
that there is still a short circuit be¬ 
tween lines. If all is in order, switch 
on the power to the interface. Again, 
if the display changes, switch off both 
the interface and the computer and 
check for wiring errors. 

The simplest test is to read the 
state of the output of IC2, using the 
PEEK command, as in Program A. 
With TRS-80, high memory addresses 
are differently coded, so that, instead 
of PEEK (60000), we use PEEK 
(-5536). The result of PEEKing de¬ 
pends on the computer. With the 
TRS-80, an open data line is read as 
high (1), so when the output of IC2 
is high, all lines are high (1111 1111) 
giving the equivalent of 255 in deci¬ 
mal. 

When the output is low, the data 
lines hold! 1111 1110, which is equiva¬ 
lent to 254. As the program runs, a 
rapid succession of 255’s or 254’s 
should scroll up the screen, changing 
from 255 to 254 and back to 255, 
about once every 2-5 seconds. 

Other computers, such as the ZX- 
81, return “0” for each open data 
line. The display will thus show al¬ 
ternating series of 0’s and l’s, 
changing from 0 to 1 and back to 0 
about once every 2-5 seconds. 

Should the program produce 


nothing but 255 or 0, switch off and 
check the circuit for wiring errors. 
It may be that you have two address¬ 
lines crossed, giving an address out¬ 
side the range of the decoder. Another 
possible fault is a dry joint on one or 
more of the lines of the ribbon cable. 

CALIBRATING 

The simplest way of using the 
thermometer is to use a BASIC pro¬ 
gram such as Program B. This reads 
the output from IC2 over and over 
again, waiting for it to go low (line 
40). As soon as it goes low it begins 
to count how many times it reads a 
low output (line 50). It continues to 
do this until it goes high again. The 
number N is proportional to the time 
spent in the low condition. All that is 
then needed is to multiply N by a 
constant factor to convert it to a tem¬ 
perature expressed in degrees. 

It was found in trials that when 
the probe was immersed in water 
containing melting ice (0 degrees C), 
the count N was 111. The count in¬ 
creased to 119 when the probe was 
allowed to warm up to room tempera¬ 
ture (20 degrees C). This is as ex¬ 
pected, since the count is proportional 
to absolute temperature and 20 de¬ 
grees C is 293 K degrees. 

The effect of an increase of tem¬ 
perature of 20 degrees is an increase 
of only 8 counts. There can be only 
eight different readings between 0 
and 20 degrees C, giving a resolution 
of less than 2 degrees. This may be 
sufficient precision for triggering 
alarm systems, but is not good enough 
when temperature is to be precisely 
known. To obtain maximum counts 
the program has been written so as 
to run as fast as possible. 

Variables A to Y are defined as 
integers and it uses variables instead 
of constants (line 20). Even so, re¬ 
solution needs to be improved. 

The simplest way to do this is that 
adopted in the program. The reading 
operation is repeated six times, the 
first being ignored, for it might have 
been begun during a low pulse. The 
total of the five remaining counts is 
M (line 70). Tests showed this total 
to be 555 at 0 degrees C, so that the 
temperature, x, in degrees Celsius is 
given by: . 

X -(^) 

This is the basis of the calculation 
in line 100, in which the result is 
rounded to the nearest whole degree. 

To calibrate the Interface, a means 
of taking a spot temperature reading 
of the sensor is required. In the 
absence of a thermometer, a beaker 
of melting ice and salt could be 
used. This is at a temperature of 
273°K (0°C). With this and any other 
liquid bath method, the sensor must 
be isolated from the liquid, see Fig. 9. 
Run program B. 


The total count will be displayed 
with the calculated temperature. 
Allow the sensor temperature to 
stabilise. This will be evident from 
the display. Mentally note the con¬ 
stant total count and run program B 
from line 200. After typing in new 
value for Z, run the program to read 
the sensor temperature in degrees C. 
Line 95 may be deleted if not 
required. 


HIGHER RESOLUTION 

To achieve even higher resolution 
we need read the output of IC2 many 
more times during each low pulse. 
This can readily be done by using 
the machine-code routine of Program 
C. The program waits until output 
goes low, then reads and counts until 
it goes high again. This routine is 
extremely fast, giving a count in the 
region of 44000 at 0 degrees C. The 
program is written in Z80 machine 
code and can be accessed by the 
USR(0) command in the TRS-80. Pro¬ 
gram D shows how this may be done. 
A count of 44000 at 0 degrees C in¬ 
creases to 47223 at 20 degrees C, an 
increase of 161 counts per degree. 
Thus it is possible to resolve tem¬ 
peratures to the nearest 0.01 degrees. 
At this speed it becomes possible to 
detect occasional spikes on the out¬ 
put, which give rise to very low 
counts. Such counts are excluded by 
line 80 of program D. 

A complication arises because the 
value returned from the routine is 
treated as a “signed number”. Being 
greater than 32767 (0111 1111 1111 
1111), its most significant digit is ‘1’, 
and it is taken to be a negative num¬ 
ber. The remaining 15 digits are 
evaluated as if it were positive, and 
a negative sign is placed in front of 
the result. To obtain its true value 
as a 16-digit number, we subtract it 
from 65536 (line 90). The calculation 
of temperature, rounded to the near¬ 
est hundredth of a degree, is done 
on the same line. The equation on 
which this is based is: 


44000 

= (0-06245 X —273) degrees K 


Derive and insert the calibration 
factor as described in the previous 
section. The routine from line 200 
calculates Z. 

With this established, other tem¬ 
peratures are obtained by the 
calculation in the program. It is im¬ 
portant to allow the sensor several 
minutes to come to a steady tempera¬ 
ture before attempting to calibrate it. 
The output of the i.c. is upset by 
sudden changes of temperature, pre¬ 
sumably because some parts of the 
i.c. acquire the new temperature be¬ 
fore other parts and the balance of 
the circuit is disturbed. □ 


602 


Everyday Electronics, September 1982 






TEMPERATURE INTERFACE SOFTWARE 


Program A: Testing 

10 PRINT PEEK (-5536) 

20 GOTO 10 

Program B: Reading Temperature 

1 CLS 

5 Z = 1 

10 DEFINT A - Y 

20 X = -5536: Y = 255 

30 FOR K = 1 TO 6 

40 IF PEEK (X) = Y THEN 40 

50 IF PEEK (X) < > Y THEN N = N + 1: GOTO 50 
60 IF K = 1 THEN 80 
70 M = M + N 
80 N = 


100 


NEXT K 
PRINT M; “ 

PRINT “TEMPERATURE = INT (Z*M -272-5) 


110 M = 

120 GOTO 30 
200 CLS 

210 PRINT “TO CALCULATE CALIBRATION FACTOR, Z” 
220 PRINT “TEMP. IN DEGREES K = 

230 INPUT A 

240 PRINT “COUNT = "; 

250 INPUT B 

260 PRINT "Z = A/B 

270 PRINT “ENTER THIS VALUE FOR Z IN LINE 5“ 


Program C 


7D00 

7D00 

7D03 

7D06 

7D08 

7D0B 

7D0E 

7D10 

7D13 

7D14 

0000 

00000 

Count 

Wait 


210000 
3A60EA 
FEFF 
CA037D 
3A60EA 
FEFE 
C29A0A 
23 

C30B7D 

00210 

TOTAL ERRORS 

7 DOB 

7D03 


00100 

00110 

00120 

00130 

00140 

00150 

00160 

00170 

00180 

00190 


; PROGRAM TO READ THERMOMETER 


COUNT LD 


7D00H 
HL, 00H 
A, (0EA60H) 
0FFH 
Z, WAIT 
A, (0EA6OH) 
0FEH 

NZ, 0A9AH 
HL 

COUNT 


MODIFICATIONS 

There is an error on the circuit 
diagram, Fig. 2 published last 
month. The “short" across R2 
should of course not be there. 

It was found necessary to use 
screened cable to connect the 
sensor to the unit in order to 
minimize the effect of e/m noise 
pick-up giving rise to false trigger¬ 
ing of IC2 and thus “odd" calcula¬ 
tions. Consequently, PL1 and SKI 
have been re-specified as 3-pin 
DIN plug and socket respectively. 
Twin screened cable should be 
added to the Component List. The 
circuit diagram front end should be 
modified as shown below. The 
likelihood of false triggering may 
be further reduced with C2 
2 2/.F. 



; CLEAR HL REGISTER 

; READ OUTPUT 
; IS OUTPUT HIGH? 

; STILL HIGH, SO WAIT 

; READ OUTPUT 
; IS OUTPUT LOW? 

; HIGH AGAIN, RETURN TO BASIC 
; COUNTING NO. OF LOW READS 
; FOR NEXT READ 


Program D: Reading Temperatures to 0-01 degrees 


10 DATA 33, 0, 0, 58, 96, 234, 254, 255, 202, 3,125, 58, 96, 234, 254, 254,194,154,10, 35, 195, 11, 125: ' Program in decimal 
20 FOR J = 0 to 22: ’ Putting program into memory 
30 READ D 
40 POKE 32000 + J, D 
50 NEXT J 

60 POKE 16526, 0: POKE 16527, 125: 'Starting address of program 
70 X = USR(0): ’ Go to program 

80 IF X <50 AND X> 0 THEN 70: ’ Reject small counts 
85 PRINT (65536 + X); " 

90 PRINT “TEMPERATURE = INT( Z*(65536 + X) - 27299• 5)/100 
100 GOTO 70: 'To take next reading 

200 CLS 

210 PRINT "TO CALCULATE CALIBRATION FACTOR, Z” 

220 PRINT “TEMP. IN DEGREES K = 

230 INPUT A 

240 PRINT "COUNT = 

250 INPUT B 

260 PRINT “Z = (100*A)/B 

270 PRINT "ENTER THIS VALUE FOR Z IN LINE 5” 


Everyday Electronics, September 1982 


603 































A continuity tester is a valuable 
** piece of equipment used to 
determine whether a component or 
circuit has a broken connection. 
That is, it tests for the continuity of 
current flowing. 

It can also be used to check fuses, 
bulbs, wires and certain semiconduc¬ 
tors, including diodes and transistors. 


DESIGN CONSIDERATIONS 

This circuit has been designed' to 
overcome difficulties often associated 
with basic continuity testers. These 
are: 

1. Difficulty in establishing if con¬ 
tinuity has been made. 

2. High probe current damaging 
semiconductor junctions. 

3. Inability to predetermine the 
maximum resistance that the 
tester “sees” as continuity. 

4. Variation in the tone produced 
by the tester according to the 
resistance being checked. 

5. High stand-by current. 

The first problem was simply over¬ 
come by giving the Continuity Tester 
an audio output, not a visual indica¬ 
tion, thus allowing the user to con¬ 
centrate on the component under 
test, and with a probe current of less 


than 1mA, the second problem was 
also removed. 

Provision of a potentiometer in the 
tester input circuit permits the user 
to programme the resistance at which 
the unit registers continuity and by 
incorporating an oscillator with a 
tone frequency independent of test 
resistance, difficulties 3 and 4 were 
erased. 

The standrby current was reduced 
to an almost negligible level by using 
ultra-low power cmos circuitry thus 
overcoming the last problem. 


CIRCUIT DESCRIPTION 

The tester employs a cmos 4011 i.c, 
consisting of four nand gates, three 
of which form an oscillator, the 
remaining gate acts as an inverter 
to enable the oscillator. See Fig 1. 

IClb, c and d form the “ring of 
three” oscillator, the frequency of 
which can be calculated with the 
formula: 

frequency (f)=-Hz 

R2 X Cl 

where R2 = resistance in ohms 
Cl = capacitance in farads 

With the components used in this 
circuit, the frequency will be approxi¬ 
mately 1kHz. 

The last nand gate, ICla, is made 
into an inverter by wiring its two 
inputs together. Its output is con¬ 
nected to the oscillator which will 
only oscillate when the output of 
ICla is high (logic 1). This will only 
occur when a low resistance path is 
connected between SKI and SK2, 
thus forming a potential divider 
along with R1 and VR1. 


TRANSDUCER 

The oscillator drives WD1, a piezo 
ceramic transducer. This type of de¬ 
vice consists of a piece of ceramic 
material deposited on a circular brass 
“resonator” and it produces a very 
high output when energised. This 
makes it more efficient for this 
application than a traditional speaker, 
particularly considering its low power 
consumption and diminutive size. 


CIRCUIT BOARD 

Fig. 2 shows the construction of 
the circuit board on 01 inch strip- 
board, 12 strips by 19 holes. It is 
best to start with the i.c. holder for 
IC1 and then solder in the wire links 
(9 in all), VR1, the resistors and the 
capacitor. 



604 


Everyday Electronics, September 1982 






















Next, the flying leads of 7/0-2mm 
equipment wire should be added, and 
these should be about 100mm long. 
Finally solder the leads from the 
piezo ceramic transducer into posi¬ 
tion, taking great care as these tend 
to be a little fragile, andi to complete 
this stage of construction, add the 
battery clip (remember that the red 
lead goes to switch SI). 


THE CASE 

The tester is now ready to go into 
its case, and the prototype was fitted 
into a plastic box, 75 X 55X 35mm. 
The red and black banana sockets 
and the rocker switch are mounted 
on the outside of this case as shown. 

The piezo ceramic transducer 
(WD1) is glued) onto the inside of the 
case with a contact adhesive (such as 
“Evostick”) and the battery B1 is 
taped to the inside of the lid. When 
all the flying leads have been 
soldered in place, the unit can be 
completed by slotting the board into 
the space between the battery and 
the plastic body of the rocker switch. 


SETTING UP 

To set up the Continuity Tester, a 
small screwdriver and 1-5 kilohm 
resistor are required. 

With the unit open so as to give 
access to VR1, switch on andi connect 
the 1 - 5 kilohm resistor between the 
two probes. A 1kHz tone may be 
heard but don’t worry if it is not at 
this stage. 

Adjust VR1 until the tone just 
ceases and if the unit was not initially 
making the tone, VR1 must be rotated 
until it does and then adjusted to the 
point where it ceases. It is important 
that this adjustment is exact so that 
the tester is sensitive to this 
resistance. 

The value 1-5 kilohm was chosen 
as the maximum resistance at which 
the Continuity Tester “sees” as con¬ 
tinuity for the prototype as it is a 
practical value. However, the con¬ 
structor may wish to substitute a 
different value here. 

The unit is now ready to test com¬ 
ponents and circuits, and Fig. 3 and 4 
show how it can be used to check 
diodes and transistors H 


COMPONENTS 


Resistors 

R1 4-7W1 R2 10kil 
All }W carbon ±5% 

Capacitors 

Cl 0-I^F polycarbonate 
Semiconductors 

IC1 401 IB CMOS quad 2-input 
N AND gate 

Miscellaneous 

RV1 10k£l miniature horizontal 

SI on/off rocker switch 
B1 9V PP3 battery 
SKI 4mm banana socket, red 
SK2 4mm banana socket, black 
WD1 PB2720 piezo ceramic 
transducer 

Shipboard, 0-1 inch matrix, 12 
strips by 19 holes; plastic case 
75 x 55 v 35mm; 14 pin d.i.l. 
holder; battery clip; 7/0-2mm 
equipment wire; probes (2 off, 
one red, one black) on 4mm 
banana plugs. 

M— 



SEMICONDUCTOR TESTING 


aii :: 





Fig. 3. Method of testing a diode with the Continuity 
Tester unit. If the cathode (k) is fi.st connected to the 
positive (red) terminal as shown, the diode should not- 
conduct. By reversing the diode across the terminals as 
shown in the right hand diagram, the tone should sound, 
indicating conduction. It the tone sounds for both tests 
or does not sound at all, then the diode is defective. 





pnp 


Fig. 4. When testing a transistor with the Continuity 
Tester, the device must be thought of in terms of the 
equivalent circuits shown. For example, an npn transistor 
is equivalent to two diodes, connected in series by their 
anodes (a), the base (b) being at this junction. Then by 
testing each "diode” in turn using the procedure given in 
Fig. 3, that is the base-collector junction and then the 
base-emitter junction, thus checking the function of the 
transistor. The pnp transistor is tested in a similar manner, 
remembering that the diodes in the equivalent circuit 
are reversed. 


Everyday Electronics , September 1982 


605 

























































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606 


Everyday Electronics, September 1982 

























































S o what is this mysterious form of 
energy called! electricity? 

It’s not mysterious at all, and the 
voltage and current of an electricity 
supply can be explained quite simply 
if we use the example of water flow¬ 
ing in a pipe, for instance in a basic 
central heating system. 

In our central heating system, 
three points must be remembered. 
Firstly, if water is to flow in the 
pipes, they must be connected 1 into 
a complete loop or “circuit”. Secondly, 
the water must be pumped around 
the system to maintain the flow, and 
lastly, the same water must be mov¬ 
ing round all the time; no water is 
added or removed. 

VOLTAGE AND CURRENT 

The voltage in an electric circuit 
can be thought of as the pressure at 
which the water is pumped around 
the system and the current is the 
amount of water actually flowing 
through it. 

Note that if you were to cut the 
water pipe and block off the two ex¬ 
posed ends (in effect, break the cir¬ 
cuit), no water could flow. Similarly, 
in an electric circuit, if the circuit 
is broken, no current would flow. 

However, in the water system, the 
pump could still be operating at the 
same pressure even though no water 
is actually moving. Again, in our 
electric circuit, the voltage can still 
be present even if the current flow 
is interrupted. 


Fig. 1. Demonstration of current flow. 

This is illustrated in a simple circuit 
consisting of a battery (a voltage 
source), a small light bulb (to indicate 
the flow of a current) and! two wires 
to make the “circuit”. 


Everyday Electronics, September 1982 


When these components are con¬ 
nected as shown in Fig. la, a current 
flows and lights the bulb. It is the 
battery that provides the voltage (or 
pressure) for this current to flow. 

If one wire is now removed from 
the battery as shown in Fig. lb, no 
current can flow and the bulb goes 
“out”. However, the voltage is still 
present on the battery. 

UNITS OF ELECTRICITY 

The voltage (V) is measured in 
volts (V). As a voltage is a “pressure” 
or forces it is also referred to as an 
electromotive force (e.m.f.). 

To give an idea of various voltages, 
the cylindrical torch light battery 
(say an SP11) produces l^V, the PP3 
transistor radio type battery can 
supply 9V and the mains voltage is 
240V. 

These high voltages are potentially 
very dangerous and must be treated 
with great care. 

The current (I) flowing in a circuit 
is measured in amperes (A). The 
abbreviation for amperes is not amps 
as this could cause confusion with 
the abbreviation for an amplifier. 

Again to give some idea of practical 
current values, a torch bulb may 
require as little as one twentieth of 
an ampere (0-05A), whereas a two 
bar electric fire uses as much as 12A! 

RESISTANCE 

Having established the basic con¬ 
cepts of voltage and current, we can 
look at another property affecting the 
electric.circuit; that of resistance (R). 
Resistance in a circuit literally “re¬ 
sists” the flow of current through the 
circuit and if we return to the central 
heating system comparison, resistance 
can be thought of as the bore (the 
inside diameter) of the pipes. 

So a small bore pipe restricts the 
flow of water, meaning it has a high 
resistance, and a large bore pipe 
permits a lot of water to flow as it 
has a low resistance. Similarly in the 
electric circuit, a high resistance 
limits the current flow and a low 
resistance allows more current to 
flow. 

The unit of resistance is the ohm 
(given the greek symbol omega, fl) 
and a resistor is a component that 
has a definite known value of resis¬ 
tance. It is the most widely-used Type 
of component and is commonly avail¬ 
able in resistance values ranging 
from one ohm to ten million ohms. 

OHM’S LAW 

The most important fundamental 
law governing all electric and elec¬ 
tronic circuit theory is Ohm’s law. 
This concerns the relationship be¬ 
tween voltage, current and resistance 
in a circuit and! states that the cur¬ 
rent flowing through a circuit is pro¬ 
portional to the voltage applied to 
that circuit and inversely proportional 
to the resistance in that circuit. 




What this means in real terms is 
that voltage, current and resistance 
are governed by the formula: 
voltage(V) = current (7) X resistance^) 
It also follows that: 

1=V+R and R = V=7 
These three formulae derived from 
Ohm’s law allow us to calculate an 
unknown quantity in a circuit if we 
know the other two quantities. For 
example, if a 10 ohm resistor is con¬ 
nected across a 12V car battery, the 
current through it can be calculated 
from I=V/R. Therefore 7= 12/10 = 
1-2A. 

A simple way to remember this 
relationship between V, I and R is to 
draw a triangle as shown in Fig. 2. 

Then by covering the unknown 
quantity with a finger, the relation¬ 
ship between the other two quantities 
will be exposed in the triangle. 



Fig. 2. Ohm's law triangle. 

POWER 

The power (P) developed in a 
circuit is the product of the voltage 
and the current and is measured in 
watts (W). So in our previous ex¬ 
ample, the power developed in the 
10 ohm resistor is equal to 12V X 
1-2A = 14-4W. 

A similar triangle can be construc¬ 
ted to remember the relationship be¬ 
tween P, V and I and this is shown 
in Fig. 3. 



Fig. 3. Power equation triangle. 

MULTIPLES AND SUB-MULTIPLES 

When dealing with voltages, cur¬ 
rents, wattages and resistances, some¬ 
times the values are very large or 
very small numbers and to make 
these numbers a little easier to under¬ 
stand they are expressed as multiples 
and sub-multiples. 

The following are the most com¬ 
monly used: 

micro 00=one millionth of 

(X0-000001 or X10- 6 ) 

milli (m) = one thousandth of 

(XO OOl or 10- 3 ) 

kilo (k) = one thousand times 

(X 1,000 or X10 3 ) 

mega (M) = one million times 

(X 1,000,000 or X10 6 ) 

For example: 

one microvolt (lj»V) = 0 • 000001V 
four milli amperes (4mA)=0-004A 
six kilowatts (6kW) = 6,OOOW 
two megohms (2Mft) = 2,000,000ft 


607 



















MASTER 

ELECTRONICS 

NOW! 

The PRACTICAL wav! 


This new style course will enable 
anyone to have a real understanding 
of electronics by a modern, practical 
and visual method. No previous 
knowledge is required, no maths, and 
an absolute minimum of theory. 

You learn the practical way in easy 
steps mastering all the essentials of 
your hobby or to start or further a 
career in electronics or as a self- 
employed servicing engineer. 

All the training can be carried out in 
the comfort of your own home and at 
your own pace. A tutor is available to 
whom you can write personally at any 
time, for advice or help during your 
work. A Certificate is given at the end 
of every course. 

You will do the following: 

• Build a modern oscilloscope 

• Recognise and handle current electronic 
components 

• Read,draw and understand circuit diagrams 
•Carry out 40 experiments on basic 

electronic circuits used in modern 
equipment 

• Build and use digital electronic circuits 
and current solid state 'chips' 

• Learn how to test and service every type 
of electronic device used in industry and 
commerce today. Servicing of radio, T.V., 
Hi-Fi and microprocessor/computer 
equipment. 


b^SEEINGfc 



Newdob?Ifewr(^ 


E NAME 



Please send your brochure without any obligation to 


am interested in: 

COURSE IN ELECTRONICS 
as described above 
RADIO AMATEUR LICENCE 
MICROPROCESSORS 
LOGIC COURSE 


I 


. OTHER SUBJECTS 


BLOCK CAPS PLEASE 


asj 

Everyday Electronics, September 1982 

























makes soldering easy 
fast & reliable 


Ersin Multicore 

Ersin Multicore, solder contains 5 cores of non- 
corrosive flux, instantly cleaning heavily oxidised 
surfaces. No extra flux is required. 

Comes in handy dispensers and tool box reels in two 
different alloys 40/60 tiiVlead for general purpose 
electrical soldering and 60/40 tin/lead ideal for 
small components and fine wire soldering. 




tin/lead 

pack 0.028mm dla 

Multicore Savbit 

Multicore Savbit, solder increases the life of your 
soldering bit by 10 times, for better soldering 
efficiency and economy. 

Comes in two handy dispensers and tool box reels. 


Size 3 40/60 tin/lead 
£4.37 Per reel 16mm dia 
Size 10 60/40 tin/lead 
£4.37 Per reel o.7immd„ 



Multicore Alu-Sol 

Multicore Alu-SoL solder contains 4 cores of flux, 
suitable for most metals especially aluminium. 
Comes in handy dispensers on tool box reels. 




£2.07 


All prices inclusive of VAT. 

Available from most electrical and DIYs stores. If 
you have difficulty in obtaining any of these products 
send direct with 50p for postage and packing. For 
free colour brochure send S.A.E 



Multicore Solder Wick 

Multicore Solder Wick, absorbs solder instantly from 
tags and printed circuits with the use of a 40 to 50 
watt soldering iron. 

Quick and easy to use, desolders in seconds. 

Size AB10 Solder Wick 
£1.43 Per pack 



Multicore Tip Kleen 

Multicore Tip Kleen, soldering iron tip wiping pad. 
Replaces wet sponges. 


Tip Kleen 
Per pack 



Bib Wire strippers and cutters 
Wire strippers and cutters, with precision ground 
and hardened steel jaws. Adjustable to most 
wire sizes. With handle locking-catch and easy-grip 
plastic covered handles. 


Size 9 Wire Strippers 

£2.69 Per pair 


Bib Audio/Video Products Limited 

(Solder Division), Kelsey House, 
Wood Lane End, Hemel Hempstead, 
Hertfordshire, HP2 4RQ. 

Telephone: (0442) 61291 
Telex: 826437 






'.'..the quality of the colour display is excellent ". Popular Computing Weekly. 
"The graphics facilities are great fun". Personal Computer world. 

"...the Spectrum is way ahead of its competitors" Your computer 


//■ 


'The world's best 
personal computer 
forunder £500" c _ 

Sindair ZX Spectrum 

16K RAM £125,48KRAM £175. 


This is the astonishing new ZX Spectrum 
- a powerful professional’s computer in 
everything but price! 

There are two versions - 16K or a 
really powerful 48K. Both have a full 
8 colours, sound generation, a full-size 


The ZX Printer - available now 

The printer offers ZX Spectrum 
owners the full ASCII character set - 
including lower-case characters and 
high-resolution graphics. 

Printing speed is 50 characters per 


moving-key keyboard and high-resolution second, with 32 characters per line and 


graphics. Plus established Sinclair 
features such as ’one-touch’ keyword 
entry, syntax check and report codes! 

Key features of the Sinclair 
ZX Spectrum 

Full colour - 8 colours plus flashing 
and brightness-intensity control. 

Sound - BEEP command with 
variable pitch and duration. 

Massive RAM - 16K or 48K. 

Full-size moving-key keyboard - all 
keys at normal typewriter pitch, with 
repeat facility on each key. 

High resolution - 256 dots horizontally 
x 192 vertically, each individually 
addressable for true high-resolution 
graphics. 

ASCII character set - with upper- and 
lower-case characters. 

High speed LOAD & SAVE - 16K in 100 
seconds via cassette, with VERIFY and 
MERGE for programs and separate 
data files. 


9 lines per vertical inch. 

ZX Microdrive - coming soon 

Each Microdrive will hold up to 100K 
bytes on a single interchangeable 
microfloppy - with a transfer rate of 
16K bytes per second. And you’ll be 
able to connect up to 8 ZX Microdrives 
to your ZX Spectrum - they're available 
later this year, for around £50. 

(T 0 ; 


irley, Surrey, GU15 3BR. 


How to order your ZX Spectrum 

BY PHONE - Access, Barclaycard or 
Trustcard holders can call 01-200 0200 
for personal attention 24 hours a day, 
every day. 

BY FREEPOST - use the coupon 
below. You can pay by cheque, postal 
order, Access, Barclaycard or Trustcard. 

EITHER WAY - please allow up to 28 
days for delivery. And there’s a 14-day 
money-back option, of course. We want 
you to be satisfied beyond doubt - and 
we have no doubt that you will be. 

Sinclair - 

ZX Spectrum 

Sinclair Research Ltd, 

Stanhope Road, Camberley, Surrey, 
GU15 3PS. Tel: Camberley (0276) 6853U 


FREEPOST - no stamp needed. Prices apply to UK only. Export prices on application. EVE 8 09 | 


























Guess who builds this grea t 

■HI B 

Logic Probe.. YOU!^£12.50 

With this easy-to-build Logic Probe Kit from GSC and just a 
few hours of easy assembly - thanks to our very descriptive 
step-by-step manual - you have a full performance logic 
probe. 

With it, the logic level in a digital circuit is indicated by light 
from the Hi or Lo LED; pulses as narrow as 300 nanoseconds 
are stretched into blinks of the Pulse LED, triggered from 
either leading edge. You'll be able to probe deeper into logic 
with the LPK 1, one of the better tools from GSC. 

GLOBAL SPECIALTIES CORPORATION ■ Wm H M mi m 

ESEl-.= 




G.S.C. (UK) Limited. Dept. 42 
Unit 1. Shire Hill Industrial Estate, 

Saffron Walden, Essex. CB11 3AQ. 
Telephone: Saffron Walden (0799) 2168 
Telex: 81 7477. 


I 

I 

2 L 


IncP&P and 15% VAT 


LPK-1 £15.52 



■ 

j 


Everyday Electronics , September 1982 


611 










































































Everyday Electronics, September 1982 
















































Make theconnection with Access 


and receive a regular postal delivery of Everyday 
Electronics. It’s easy, it’s straightforward and it’s quick. 
Just use the subscription order form to get your Access 
card account charged with the price of a subscription or 
order your subscription through Access on the phone: 
(01) 886 6433. If you pay by cheque or postal order, use 
the subscription order form in the usual way. 



SUBSCRIPTION ORDER FORM 

I wish to become a subscriber to Everyday Electronics for one year and enclose cheque/ 
postal order value. no.made payable to IPC Magazines Ltd. 


I I I I I I I I I I I I I I I I I 


Subscription Rates: 

UK, Isle of Man, Channel 
Islands and Irish 
Republic £11 

Overseas £12 

Unless you are phoning your 
order, complete and post this 
order form to: 

Everyday Electronics, 

2613 King’s Reach Tower, 
Stamford Street, 

London SE1 9LS. 


TECHNICAL TRAINING 
IN ELECTRONICS, 
TELEVISION AND AUDIO 

IN YOUR OWN HOME-AT YOUR PACE 



ig (Joint C&G/ICS) 


■HHHHP 

Radio & Amplfier Construct_ 

Electronic Engineering* and Maintenance 
Computer Engineering* and Programming 
Microprocessor Engineering* 

TV, Radio and Audio Engine 





JOIN UP WITH LITESOLD 



Elements are enclosed in Stainless Steel shafts, 
insulated with mica and ceramic. Non- 
interchangeable bits, choose from 
‘copper’ or 'long life'. A very special 
tool at a very special ‘direct’ price. 

Just £6.58 for iron fitted with 3.2mm 
copper bit. Just £2.40 for 3 spare 
copper bits (1.6; 2.4; 4.7). 

A mere £4.38 for 
professional spring 


AU prices inc. VAT 
Please allow 14 days delivery. 

Write today. Send Cheque/P.O. to Litesold, 97-99 Gloucester Road. Croydon CRO 2DN 
or phone 01-689 0574 for Barclaycard/Access sales. 


TITAN TRANSFORMERS AND COMPONENTS 

CENTRAL HALL CHAMBERS, 
GRIMSBY DN32 7EG 

Mail Order only — Prices include 15% VAT 

Inverters—high quality frequency stable 12 volts DC. 240 
AC/SOHz/100, 250 and 300 watt. 24 volts DC/240 AC/ 
S0Hz/500 watt. 

Voltage Stabilisers—Coarse or fine regulation. High speed 
Null switching I70/260V/50HZ. I00VA to 5KVA plus other 
sizes. 

Adaptors—Switchable 6-7-5-9V 300MA unregulated. 

6-7-5-9V 250 MA regulated. 

Battery chargers—2 volts & 6 volts, power supplies and a 
full range of transformers. 

Send S.A.E. for details of product you are interested in. 


613 





















































CLASSIFIED 


The prepaid rate for classified 
advertisements is 31 pence per word 
(minimum 1 2 words), box number 
60p extra. Semi-display setting 
£7 -24 per single column centimetre 
(minimum 2-5cm). All cheques, 
postal orders, etc., to be made pay¬ 
able to Everyday Electronics and 
crossed "Lloyds Bank Ltd." Treasury 


notes should always be sent 
registered post. Advertisements, 
together with remittance, should be 
sent to the Classified Advertisement 
Department, Everyday Electronics, 
Room 2612 , IPC Magazines Limited, 
King's Reach Tower, Stamford St., 
London SE1 9LS. (Telephone 
01-261 5942). 


Receivers and Components 


P.C. BOARD S.S. 12in X 12in 3 for £2 00, 
glass fibre P.C board S.S. or D.S. 12inX12in 
£1-00 each. Add 60p P&P any quantity. 
COOPER, 16 Lodge Road, Hockley, Birm- 
ingham B18 5PN. _ 

POWER SUPPLIES, chassis mounted (un¬ 
boxed). Output fully variable between 5 
Volts and 27 Volts. Maximum current 1 
Amp between 11 and 15 Volts. £8 post paid. 
RUSSELL, 33 Longridge Avenue, Staly- 
bridge, Cheshire. 

BUILD YOURSELF a Valve Radio or Ampli- 
fier! We can supply most parts and data. 
Lists 65p or SAE enquiries. New Valves! 
ECH81/80i 2 p, EF80/67p, EF85/69p, EF86/83p, 
ECC83/86p, EL34/£2 • 18‘ 2 p, EL84/80‘ 2 p, 
EZ80/75p. P&P/75p. Holders (B7G/PCB, 

23p), Transformers, Capacitors, Resistors, 
etc. Callers welcome. Letchworth Electronic 
Components, Spirella Building, Bridge 
Road, Letchworth, Herts SG6 4ET. Tel: 
(04626) 70354. 


SCOOP PURCHASE! TELEPHONES 

HAVE YOU 0 IeeN THE GREE^CAT’^OMs'oV’nsw 


300 SMALL COMPONENTS, transistors, 
diodes. £1-70 71bs assorted components £4-25 
lOlbs £5-75. Forty assorted 74 series ICs on 
panel £1-70. 500 capacitors £3-20. List 20p 
refundable. Post 60p. Optionable insurance 
20p. J. W. B. RADIO, 2 Barnfield Crescent, 
Sale, Cheshire M33 1NL. 



Receivers and Components 

TURN YOUR SURPLUS capacitors, tran¬ 
sistors etc., into cash. Contact Coles Hard¬ 
ing & Co., 103 South Brink, Wisbech, 
Cambs. 0945-584188. Immediate settlement. 
AERIAL BOOSTERS trebles incoming sig- 

nal, price £7 00. SAE leaflets. VELCO 
ELECTRONICS, Ramsbottom, Lancashire 
BL0 9AG. 

CHEAP COMPONENTS SUPPLIED, tele¬ 

phone J. G. Electronics day or evening 
Medway 250271 for prices and details. 


For Sale 


NEW BACK ISSUES OF “EVERYDAY 
ELECTRONICS”. Available 85p each Post 
Free, cheque or uncrossed PO returned If 
not in stock. BELL’S TELEVISION 
SERVICES, 190 Kings Road, Harrogate, 
Yorkshire. Tel: (0423) 55885. 


Veteran & Vintage 



Educational 


COURSES—RADIO AMATEURS EXAMINA¬ 

TION. City and Guilds. Pass this important 
examination and obtain your licence, with 
an RRC Home Study Course. For details of 
this and other courses (GCE, profes¬ 
sional examinations, etc) write or phone— 
THE RAPID RESULTS COLLEGE, DEPT 
JR2, Tuition House, London SW19 4DS. Tel: 
01-947 7272 (9 am-5 pm) or use our 24 hour 
Recordacall Service: 01-946.1102 quoting 
Dept JR2. 



MANUFACTURERS SURPLUS new com¬ 
ponents, ICs, transistors, diodes, capacitors, 
resistors, etc. 100 assorted £1-25 post free. 
U.H.A. Ltd, 62 Wellington Road South, 
Stockport, Cheshire. _ 

P.C. BOARD D.S. 7inX7in 30p; 9inX9in 40p; 
9inX13in 50p; S.S. 15inX7in 40p; 18inX6in 
50p. P&P lOp per £1-00. TULETT, Kempes 
Corner House, Boughton Aluph, Ashford, 
Kent. 



ORDER FORM PLEASE WRITE IN BLOCK CAPITALS 

Please insert the advertisement below in the next available issue of Everyday Electronics for. 

insertions. I enclose Cheque/P.O. for £. 

(Cheques and Postal Orders should be crossed Lloyds Bank Ltd. and made payable to Everyday Electronics) 


NAME . 

ADDRESS...... 

Company raplstered In Enplond. Registered No. 93S». Registered Office: King’s koech Tower, Stamford Street, Lo 


EVERYDAY ELECTRONICS 


614 


Everyday Electronics, September 1982 
































































615 















































V INDEX 


ADVERTISERS 


Ambit . 

.. 598 

Bib-Audio. 

.. 609 

Bi-Pak . 

.. 554 

B.K. Electronics .. 

Cov. Ill 

B.N.R.E.S. 

.. 608 

Bradley Marshall .. 

.. 554 

Chordgate. 

.. 615 

Cricklewood Electronics.. 

.. 612 

Dziubas, M. 

.. 556 

Electronic Hobbies Fair .. 

.. 579 

Electronize Design 

Cov. II 

Electrovalue 

.. 606 

Enfield Electronics 

.. 556 

Global Specialties 

.. 611 

Greenweld. 

.. 616 

Heath-kit. 

.. 612 

Intertext-ICS 

.. 613 

Lightning Components .. 

.. 616 

Literacy . 

.. 616 

Litesold . 

.. 613 

Magenta Electronic Supplies Ltd. 555 

Maplin Electronics 

Cov. IV 

Phonosonics 

.. 612 

R & T.V. Components 

.. 606 

Radio Component Specialists 

.. 615 

Rapid Electronics .. 

.. 557 

Sinclair Research .. 

.. 610 

Titan Transformers 

.. 613 

T.K. Electronics .. 

.. 611 

Watford Electronics 

.. 558 

Wilmslow Audio .. 

556, 606 


SUMMERTIME SALE!! 


AM price, inc.ude SCOOP) 



Wi,H " 



For further information contact: 

I Literacy A Bade Skill. Unit, King.boume Hou*e, 
229/231 High Holborn, London WC1V 7DA. 

_ Telephone: 01 -405 4017 J 

































B.K. ELECTRONICS ^ Oo "> 

A SOUND CHOICE Sr %%pr 

★ PROMPT DELIVERY ★ PRICES INCLUDE V.A.T. * AMPLE STOCKS 
A PERSONAL SERVICE FROM A SMALL EXPANDING COMPANY 


STEREO CASSETTE TAPE DECK MODULE 


1 K-WATT SLIDE DIMMER 

★ Controls loads up to 1KW. 

★ Compact Size 4}" ' lii' > 2i". 

•k Easy snap in fixing through panel/cabinet cut out. 
ic Insulated plastic case. 

★ Full wave control using 8 amp triac. 

★ Conforms to BS800. 

ir Suitable for both resistance and inductive loads. 
Innumerable applications in industry, the home, 
and disco's/theatres, etc. 

Price £11 70 each + 50p P4P. (Any quantity.) 


v oic e ^f,t' Re so ruiju ^ tequencj *4F r e n™ y 6 * ^ "'“"bKHz! ^ 



TVPEB^Pf S5A 




sSSSi*- 
sST- M 


(KSN1057A) Cas 


B.K. ELECTRONICS .. ..*7 


37 Whitehouse Meadows, Eastwood, Leigh-on-Sea, Essex 5 

k SAE for current lists. ★ Official orders welcome. ★ All prices include VAT. ★ Mail order c 


5TY 


All items packed (where 


applicable) in special energy absorbing PU foam. Callers welcome by prior appointment, please phone 0702-527572. 



















it with mwim 

KEYBOARD KIT WITH ELECTRONICS FOR ZX81 

* A full size, full travel 43-key keyboard that's simple to add to your ZX81 (no soldering in ZX81). 

* Complete with the electronics to make "Shift Lock", "Function” and "Graphics 2" single key selections making entry far ea: 

* Powered from ZX81’s own standard power supply - with special adaptor supplied. 

* Two-colour print for key caps. 

* Amazing low price. 

Full details in our projects book. Price 60p. Order As XA03D. 

Complete kit for only £19.95 incl. VAT and carriage. Order As LW72P. 


25W STEREO MOSFET AMPLIFIER 

AwperbnMV amplifier a, a remarkably low price. 


ssS£!2& 

Complete kits availahlp 
^ ectr onics~£2gggc 
Cabinet- £gg c n \ ' 

^oca^? 0 . fc f r . r,a 9^«ra). 


* Over 26W P e, 

* Extremely easy to 

cutting intetwirmg to |ust 7 wires tpms 

and mamsleadtem,mat,onsl h ^ ^ ^ jncludjng p[e . 

* Sffiptintedc^and^encabinet 



MAPLIN S FANTASTIC PROJECTS 

Full details in our project books only 60p each. 

In Book 1 (XA01B) 120W rms MOSFET Combo-Amplifier • Universal Timer with 18 
program times and 4 outputs - Temperature Gauge - Six Vero Projects 
In Book 2 (XA02C) Home Security System ■ Train Controller for 14 trains on one circuit • 
Stopwatch with multiple modes - Miles-per-Gallon Meter 

In Book 3 (XA03DIZX81 Keyboard with electronics • Stereo 25W MOSFET Amplifier • 
Doppler Radar Intruder Detector - Remote Control for Train Controller 
In Book 4 (XA04E)* Telephone Exchange expandable up to 32 extensions - Ultrasonic 
Intruder Detector • Frequency Counter 10Hz to 650MHz ■ Remote Control for 25W Stereo 
Amplifier 


MORE GREAT KITS FROM MAPLIN 

Matinee Organ (see box above) 

Spectrum Synthesiser. Full details in book XH56L. Price £1.00 

Kfinnc S c n,h r' Ser 1 Ful1 de,ails in book XF11M - Price £2 - 00 

5600S Synthesiser J 

150W Power Amp Kit LW32K. Price £17.95" 

75W MOSFET Power Amp Kit LW51F. Pric? £11.49* 

50W Power Amp Kit LW35Q. Price £14.95* 

15W Power Amp Kit YQ43W. Price £6.45* 

8W Power Amp Kit LW36P. Price £4.45* 

(Power supply not included - details with kit/. 


HOME SECURITY SYSTEM 

Six independent channels-2 or 4 wire 
operation. External horn. High degree 
of protection and long term reliability. 


Map/in launches MAPCARD 

of buying fromMaS CARD 1 

APPLY NOW, 



NEWSHOP IN BIRMINGHAM 

Visit our brand new shop in Birmingham for our __ 

Come and see us at Lynton Square, Perry 
Barr, Birmingham (just off the junction of the J * J. 

Outer Ring Road A4040 and Birchfield Road ftf I 

A34). Tel: 021-356-7292. Excellent free Pi 
parking. A 

Opening Tuesday 24th August, 1982 


On sale now in all branches of WHSMITH^i price £1. 
320 big pages packed with data and pictures ol over 


Post this coupon now) 

Please send me a copy of your 320 page catalogue. I enclose £1.25 line. 25p p&p). II 
I am not completely satisfied I may return the catalogue to you and have my money 



AH mail to: 

P.O. Box 3, Rayleigh, Essex SS6 8LR 

Tel: Sales (0702) 552911 General (0702) 554155