-

V ■

A 1

CONTENTS

SPECIAL FEATURES

907 EDITORIAL: Hacking( Fun or Evil?

9.18 Al Transplant

9.27 The changing role of public telecom operators

9.30 Medical electronics from L & T

9.41 Finding faults, No problem

9.45 News

9.120 New products

AUDIO & HI-FI

9.50 Balance indicator (001 )

9.55 Recording control (009)

9.57 4-channel mixer (01 4)

9.59 Bucket brigade delay line (01 9)

9.62 T uneable band-pass filter (022)

9.66 Sound level attenuator (030)

9.80 A simple VOX (053)

9.80 Low-noise microphone preamplifier (054)

9.1 12 Voice-band filter (096)

CAR ELECTRONICS

9.51 Car lights monitor (003)

9.62 Car headlight control (023)

9.71 Energy control for battery charges (039)

9.72 Car alarm (041)

9.74 Psychological car lock (043)

9.79 Improved low-fuel indicator (052)

9.89 Rear window wiper coupler (067)

9.96 Rear wash-wipe control (077)

COMPONENTS

9.63 BiCMOS integrated circuits (025)

9.67 Fast unity gain opamp (031 )

9.1 17 Decoupling power rails (103)

9.1 15 R-2R resistance network in SMT (100)

COMPUTERS

9.50 Reset for the PCI 640 (002)

9.55 Child-proof reset switch (01 0)

9.58 MSX EPROM (015)

9.63 Reset protection (024)

9.68 Printer reset (032)

9.71 l/O-friendly keyboard (038)

9.77 Monitoring temperature with the C64 (049)

9.89 Pulse skipper (068)

9.91 Daylight-resistant optocoupler (070)

9.95 RS 232 Interface for C64 (076)

9.98 One or two MBIT Eprom programmer (081 )

9.104 Small I/O card (087)

9.1 10 Automatic printer sharing (095)

ELECTROPHONICS

9.51 Vocal eliminator (004)

9.68 Variable low-pass filter (033)

9.87 Guitar compressor (064)

9.99 MIDI interface for Amiga (082)

9.116 Break-jack adaptor (101)

9.1 1 8 Modular guitar amplifier (1 05)

9.54 Sound level meter (008)

9.52 Power booster for 7406/7407 (005)

9.56 "On” indicator (01 1 )

9.59 Simple temperature indicator (01 8)

9.64 Programmable switch (026) /

9.69 Automatic switch (034) ( /

ANNUAL NUMBER
SEPTEMBER 1989
VOLUME-7
NUMBER-9

9.70 Head/tail lights for model railway (037)

9.73 Twilight switch (042)

9.75 X-Y plotter interface (044)

9.75 Timer with audible warning (045)

9.79 Heating timer (051)

9.81 High-volume alarm (055)

9.82 Mains-powered timer (056)

9.83 Single-chip melody generator (057)

9.83 Infra-red microphone (058)

9.84 Four-quadrat dimmer (059)

9.84 Sensor switch and clock (060)

9.85 Mini-drill control (061)

9.86 Mains failure indicator (063)

9.90 Flashing-light control (069)

9.92 Improved economical porch light (071)

9.92 Ringing pruner (072)

9.94 EEPOT (074)

9.95 LED Voltmeter in SMT (075)

9.97 Differential amplifier (078)

9.100 Automatic charger add-on unit (083)

9.101 Smoke detector (084)

9.105 Liquid-level monitor (089)

9.106 ‘On’ indicator for gas-operated fridges (090)

9. 107 Automatic fog horn (091)

9.1 12 Slide fader update (097)

9.1 14 Time-delayed flash (099)

9.1 18 Light (S) out? (104)

9.1 19 Donald duck generator (106)

POWER SUPPLIES

9.52 78xx monitor (006)

9.56 9-volt supply (01 2)

9.65 Switch-mode voltage regulator (027)

9.69 Low dissipation regulator (035)

9.76 High-power zener diode (047)

9.78 Simple variable power supply (050)

9.93 Small step-up converter (073)

9.98 Power supply with EEPOT (080)

9.109 Symmetrical power supply (094)

9.113 Reference-voltage source with indicator (098)

9.116 Maximum/minimum voltage indicator (102)

RADIO & TV

9.53 Radio beacon converter (007)

9.58 Universal squelch (016)

9.60 Fast envelope sampler (020)

9.65 2-metre transmitter (028)

9.86 Call tone generator (062)

9.102 VFO stabilizer for up to 100 MHZ (085)

9.107 Headphone amplifier with scart plug (092)

TELECOMMUNICATIONS

9.70 Duplex audio link (036)

TEST & MEASUREMENT

9.56 ABS/RMS/LOG converter (013)

9.59 48 MHz CMOS oscillator (017)

9.61 Noise generator (021 )

9.66 Meter-scale magnifier (029)

9.72 TTL supply monitor (040)

9.76 HCMOS square wave generator (046)

9.77 Voltage-controlled oscillator (048)

9.88 LC sine wave generator (065)

9.88 Shunt for multimeter (066)

9.97 HF probe for oscilloscope (079)

9. 103 Crystal tester (086)

9.104 Voltage tracer in SMT (088)

9.109 Digital trigger for oscilloscopes (093)

elektor india September 1989 9.05

Publisher : C.R. Chandarana
Editor : Surendra Iyer
Circulation : Advertising : J. Dhas
Production : C.N. Mithagari

Address :

ELEKTOR ELECTRONICS PVT. LTD.

52, C Proctor Road, Bombay-400 007 INDIA
Telex: (011) 76661 ELEK IN

OVERSEAS EDITIONS

Elektor Electronics
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Editor: Bill Cedrum

The Circuits are domestic use only. The submission of
designs or articles implies permission to the publisher
to alter and translate the text and design and to use the
contents in other Elektor Publications and activities.
The publishers cannot guarantee to return any material
submitted to them. Material must be sent to the Holland
address (given above!. All drawings, photographs,
printed circuit boards and articles published in elektor
publications are copyright and may not be reproduced
or imitated in whole or part without prior written
permission of the publishers.

Patent protection may exist in respect of circuit devices,
components etc. described in this magazine.

The publishers do not accept responsibility for failing to
identify such patent or other protection.

Printed at : Trupti Offset Bombay - 400 01?
Copyright ® 1989 Elektuur B.V.

ANNUAL NUMBER
SEPTEMBER 1989
VOLUME-7
NUMBER-9

HACKING: FUN OR EVIL?

There are indications that hacking, the nefarious accessing of computers, is
becoming more widespread. There are many who in their naivity consider that
this anti-social behaviour is nothing but a bit of fun whose perpetrators do a
useful job in testing the security of computer systems. Nothing could be further
from the truth.

A hacker is someone who, without permission, enters your home as it were and
goes through your personal belongings. While doing so, he may, innocently or
intentionally, take copies of your computer files. He may destroy or corrupt
them. He may unwittingly plant an electronic time bomb that does not go off til
much later.

There is no way of knowing exactly the harm and damage caused to private
computer systems, but it is estimated, and in a number of countries a
documented fact, that industrial and commercial losses world-wide amount to
thousands of millions of pounds. More seriously, deaths have already occurred
owing to the (hopefully inadvertent) changing of medical records by hackers
infiltrating a French hospital computer system. And nobody knows how many
cases of blackmail have been, or are being, conducted as the direct result of a
hacker’s activities.

It is, of course, true that many computer owners to a large extent have only
themselves to blame. After all, you don't leave your car unlocked parked
anywhere in a big (or even small) town nowadays. Nor do you leave the front
door of your home open so that people can just walk in. That is, of course, no
reason for anybody to take your car or enter your home, but it does hand them
the opportunity of so doing on a plate. Considering the cost of a computer
system, and the possible losses caused by hacking, the cost and trouble of
making the system secure are relatively small.

Many may argue that the laws should be changed to deal with hackers, but that
will not be the whole answer any more than it is in the case of other kinds of
crime. Nevertheless, a change in the law is overdue, if only because it will at
least reflect the attitude of the nation to hacking, an activity that is at best
irresponsible, invariably harmful, and often destructive.

elektor india September 1989 9.07

BALANCE INDICATOR

If your amplifier is fitted with
two balance controls (as, for
instance, the Elektor Electronics
Preamplifier for Purists - Ref. 1 ),
it actually offers you a balance
control and a level control. A
drawback of this is that it is quite
difficult to set the balance prop-
erly. This may be obviated, how-
ever, by replacing the two mono
potentiometers by stereo ver-
sions, PI and P2 in the diagram.

One half of the pair, Pla and
P2a, assumes the tasks of the
removed components. The other
half is connected in a bridge cir-
cuit. The voltage between the
wipers of the potentiometers is
then a measure of the balance between the two channels. The
lower the potential, the better the balance. If you are interested in
knowing the degree of unbalance, connect a centre-zero moving
coil meter with a bias resistor between A and B. With this
arrangement, zener diodes D1 and D2 may be omitted: they are

necessary only with the led indi-
cator shown in the diagram to
prevent the input voltage of the
opamp getting too close to the
level of the supply voltage.

The circuit around IC1 is a
classical differential amplifier.
Resistors R5 and R6 provide a
virtual earth for the LEDs, which
is necessary to ensure that in
spite of the asymmetrical supply
voltage a positive and a negative
output is obtained.

Since the leds have been
included in the feedback loop of
the indicator, the circuit is pretty
sensitive. At only 40 mV, that is,
just one fourhundredth of the
supply voltage, one of the leds begins to light already. The maxi-
mum current drawn by the leds is determined by the values of R5
and R6.

Ref. 1. “Elektor India November 1988

| 002

COMPUTERS

1

RESET FOR THE PC 1640

The PC1640 is one of Amstrad's popular and successful series
of compact pcs. It has, unfortunately, a serious deficiency: there is
no reset control. Luckily, it is not too difficult to fit this control ret-
rospectively, since every pc, and the 1640 is no exception, has a
reset circuit.

In the 1649, the circuit shown below monitors the level of the
supply voltage. This voltage is sampled by potential divider
R151-R152-R176-R188. If the supply is too low, the Q105, and
consequently the Q104, switches off. Transistor Q106 is also pro-
vided with too little bias voltage to conduct. The enable input of
the CMOS memory, IC134, is linked to the collector of Q105, so that
when Q105 is turned off, the memory is disabled. At the same
time, the supply to the memory is switched off since Q104 is off.

CE

IC134

Only when the supply voltage is at the correct level does Q105
conduct. At the same time, Q104 and Q106 are provided with suf-
ficient base current to switch on. This causes the supply to 1034
to be switched on, which results in the removal of the inhibit on
the enable input. The processor receives a signal, pwrok, indicat-
ing that the supply level is all right, so that the reset cycle can
start. The entire computer is initialized, while all i/o lines are set
as required.

To fit a reset control, use is made of the pwrok signal in the fol-
lowing manner. If R152 is short-circuited, pwrok goes low and the

processor is reset. Only when pwrok has gone high again will the
processor start the reset routine. This is exactly the same cycle that
occurs when the computer is restarted. In other words, all the
reset control needs to do is to short-circuit R152.

The reset facilicity is incorporated fairly easily by fitting at
some covenient place on the computer a simple push-button
switch with a make contact. Connect the two terminals of the
switch to the two ends of R152 (clearly marked on the pcb) via two
short lengths of flexible wire and that’s all.

CAR LIGHTS MONITOR

A defect car light is at best a
nuisance and at worst a danger.

Fortunately, most new cars are
equipped with suitable monitors
thast indicate on the dashboard
whether a light is not working.

There are, of course, millions of
older cars that have no such
sophistication and it is for these
that the present monitor is
intended.

Two special ics are available
from Telefunken that are de-
signed for measuring the current
through a light bulb. In practice,
detecting whether a current
flows through a bulb or no is a
most suitable way of determin-
ing whether the bulb still works.

If a small resistance is con-
nected in series with the bulb, a
small voltage drop will develop
across it when the bulb lights (R1 and R2 in the diagram). Each ic
can cope with only two bulbs, so that per car three or four ics are
needed. The junction of the bulb and resistor is connected to one
of the inputs (pin 4 or pin 6) of the ic. The potential across the
resistor is compared in the ic with an internal reference voltage.

Depending on which of the two
ics is used, the voltage drop must
be about 16 mV (U477B) or 100
mV (U478B). This voltage drop is
so small that it will not affect the
brightness of the relevant bulb.

The value of the series resis-
tor is determined quite easily. If,
for instance, it is in series with the
brake light (normally 21 W), the
current through the bulb, assum-
ing that the vehicle has a 12 V
battery, is 21/12=1.75 A. The
resistance must then be 16/1.75=
9 m£2 (U477B) or 100/1.75=
57 mO (U478B).

These resistors may be made
from a length of resistance wire
((available from most electrical
retailers). Failing that, standard
circuit wire of 0.7 mm dia. may be
used. This has a specific resis-
tance of about 100 ml) per metre. However, in most cars the exist-
ing wiring willl have sufficient resistance to serve as series resis-
tor.

LEDs may be connected to the outputs of the ic (pins 3 and 5).
These will only light if the relevant car light fails to work properly.

ELECTROPHONICS

VOCAL ELIMINATOR

Otherwise properly mixed sounds often suffer from a predom-
inant solo voice (which may, of course, be the intention). If such a
voice needs to be suppressed, the present circuit will do the job
admirably.

The circuit is based on the fact that solo voices are invariably
struated "at the centre" of the stereo recordings that are to be
mixed. This means that the voice levels in the left- and right-hand
channels are about equal. Arithmetically, therefore, left minus
right is zero, that is, a mono signal without voice.

There is, however, a. problem: the sound levels of bass instru-
ments, more particularly the double basses, are also just about the
same in the two channels. This is becausae on the one hand low-
frequency sounds are virtually non-directional and on the other
hand, the recording engineers purposely use these frequencies to

give a balance between the two channels.

However, the bass instruments may be recovered by adding
those appearing in the left+right signal to the left-right signal.
The whole procedure is easily followed in the circuit diagram.
The incoming stereo signal is buffered by A1 and A2. The
buffered signal is then fed to differential ampolifier A3 and subse-
quently to summing amplifier A5. The latter is followed by a low-
pass filter formed by A6. You may choose between a first-order
and a second-order filter by respectively omitting or fitting C2.
Listen to what sounds better.

The low-frequency signal and the difference signal are applied
to summing amplifier A4. The balance between the two is set by
PI and P2 to individual taste.

You may have noticed that the circuit does not contain input

elektor india September 1989 9.51

or output capacitors. If you wish,
output capacitors may be added
without detriment. However, the
fitting of input capacitors is not
advisable, because the conse-
quent phase shift would adverse-
ly affect the operation of the cir-
cuit.

(A. Roelen)

L

POWER BOOSTER FOR 7406/7407

It often happens that a digital
signal is required for controlling a
relay, a stepper motor, or other kind
of relatively heavy load. This makes
it necessary for both the output cur-
rent and the output voltage of the
relevant device to be increased.
Some logic devices are provided
with constant-voltage open-collec-
tor outputs, but these are invariably
restricted to 15 V or 30 V.

With a little dexterity, it is possi-
ble to provide a 7406 or 7407 with a
dedicated open-collector output -
see Fig. 1. If you aim for the con-
struction in Fig. 2, the result will
take not much more space than a
standard 7406. Since the output
stage inverts, it is necessary to use
the non-inverting 7407 to obtain an
inverted output.

The transistor must be chosen in
accordance with the wanted output.
For most general purposes, the
BC546 is perfectly satisfactory (200

mA at 65 V). - . „ , „

(A. Schaffert)

78XX MONITOR

When a voltage regulator is supplied from a mains adapter, it
sometimes happens that its output is too low (because the output
from the adapter is too low, or because the voltage has dropped to
a low value owing to an overload). It is useful if a warning of that

situation is indicated.

The proper operation of the 78xx series of regulators depends
on the difference between input and voltage voltage, which must
be not less than 3 V (worst case; many regulators are much better).

9.52 elektor india September 1989

The voltage drop across the regulator
is monitored by IC1. The input and out-
put voltage of the regulator are supplied
to IC1 via potential dividers. If the input
voltage to the regulator is too low, IC1
goes high, which causes Cl to charge
and this turns on Tl, so that D2 lights.
You may, of course, use a buzzer instead
of D2 and R7. The charge on Cl ensures
that the LED lights for at least 10 ms. This
means that the circuit will react to even
very short voltage drops at the input of
the regulator. A large ripple that results
in a too low input voltage is therefore
clearly indicated. The circuit is based on
the 7805; the value of R1 must be redi-
mensioned for other members of the
78xx series by the following:

Rl=[(2dU/Ur) + l|/R2

where dU is the voltage drop across the
regulator and Ur is the characteristic
output voltage of the regulator. It is nec-
essary that dU is chosen somewhat larg-
er than the actual minimum voltage
drop across the regulator to prevent
non-operation of the circuit. This is so
because the minimum voltage drop
across the regulator is constant when the
device ceases to function properly until
the input voltage returns to normal. In
other words, the monitor must be able
to react to a voltage drop that is slightly
higher than the minimum voltage drop.

RADIO BEACON CONVERTER

.1 = 10 MH* ory<sisL.30 pF peraltei
resatwcs

Capacitors:
iCl = 68 pF
C2; C6; C15 = 27 pF
C3; C16 = 1 nF
C4 = 120 pF
C5 = 82pF
C7 = 22QpF

C8; C13 = 40 pF trimmer
C9 = 15 pF

CIO; Cll = 150 pF s tyro flex
C12 = 100 nF
C14 = 330 pF
07= 1 fiF;16V

inductors:

LI; 125 = 2.2 mH
L2;L4;L6 = 4.7mH
L3= 1.5mH

;D1 = zener diode 82 V
ICl = NE602

The radio beacon band extends
from 280 kHz to 516 kHz. Each
beacon has its own characteristic
AM modulated morse-coded call
sign that is transmitted on a specif-
ic frequency. To be able to receive
distant beacons, the aerial signal is
passed through a band-pass filter
that effectively suppresses long-
wave and medium-wave signals.

The filter also converts the aerial
impedance, Zin, from about 10 kQ
to the inpout impedance of mixer
ICl, which is about 1 kfl.

The mixer adds or subtracts the
received signal to/from the local
oscillator signal, so that the beacon
signal can be received on a normal
short-wave receiver. The resulting
frequencies he in the range
9.72-9.484 MHz or 10.280-10.516
MHz.

In the construction of the converter some
components must be surrounded by a metal
shield as indicated by dashed lines on the pcb
layout.

The circuit is aligned with the aid of an
ssb receiver to which the output of the con-
verter is connected. Tune the receiver to 10
MHz and adjust the oscillator frequency of
the converter by means of C8 for zero beat.
Next, detune the receiver slightly until a
pleasant whistle is heard, which is adjusted
for minimum level with the aid of PI. Finally,
tune to a beacon transmitting at or about 300
kHz and adjust C13 for maximum sound
output.

Resistors:

Rl;R2 = 10ka
R3=470O

PI = 25 kG preset potentiometer

n, I

1

22 Op L_

1

{y

d

R 2 |

100nL_

1 2

1

XI

~_.n.

ICl

NE602

1SOp c 12l

40P TTso

-QtoHJ-0— 0

8...12V
<10mA

elektor india September 1989 9.53

SOUND LEVEL METER

Although the NE604 is, strictly speaking, intended primarily
for hi. applications, it may be used for a number of other purpos-
es. One of these is a sound level meter for audio applications as
presented here.

Use is made of the IC's signal-strength indicator that is based
on an internal logarithmic converter. This enables us to obtain a
linear decibel scale, so that the moving-coil meter shown in the
diagram may be replaced by a digital instrument.

The signal source is assumed to be an electret microphone that
converts ambient noise into an electriocal signal. Since this type of
microphone normally contains a buffer stage, R7, R8 and C13
have been included to provide the supply voltage for this stage.

The NE604 delivers an output current (at pin 5) of 0-50 pA,
which causes a potential difference across R2+R3 of 0-5 V. The
range over which the relation between input and oujtput signal is
logarithmic is, however, slightly smaller: about 0-4 V. This is
equivalent to a sound range of 70 dB.

To compensate the effects of temperature changes, the required
resistance of 100 kfJ is formed by two resistors, R2 and R3, and a
diode, D1 .

Any ripple remaining on the output voltage is removed by R4-
C9-C10 before the output is buffered by 1C2.

The indicating instrument, here a moving-coil meter, is con-
nected to the output (pin 6) of IC2 via a series resistance, R6+P1,
The preset is adjusted to give full-scale deflection (f.s.d.) for an
output voltage of 4 V.

Calibrating the meter is a little tricky, unless you have access to
an already calibrated instrument. Otherwise, if you know the effi-
ciency of your loudspeaker, that is, how many decibels for 1 W at
1 metre, you can use that as reference. The scale of the meter can
then be marked with the (approximate) value. In any case, the
meter deflection must at all times be seen as an indication, not as
an absolute value: it was not thought worthwhile to add a filter to
the circuit to enable absolute measurements to be made.

Resistors:

Rl; R6 = 2k2

R2 = 60k4 (E96) or two 120 kft
resistors in parallel
R3 =* 40k2 or 39 kQ in series
with 1 kX2
R4;R5 = 1M

R8=T0k

PI * 10 k preset potentiometer

Capacitors: i-mr-m-::

Cl =4p7/63V radial
C2 =» 15 n

C3 « 220 p/ 16 V radia
C4-C7 inch - 10 p/16
C8 = 47p/16V - v

radial

C9; CHc 100 h
CIO - 4n7 : "
Cll; 02 * 1 n
C13«47p/10V

ooo

D1 = 1N4148

Miscellaneous:

SI = push-button switch with
one make contact
Ml = moving-coil meter; 1 mA

9.54 elcktor india September 1989

RECORDING CONTROL

The circuit presented here is intended as a recording
level control for cassette recorders. It enables the reading
on the vu meters to be kept out of the red sector without
the necessity of the recorder's level control to be adjusted.

This is particularly useful when speech is recorded.

The recording signal is controlled via two voltage
dividers, each containing a light-dependent resistor - i.dr.
Unfortunately, these devices introduce slight distortion, but
that is still better than a lot of distortion through overload-
ing of the tape.

The circuit monitors the signal level at the headphone
output of the recorder. The signal there is rectified (half
wave) and then applied to Tl. The speed with which T1
can react is determined by Cl. As shown, the attack time is
set at 3 ms. The release time of the circuit is determined by
R4-C4 and is much longer than the attack time: about 100
ms.

The voltage across C2 determines to what extent the signal to
be recorded is attenuated. The charging current through R4
depends on the voltage across the capacitor, so that the light
intensity of the LEDS is also dependent on the signal strength.

The light from D1 and D2 falls on to R7 and R8 , which form
part of the potential dividers that are added to the input circuit of
the recorder.

The circuit is easily disabled by opening SI .

Owners of cassette recorder with separate recording and play-
back heads should bear in mind that the circuit does not work
well if the internal monitor is switched on. In that situation, the
signal takes a finite time to reach the headphone socket: too late
for correcting the input signal level.

(S.G. Dimitriou and F.P. Maggana)

•

010

COMPUTERS

CHILD-PROOF RESET SWITCH

The reset switch on a com-
puter is a very important con-
trol. If an operating instruction
threatens to wreck the internal
management of a computer, the
reset button is often the only
way of avoiding a possible dis-
aster. On the other hand, it may
also be the cause of a disaster.
After all, one touch on it and
hours of work may be negated
in an instant, that is, if you do
not save your work every fif-
teen minutes or so. None the
less, anyone can have an acci-
dent, but it is particularly
important that children or pets
can not inadvertently operate
the control.

The circuit proposed here
should put an end to your wor-
ries in this respect. Instead of
one reset switch, it is necessary
to press four switches simulta-

or pet are so
ligible.

The four switches are placed

in positions that make it impos-
sible to operate them all with
one hand. Instead, two of them
can be operated with the fingers
of one hand and the other two
with the fingers of the other
hand.

As shown, the four switches
are connected in series and are
intended to replace the existing
switch.

SI S2__ S3_ S4

JL JL JL JL

O — o o — o o — o o — o o Q

894011-11

on

GENERAL INTEREST

“ON” INDICATOR

Battery-operated equipment can
work on one set of batteries for a
long time nowadays. But if it is left
on inadvertently that 'long time' is
over very quickly. Moreover, flat
batteries are always found at the
wrong moment. The circuit pro-
posed here is a sort of aide-mem-
oire. Every two minutes it emits
5-10 pips to indicate that the equip-
ment is still switched on.

Basically, the circuit consists of
three rectangular-wave generators and an inverter. The first of the
generators is formed by N1 and provides a signal with a period of
about two minutes and a pulse duration of around ten seconds.
During those ten seconds, the second generator starts operating in
a one-second rhythm. Thus, N2 outputs ten pulses every 2 min-

utes. That output is inverted so that
N4, like N2, can only be enabled
during the 10-second pulse train
from Nl. There is a difference,
though: during those 10 seconds, N4
is enabled and inhibited ten times
and this is what causes the pips.

Do not take the times and num-
ber of pulses too literally, because
there are wide variances between ics
from different manufacturers. On

the other hand, component values are not critical, so that it is fair-
ly easy to adapt the circuit to personal taste or requirements.

The buzzer may be a standard Toko type or equivalent.

Finally, the current drawn by the circuit is negligibly small.

(R. Kambach)

9-VOLT SUPPLY

The TL496 is an 1C that is intended to produce a nominal 9-volt,
that is 7-10 V, supply voltage from a variety of sources. This sup-
ply is perfectly adequate for many circuits.

The voltage source may be a 1.5 V battery, or two of them, or
the secondary of a mains transformer.

To provide 7-10 V supply from these, the 1C contains a series
regulator and a switching regulator. The series regulator is con-
nected actively like a transformer. A diode for half-wave rectifica-
tion is also on board. When the series regulator delivers voltage at
a satisfactory level, the switching regulator is inhibited. When,
however, the output from the series regulator falls below require-
ment, the switching regulator comes into action. It is capable of
generating a sufficiently high voltage level at the output from
only one or two 1.5 V batteries.

The diagram shows how the TL496 must be connected for
operation from one 1.5 V battery. For operation from two 1.5 V
batteries, some modifications are necessary: pins 1 and 3 of the ic
must be disconnected and left open, and the negative terminal of
Cl must be disconnected from earth and instead be connected to
pin 2 of the ic.

The circuit is eminently suitable for use with equipment that
can also be supplied from the mains: normally, such equipment
uses NiCd batteries. With the present circuit connected to the rele-
vant terminals of the equipment, these batteries are recharged via
Dl. This occurs during the half cycle that is not used by the

TL496. The charging current is determined by the internal resis-
tance of the transformer (about 11 Q, but note that this can not be
measured with an ohmmeter). If you are in any doubt, connect a
10 resistor in series with Dl.

Finally, do not use Dl with dry batteries, because that would
not do the batteries any good.

013

TEST & MEASUREMENT

ABS / RMS / LOG CONVERTER

The converter is specially intended for use in audio applica- larly suitable. The ic derives three voltages from the input signal:
tions for which the ic used, a Type SSM2210 from PMl, is particu- one corresponding to the absolute — abs — value; one correspon-

9.56 elektor indta September 1989

ing to the root-mean square — rms — value; and the third corres-
ponding to the logarithmic — log — value of the input current,
since the ic operates with currents. This makes possible a wide
dynamic range: something like 100 dB. Translated into terms of
input current, that gives 3 mA (p-p) to 30 nA (p-p). This makes it
vital that Cl is a type with a very small leakage current.

The input voltage is converted into current by Rl. With a value
as shown for this component, the nominal input level is 0 dBV
with a reserve up to +20 dBV.

Like the input signal, the output signals are currents that are
converted into voltages by opamps Al, A2 and A3, which are con-
nected as current/ voltage amplifiers.

To enable the decibel scale to be calibrated accurately, a preset
potentiometer, PI has been included in the A3 stage.

With component values as shown in the diagram, the output
voltage from A3 varies at about 33 mV/dB. It should be noted,
however, that the dynamic range of the LOG output at 80 dB is
rather narrower than the maximum possible 100 dB.

©

A1...A4 = IC2 = TL074CN

014

AUDIO & HI-FI

4-CHANNEL MIXER

The proposed mixer is designed around four current-driven
transconductance amplifiers contained in a Type SSM2024 from
Precision Monolithics Inc — rMi. To obtain a low off-set and high
control rejection, the four inputs should have an impedance to
earth of about 200 12. These impedances are obtained from resist-
ors R5-R8 that also form part of a potential divider at each input.

With values as shown in the diagram, the nominal input signal
is 1 V (0 dBV). Distortion at that level is about 1% and at lower
levels not more than 0.3%.

The amplification of the current-driven amplifiers — ccas — is
determined by the current fed into the control inputs. These
inputs form a virtual earth so that calculating the values of the
bias resistors (to transform the inputs into voltage-driven inputs)
is fairly simple.

With a value for R1-R4 of 10 k!2, the ccas are switched off if
the potential at the control inputs is lower than 200 mV. Maximum
amplification is obtained at a drive current of 500 pA. The voltage
at the control inputs is then slightly higher at 0.5 V, so that a maxi-
mum control voltage of 5.5 V is needed.

The output currents of the amplifiers are summed by simply
linking the output pins (it is that simple with current outputs and
completely in agreement with Kirchhoffs rules).

The current-to-voltage converter, IC2, translates the combined
output currents into an output voltage. The value of R13 ensurest
that the amplification of IC2 is unity.

elektor indie September 1989 9. 57

The current drawn by the mixer depends on the setting of the
four ccas and lies between 5 and 9 mA.

The signal-to-noise ratio of the mixer is about 90 dB, while the
bandwidth is of the order of 130 kHz. The bandwidth is limited
mainly by Cl, which is essential to ensure good stability of the

current-to- voltage converter.

The SSM2024 operates satisfactorily with a supply voltage
between +9 V and ±18 V, but best results are obtained when it is
±15 V.

MSX EPROM

The "64 Kbyte ram extension for msx computers" (Ref. 1) can
house eproms, but there is a small problem. To keep the board lay-
out as simple as possible, the data and address lines on the exten-
sion board were not connected sequentially. With rams that does
not matter too much and, in principle, not with eproms either, but
their programming becomes a bit of a tangle. To prevent that, it is
possible to use the auxiliary board shown here, which ensures
that the address and data lines are connected to the correct pins of
the ic. The board may be adapted to two types of eprom by means
of a wire link. With the link between A and B, a Type 27128 eprom
(16 K) may be used; if a 32 K eprom is to be used, the link should
be laid between A and C.

The two rams on the extension board each take two pages (a
total of 32 K) in the address memory (pages 0/1 and 2/3 respec-
tively). That means that when a Type 27128 eprom is used, the

data occurs twice on both pages.
Ref. 1 . “Elektor India October 1988

UNIVERSAL SQUELCH

This squelch circuit is simple, universally usable and provides
a large enough gain to enable it being incorporated in the auto-
matic-gain control (agc) circuit of a variety of receivers.

The input signal derived from the agc circuit in a receiver is
attenuated by network R1-R2-P1. The signal at the wiper of PI is
taken to the inverting input of opamp Al, which is connected as a
comparator. The non-inverting input is provided with a reference
voltage of 200 mV by potential divider R9-R10. The output signal
of Al is applied to a Schmitt trigger circuit, A2, via low-pass sec-
tion R2-C2. This filter ensures that small noise and other interfer-
ence signals do not affect the correct fucntioning of the squelch.

Capacitor C3 removes the steep skirt of the output signal of
A2, which renders the operation of the agc rather more pleasant
to the ear. The output of A2 is then taken to the base of output
transistor Tl via potential divider R7-R8.

The open-collector output of the squelch may be used to sup-
press the audio frequency output of the receiver.

Since the squeich draws only a small current, less than 10 mA,

its incorporation into an existing receiver should not present any
problems as far as the power supply is concerned.

(R. Lalic)

9.58 elektor indie September 1989

017

TEST & MEASUREMENT

48 MHZ CMOS OSCILLATOR

Crystal oscillators using digital gates generally do not generate
frequencies above 30 MHz, because the necessary crystals are not
allowed to oscillate on their fundamental frequency.

In the oscillator shown here, the crystal is forced to oscillate on
the third harmonic since it is connected in series with a parallel
tuned circuit that resonates at the fundamental of 16 MHz. For
digital applications, the output of N1 may be enhanced by the use
of a second inverter.

The circuit will operate satisfactorily only if non-buffered CMOS
devices are used. Gates from the hcl) family will enable operation
up to 60 MHz.

N1,N2=1/3 74HCU04

018

GENERAL INTEREST

SIMPLE TEMPERATURE INDICATOR

For the absolute measurement of tempera-
tures a thermometer is indispensable. There
are, however, many situations where an abso- ,
lute value is not needed and a relative indica-
tion suffices.The getting hot of an electric drill
or vacuum cleaner may be indicated to the
user by the lighting, or changing of colour, of
a simple indicator light. It would be a further
advantage if on such equipment a green light
would indicate that all is well as far as tem-
perature is concerned. As the temperature
rises, the light should change colour slowly to
indicate that the equipment is getting too hot.

The proposed circuit does this and has the
additional advantage that it does not need a
separate low-voltage supply: it works direct
from the mains. The indicator proper is a
two-colour LED, Dl, while the sensor is a com-
bination of a negative temperature coefficient
(ntc) and a positive temperature coefficient (PTC) resistor, R4 and
R3 respectively.

At a relatively low temperature, the value of R3 is low and that
of R4 is high. During the positive half cycle of the mains voltage, a

voltage will exist across R3-D3 that is suffi-
ciently high to cause the green section of Dl to
light. The value of R3 has been chosen to
ensure that during the negative half cycle of
the mains voltage the potential across it is too
low to cause the red section of Dl to ligh.

If he temperature rises, the value of R4
diminishes and that of R3 rises. Slowly but
surely the green section will light with lesser
and lesser brightness, while at the same time
the red section lights with greater and greater
brightness until ultimately only the red section
will light.

Resistor R2 and capacitor C3 ensure that the
current drawn by the LEDS does not become
too large. This arrangement keeps the dissipa-
tion relatively low.

Both R3 and R4 should be of reasonable
dimensions, something like 6 mm in diameter
- not less. At a temperature of 25 °C, the ntc must have a value of
22-25 kfi and the PTC one of 25-33 Si.

The circuit should be treated with great care since it carries the
full mains voltage.

(

019

AUDIO & HI-FI

BUCKET BRIGADE DELAY LINE

Although bucket-brigade devices (bbds) are not in the same
limelight as many other components, they do exist and are used.
One of them, Type MN3004, consists of 512 capacitors and switch-
ing transistors. The ic is perhaps best described as an analogue

shift register. The capacitors are provided by the drain-gate capac-
itance of the transistors.

Samples of an analogue signal taken at the input appear 256
clock pulses later at the output. The clock pulses are obtained

elektor India September 1989 9.59

MN3004

’CPI VpDCP2

894055- 11

olfoo Ot-

013 oO

ojjoo ©►

91 O0

S* OK

K3; R4; R12 = TOO k
R5;R6-5k6
R7;R8 = t50k
R9» 100 R
R10; R» = 68 k
PI =50 k preset
PCS; PC6 = PC2

• :t.

m

CapadWW:
Cl; C2: CIS
C3 = 56p

from an associated ic, a type MN3101. This ic also provides a sup-
ply voltage, Ugg, for the source followers in IC1. Note that the
connections of the supply voltages to IC1 and IC2 have not been
drawn incorrectly: Vdd must be negative with respect to earth
(GND pin).

The MN3101 also makes it possible to construct an rc oscilla-
tor. Trimmer C9 enables varying the clock frequency and thus the
delay time. It is also possible to connect an external oscillator to
pin 7 (but note that pins 5 and 6 should be left open).

The clock frequency of the delay line may lie between 10 kHz
and 100 kHz. With component values as shown in the diagram, it
is about 60 kHz.

Maximum dissipation of the clock generator is about 200 mW,
depending on the capacitive load of the memory. If more memo-
ries are used, it is possible to reduce the dissipation by connecting
resistors in series with pins CPI and CP2. Lowering the clock fre-
quency will also reduce the dissipation.

The delay time of the circuit is equal to half the number of
capacitors divided by the clock frequency: with values as shown,
it amounts to 2.56-25.6 ms. The bandwidth of the delay line is
roughly 0.33 times the clock frequency. Thus, with a clock fre-
quency of 60 kHz, the bandwidth is 20 kHz and the delay more
than 4 ms. In the prototype, these values yielded a signal-to-noise
ratio of more than 70 dB and a distortion not exceeding 0.3% (at 1
kHz; 0 dBV). The current drawn was just over 6 mA, but this may
increase to 14 mA with increased clock frequencies.

Apart from the delayed input signal, the output signal contains
mixing products of the input and the clock. In the prototype (cir-
cuit as shown), with a clock of 60 kHz, these products were sup-
pressed in the audio range by 60 dB (R8-C3 and R9-C5). It is,
however, advisable when a lower clock is used to filter both the
input and the output with a filter of at least the fourth order. Try
to minimize the distortion by adjusting PI.

The delay line should find application in units for echo, tremo-
lo, vibrator, chorus, reverb, and so on.

Another possibility is its use in a compressor to suppress, or at
least attenuate, the short periods of overdrive. To that end, delay
the input signal prior to the voltage-controlled amplifier (vca) and

drive the circuit that provides the control voltage for the vca
direct. In that application, the delay should be at least equal to the
attack time of the compressor.

Resistors:

Rl; R2; Rl3 10 k

7mA mv

A 1 , A2 = IC3 = TL072

RADIO & TV

FAST ENVELOPE SAMPLER

When a signal is being amplitude-modulated, constant moni- quency does not exceed half the carrier frequency. The fast enve-
toring is necessary to ensure that the maximum modulating fre- lope sampler presented here is basically an advanced AM

9.60 elektor India September 1989

The output of the sampler is in accordance with the potential
across C6. Basically, the circuit is frequency-independent since
the clock signal is derived from the carrier. This is the reason
that the sampler may be used in such diverse applications as
satellite facsimile, radio receivers and speech processors.

The circuit may also be used to good effect in an agc loop,
because the classical problems regarding attack and delay asso-
ciated with diode detection and LF filter systems do not occur in
this type of demodulator.

Since in some applications a direct voltage is needed, for
instance, in an agc loop, the proposed circuit has a DC as well
as an ac terminal.

The sampler is intended for operation from a ±5 V supply
and draws a current of about 20 mA.

TEST & MEASUREMENT

demodulator that can be used with modulations where the well-
known diode detector with lf filter can not. Phase errors that
occur with diode detection are absent in this circuit.

Amplifier IC1 is a buffer with variable (1-11) ac amplification
Opamps A1 and A3 form an active half-wave rectifier that
charges capacitor C5 up to the maximum signal level in a half
period.

During the zero crossing, detected by A2, the network
around N2 generates a short pulse. This pulse ensures that the
potential across C5 is applied to C6 via electronic switch ES4.
After ES4 has been opened, so that the charge is stored safely in
C6, capacitor C5 discharges via parallel-connected switches
ES1-ES3 (which are actuated via the network around N3 and
N4).

NOISE GENERATOR

The noise generator presented here provides constant
noise energy over its bandwidth, resulting from the non-lin-
ear behaviour of its switching components, more particu-
larly T4. It is very useful for measurements where limited
noise bands are required. Varying the ratio R6:R7 and the
clock frequency enables the generated noise to be adapt-
ed to specific requirements.

Transistors T2 and T3 are current sources. The current
through T2 is about ten times the level of that through T3.

Assuming that T4 is on and that the clock input is low,

T1 is off and C2 discharges. The capacitor is pulled to
about half the supply voltage by the two current sources.

When that state is reached, stability ensues because the
potential then present at the gate of T4 keeps the fet
switched on.

When the clock goes high, T1 is switched on so that C2 is
connected between the gate of T4 and earth. Since C2 is only
partly charged, the fet is switched off. Transistor T1 is kept
switched on by or gate D1-D2 so that the clock pulses are
blocked. Capacitor C2 then charges via T3 until the potential
across it becomes high enough to switch on T4. Transistor T1 is
then switched off and the circuit is ready to receive another
clock pulse (or rather a leading edge of one).

Since it is not known when the clock pulse arrives, it is not

known to what potential C2 will be discharged by T2 (and coun-
tered by T3). It is therefore also not known how long it takes T3
to recharge C2. It follows that it is then not known when the next
clock pulse arrives. In other words, the pulse width of the out-
put signal is varying constantly, which is characteristic of a noise
signal.

The frequencies contained in the noise signal are limited by
the clock signal (higher frequencies than the clock can not occur,
although there are harmonics) and the maximum charge and

elektor india September 1989 9.61

AUDIO & HI-FI

CAR ELECTRONICS

TUNEABLE BAND-PASS FILTER

One of the difficulties in the design of high-
er-order tuneable band-pass filters is achieving
correct tracking of the variable resistors in the —

rc networks. The use of switched capacitor net-
works can obviate that difficulty as is shown in ml

this filter. [gl

The filter may be divided roughly into two M

stages: an oscillator that controls the electronic
switches and the four phase-shift networks that
provide the filtering proper. | M

The oscillator, based on a 555, generates a
pulsating signal whose frequency is adjustable C1

over a wide range: the duty factor varies from [S g ) || |

1:10 to 100:1. V J- i»

Electronic switches ES1-ES4 form the vari-
able resistors whose value is dependent on the
frequency of the digital signal. The operation of
these switches is fairly simple. When they are
closed, their resistance is about 60 Q.; when they

are open, it is virtually infinitely high. If a

switch is closed for, say, a quarter of the time,
its average resistance is therefore 240 Q. 41-f™

Varying the open:closed ratio of each switch
varies the equivalent average resistance. The
switching rate of the switches must be much
greater than the highest audio frequency to pre-
vent audible interference between the audio
and clock signals.

The input signal causes a given direct volt-
age across Cl, so that the opamps may be oper-
ated in a quasi-symmetric manner in spite of
the single supply voltage.

The direct voltage is removed from the output signal by capac-
itor CIO.

The fourth-order filter in the diagram may be used over the

entire audio range and has an amplification of about 40, although
this depends to some extent on the clock frequency. The band-
width depends mainly on the set frequency.

The circuit draws a current of not more than 15 mA.

CAR HEADLIGHT CONTROL

It is an annoying fact that you normally only realize that you
have left your car headlights on when you want to restart the car
only to find that the battery is flat. One of the possible ways of
preventing this happening is offered by the present control.

The circuit does not provide a warning but an action: when
you switch off the ignition, relay Rel is de-energized and the
headlights are switched off, unless you deliberately decide other-
wise. That decision is made possible by switch SI, which, when
operated, triggers silicon-controlled rectifier Thl so that Rel is
energized. Note that this is possible only when the ignition
switch, S2, is off, otherwise the voltage across Thl is so low, owing
to shunt diode Dl, that it can not be triggered. Since, however, the
headlights should not normally be switched on when the ignition
is off, in most cases SI will be used only rarely and the switch
may then well be omitted altogether.

Relay Rel should be a standard 12 V car type with contacts
that can switch up to 25 A.

(H. Huynen)

9.62 elektor india September 1989

1N4001

TIC106

\ h

;

r

1N4001

R2p

1

It

_ Rel

RESET PROTECTION

Most advanced computer programs
include preventative measures against
(possibly) hasty instructions. Responses
such as "are you sure?" and "do you real-
ly want to quit?" are familiar to most of
us. However, even the cleverest program
can not prevent the inadvertent operation
of the reset switch and the consequent
result of lost data and improperly closed
files that cause waisted clusters on the
hard disk.

The location of the reset switch on the
front panel of many computers asks, of
course, for inadvertent operation. Clearly,
some means of reset protection is no lux-
ury.

Normally, the reset switch is connect-
ed to the mother board of the computer
via two wires. One of these is at earth
potential and the other is linked to the
reset circuit. The protection, whose circuit
is shown in the diagram, is inserted
between the reset switch and the mother
board. The earth connection of the com-
puter must be linked to terminal M of the
protection circuit. The protection circuit may draw its power from
the computer supply.

When the circuit has been fitted, operation of the reset switch
will not result in an immediate restart of the computer. Instead, a
buzzer will sound to alert you to the reset operation. The buzzer
is actuated for four seconds by monostable ICla, which is trig-
gered by the reset switch. During those four seconds, the output,
pin 5, of ICla ensures that the reset function, pin 10, of IClb is dis-

abled. When then the reset switch is operated again, monostable
IClb will be triggered and this will start the reset procedure.
Transistor T2 is then switched on for half a second and the
buzzer is deactuated via Rll and D4.

The circuit around T1 and N4 ensures that ICla can accept
trigger pulses again ten seconds after the mono time of IClb has
lapsed. This arrangement prevents, say, children operating the
reset switch.

BICMOS INTEGRATED CIRCUITS

BiCMOS devices combine bipolar and CMOS technologies, pro-
viding the best features of both: the fast performance and the
48/64 mA output drive of bipolar devices and the low power con-
sumption of CMOS devices.

In active mode, BiCMOS devices operate at about half the supply
current of their pure bipolar equivalents. When disabled, they
may reduce power consumption by up to 90%. Since at present
most BiCMOS ics function as bus interfaces (that are normally dis-
abled), the result may be a system ic'-power saving of up to 25%.

Moreover, BiCMOS devices use the typical 0.3-3.5 V ttl voltage
swings at their output rather than the larger GND-to-vcc swings of
cmos devices. This smaller voltage swing reduces the overall
effects of transient voltage noise produced during the simultane-
ous switching of multiple outputs.

A number of integrated circuits are available in BiCMOS technol-
ogy, including transceivers with registers, pipeline registers,
8/9/10-bit registers, latches and parity bus transceivers. These
give the designer additional means of reducing power consump-
tion without compromising advanced performance.

(Source: Texas Instruments)

elector india September 1989 9.63

PROGRAMMABLE SWITCH

The programmable switch
may be used, for instance, to
simulate a data stream or, as
shown, to control an analogue
multiplexer, IC5. The multi-
plexer may be used as the
basis of a programmable oscil-
lator.

The circuit is based on Na-
tional Semiconductor key-
board decoder Type
MM74C922 (IC1). This device
is intended to read a 4x4
matrix keyboard in a simple
and fast manner. Apart from a
4-bit output, the ic also has a
DATA AVAILABLE Output, DA,
which is high as long as one of
the keys is pressed. Tine data
associated with the last
pressed key will remain on the
output, even after the key has
been released. The speed with
which the keyboard is scanned
is determined by Cl. When
this capacitor has a value of
100 nF, as shown in the dia-
gram, the scanning rate is 600
Hz. The anti-bounce period of
the keyboard is determined by
C2: with a value as shown, the
period is about 10 ms (rule of
thumb: C2=10C1).

The programming of the
RAM, 1C2, is fairly simple with
the aid of IC1. First, the da
output of the decoder is inverted by N1 to enable it being used
as the write pulse. After the key has been released, the ram is
disabled (da=1, we=0), and the address counter, IC3, is clocked
on by one address. Since the counter is a Type 4040, it reacts to
trailing edges and this means that WE has to be inverted again
(by N4). It is not possible to use the original DA signal, since this
could jeopardize the timing and cause non-defined states. The
delay times of the gates ensure that all processes take place in
correct sequence. Further delay is provided by the combination
of R4 w'ith the capacitance of the clock input.

The programmed data is read with the aid of a separate
clock signal generated by an oscillator based on N3. The speed
at which the data is read from the RAM is set by PI. Gate N3 is
actuated by setting SI 8 in position A. Contact bounce here is
prevented by the combination of R2 and C3, and the hysteresis
of N3 and N2.

When S18 is in position A, gates N1 and N2 ensure that the
ram is in the read mode (we=1). At the same time, N2 arranges
that the data lines of the ram are connected as outputs. Gate N4
ensures that either da or the output of N3 is used as the clock
for counter 1C3. Furthermore, S18 also arranges the disabling of
the data outputs of IC1 to prevent a bus conflict (the ram now
provides the data).

Pin 1 (Qll) of counter IC3 is connected to the clock input, pin
10, via or network R4-D1 to ensure that the counter stops after
one cycle. The stopping is indicated by the lighting of D1 . Switch
SI 7 enables the counter to be reset; it may also be used to set
the counter to zero during programming.

When, during programming, the highest counter position is

reached, the clock for the counter is disabled. Continued press-
ing of the keys will cause overwriting on to the last ram address.
To obtain a defined counter state on switch-on, network R5-C5
provides a power-on reset of the counter.

Only one half of the ram is used, since four bits suffice for the
proposed circuit. It is, therefore, possible to use a RAM with nib-
ble configuration or use two keyboard encoders.

Finally, the use of 16 separate switches may make the circuit
rather expensive: it is far cheaper to use a second-hand 4x4

9.64 elektor india September 1989

027

POWER SUPPLIES

SWITCH-MODE VOLTAGE REGULATOR

Switch-mode power supplies offer the user the benefit of a
much greater efficiency than obtainable with a traditional power
supply. The switch-mode regulator presented here has an efficien-

cy of around 85%.

An input voltage of 12-16 V DC is converted into a direct volt-
age of exactly 5 V. The use of a Type MAX638CPA enables the
design and construction of the regulator to be kept fairly simple:
only nine additional components are needed to complete the cir-
cuit.

Resistors R1 and R2 are used to indicate when the battery volt-
age becomes low: as soon as the voltage on pin 3 becomes lower
than 1.3 V, D1 lights. With values as shown for the potential
divider, this corresponds to the supply voltage getting lower than
about 6.5 V.

The output of the ic is shunted by a simple lc filter formed by
LI, C3 and D2.

The oscillator on board the tc generates a clock frequency of
around 65 kHz and drives the output transistor via two nor gates.
The built-in error detector, the 'battery low' indicator or the volt-
age comparator can block the clock frequency, which causes the
transistor to switch off.

The ic compares the output voltage of 5 V with a built-in refer-
ence (fet). Depending on the load, the fet will be switched on for
longer or shorter periods. The maximum current through the fet
is 375 mA, corresponding to a maximum output current of 80 mA

2-METRE TRANSMITTER

The transmitter was designed primarily for use by radio ama-
teurs as a radio beacon and as such it provides a good quality sig-
nal free of unwanted harmonics.

Transistor Tl, in association with crystal XI, operates as a 36

0,1. ..0,5 Hi 0.1...1 Hi 1 kHl

© J I I © _nn_n I <D ULJ _J

0.1...0.SHI 0.1..1HI IkHl 834019 • 11

MHz oscillator. Filter L1-C3 obviates any tendency of the circuit to
oscillate at 12 MHz (the fundamental frequency of the xrystal).

Circuit L2-C4 is tuned to the fourth harmonic of the oscillator
signal (=144 MHz). This signal is fed to the aerial via a buffer
stage consisting of T2, a double-gated FET. The (amplitude) modu-
lating signal is applied to the second gate of the buffer. The output
power of the transmitter has been kept low, about 10-40 mW.

The modulating signal is generated by Nl, an oscillator that
switches the transmitter on and off via transistor T3. The switch-
ing rate lies between 0.1 Hz and 0.5 Hz.

When the output of Nl is low, T3 is switched off, and the
transmitter is inoperative because the supply is disabled. When
the output of Nl is high, T3 is on and the transmitter operates
normally.

The digital pattern at the gate of T2 shapes the modulating sig-
nal. Gate N2 generates a square wave at a frequency of 0.1-1 Hz.
As long as the output of T3 is high, N4 oscillates at a frequency of
about 1 kHz. At the relevant gate of N2 there is, therefore, a peri-
odic burst-signal at a frequency of 1 kHz, and this signal is used to
modulate the transmitter.

The digital pattern at the relevant gate of T2 may be varied to
individual requirements by altering the values of the feedback
resistors in the digital chain.

The transmitter is calibrated by setting trimmers C4, C7 and C8
for maximum output power.

Inductors L2 and L3 are wound from 0.8 mm dia enamelled
copper wire: L2 = 5 turns with a tap at 1 turn from ground; L3a =
3 turns and L3b = 2 turns. The coupling between L3a and L3b
should be arranged for maximum output power.

The circuit draws a current of only 20 mA, enabling the trans-
mitter to be operated from a 9-V PP3 battery for several hours.

olektor india September 1989 9.65

METER-SCALE MAGNIFIER

The resolution of moving-coil meters is general-
ly no better than 1%, because the scale normally is
given no more than 100 marker lines for the given
dimensions: more lines might detract from the legi-
bility. Most digital meters have a resolution of
0.05% or better. The resolution of moving-coil
instruments may be improved in two ways: physi-
cally enlarging the scale, which is possible only in
the factory, or electronically enlarging the scale,
which is what this article is all about.

The circuit divides the scale into five sections,
each of which is then extended over the full scale.
This therefore gives a five-fold improvement in
resolution.

The input signal (200 mV = full-scale deflection
- f.s.d.) is amplified by 1C1 to a value of 2.5 V. The
amplified signal is fed to four comparators that
divide the input signal into segments of 40 mV.
Which segment is indicated on the meter-scale is
shown by the LEDs.

The LEDS are shunted by a 10 kfl resistor, so that
the open-collector outputs of the comparators have
a well-defined pull-up resistance.

The outputs of the comparators drive two mul-
tipliers that, depending on the magnitude of the
input signal, provide a direct voltage to buffer 1C2.
This direct voltage is always a multiple of 0.5 V
(since the output of IC1 is 5 x 0.5 = 2.5 V).

There then exists a potential between A and B
that is equal to the difference between the input
signal and a multiple of 0.5 V. This difference can
never exceed 0.5 V over the f.s.d. of the meter.
Resistor R4 must therefore have a value that causes
a f.s.d. at 0.5 V. The measured value is determined
by adding the meter reading to the multiple of
0.5 V indicated by the LEDs.

The circuit is calibrated by first setting the
meter to zero reading by P2. Next, apply a voltage
equal to one fifth of the f.s.d. (here, 40 mV) to the
input. Then, set P3 for minimum resistance, when
none of the LEDs should light. Next, set PI for f.s.d.
Finally, adjust P3 until D7 begins to light and the
meter reading falls to zero.

(R. Shankar)

SOUND LEVEL ATTENUATOR

When the radio or record player is on at a fairly high volume,
it is often impossible to hear the telephone or doorbell. A solution
to this frequent difficulty is offered by this automatic attenuator.
As soon as the doorbell or the telephone rings, it turns down the
volume of the audio equipment.

The circuit consists of an optically controlled attenuator and
the requisite electronics to connect it to, say, the telephone.

The attenuator is of fairly simple design and is based on a
TL074. Its control part, consisting of a current-driven attenuator
based on an LT2001 (a combination of an led and two light-

dependent resistors - LDRs - in a common enclosure), is incorpo-
rated in the audio equipment.

After the mains has been switched on, C9 causes the resetting
of bistable IC1. The high voltage level at pin 6 of the 741 causes T1
and T2 to switch off: this, in turn, results in D2 not conducting and
the voltage-controlled current source, T3, delivering maximum
current (30 mA) to the led incorporated in the LT2001 . The illumi-
nated ldrs will then have a value of about 1.5 kO. The voltage
transfer of the attenuators can be preset (once and for all) to exact-
ly 0 dB (at 1 kHz) by PI and P2.

9.66 elektor indie September 1989

Terminals A and B of the circuit are connected direct to the cor-
responding terminals of the telephone (this may not be allowed in
some countries - seek the advice of your local telephone manag-
er), while terminals X and Y are connected across the doorbell ter-
minals. Note that the doorbell must be fed from a 3-24 V trans-
former. If the telephone or the doorbell rings, the bistable will be
set via the relevant opto-isolator (1C4 or 1C5).

The low voltage level at the output of 1C1 will cause T1 and T2
to switch on. This in turn causes D2 to light and Cl 2 to charge via
R23. Owing to the rising potential across the capacitor, tjie output
of the current source will slowly diminish until the minimum
value, set by P3, is reached. This has the effect that the TL074
turns down the volume click-free until a reasonable sound level.

determined by the setting of P3, is reached.

Pressing switch SI resets the bistable. This will cause D2 to go
out, while the attenuation slowly drops to 0 dB. The attenuator is
connected to the control electronics by two wires, P and Q.
Thanks to the current drive, this (non-shielded) link may be up to
23 metres (75 ft) long.

The attenuator draws a current of only 10 mA and must be fed
from a symmetric ±12 V supply, which may be taken from the
audio amplifier. The control circuit needs an asymmetric +12 V
supply and draws a current of about 35 mA.

If the LT2001 is difficult to obtain, discrete components may be
used: these should, of course, be fitted in a light-tight enclosure.

031

COMPONENTS

FAST UNITY GAIN OPAMP

A number of operational amplifiers can be
used only in circuits that have a certain mini-
mum amplification, because they have been
designed with small internal compensation.

The advantage of that is that the amplifier is
faster.

If we look at two popular opamps, the
LF356 and LF357, these characteristics are well
illustrated. The LF356 may be used as a unity-
gain amplifier with a gain-bandwidth product
of 5 MHz and a slew rate of 12 V/jis. The LF357
needs an amplification of not less than 5 and
has a gain-bandwidth product of 20 MHz and a
slew rate of 50 V/gs.

It is possible to use the LF357 (and similar
opamps) with smaller amplifications by using external compensa-
tion, vet retaining most of the bandwidth. Normally, this is
achieved with a capacitor, but that is not the only, and certainly

not the best, method.

An alternative method is illustrated in the dia-
gram. Consider the LI 65 as a summing ampli-
fier of which one input is connected to earth. It
is clear that the second input then forms the
input of a unity-gain amplifier whose amplifica-
tion is determined by R1 and R3 (R3/R1 = 1).
The unused input would have provided an
amplification of 22 (R3/R2 = 22), which is rather
more than the permissible minimum amplifica-
tion of the L165. The opamp 'believes' that the
amplification is higher than required. This has
the benefit that the circuit has no tendency
whatsoever to oscillate.

The ratio R3:R2 is only of value if RI is very
much larger than R2. Otherwise the amplification is R3/(R1//R2).

Note that the L165 must be fitted on a good heat sink, in spite
of its internal thermal overload protection (lout max. = 3 A).

elektor India September 1989 9.67

COMPUTERS

ELECTROPHONICS

PRINTER RESET

When during a computer print-out something goes wrong
with the printer, such as the paper getting snarled up, the only
way to stop the operation is normally to switch the machine off.
That may be a useful, but certainly not an elegant, method: a reset
knob, on the other hand, is.

Nearly all printers with a Centronics interface have a reset
input at pin 31 of the Centronics connector (consult the hand-
book). That input is used in many ms-dos systems to set the print-
er to a defined starting state and at the same time to empty the
buffer.

The input may, of course, also be used to connect a reset switch
to. The diagram in Fig. la shows how’ such a switch may be made
quite easily. The 1 kQ resistor prevents the computer output being
short-circuited when the printer is being reset.

Users of the recently published printer buffer (Ref. 1) can fit
the switch in the buffer or expand the existing switch so that the
printer is reset at the same time as the printer buffer. The circuit is
connected to the unused contact of SI in the printer buffer. The
existing reset switch may be expanded as shown in Fig. lb.

[BC547B

Ref. 1. ‘Centromics-compatible printer buffer
Elektor India April 1989 p. 4-33

VARIABLE LOW-PASS FILTER

The Type SSM2045 ic from PM! is an active low-
pass filter whose order, Q-factor, cut-off frequency and
amplification are set with the aid of control signals.

A possible design for use in electronic music sys-
tems is shown in the diagram. To prevent distortion,
the level of the input signal is reduced by R1 and a
resistor on board the chip to one not exceeding 150 V
peak to peak.

Outputs 2 pole and 4 POLE are each connected to an
internal voltage-controlled amplifier (vca), Mixl and
mix2 respectively. For optimum off-set and control
rejection, these connections are made via resistors R4
and R5. The gain of the vcas is set by P2, which con-
trols the current that flows through the amplifiers via
pins 15 and 16. The maximum current at these control
inputs is 250 gA. The balance between the vcas, and
with it the order of the filter, is set by P4 via pin 14.
The voltage at this control input can be varied
between -250 mV and +250 mV. The input must be
driven from an impedance not exceeding 200 H. At a
drive voltage of 0 mV, the vcas attenuate the signal by
about 6 dB.

The Q-factor depends on the current flowing into
pin 17, which is controlled by PI. The input is protect-
ed by an internal 18 kfl resistor. The Q-factor may be
set so high as to cause the circuit to start oscillating.
This happens when the current is between 120 and
185 gA.

The cut-off frequency can be shifted between 20 Hz
and 20 kHz by varying the control voltage at pin 5
between +90 mV and -90 mV by P3. This voltage also
determines the frequency of oscillation, enabling a
variable oscillator to be created. The resulting sine
wave has a distortion of about 1%.

The values of C1-C4 have been chosen to give the

UVcc

5SM2045

TL071

9.68 elektor India September 1989

filter a Butterworth characteristic if the current into pin 17 (Q) is
zero.

The supply to the ic may be -5 V connected direct to pin 10, or
up to -15 V via series resistor R13, through which a current of
about 7.1 mA flows. The supply, -Vee, is limited internally by a
6.8 V zener diode.

The output current of the chip is converted into a voltage by
IC2. The output of this stage has a small off-set voltage. If this can

not be tolerated by the following equipment, the output must be
taken via a coupling capacitor.

In the proposed circuit, the values of resistors R2, R6, R8, R9
and R12 have been chosen to allow control of the ic from voltages
of 0-5 V or ±5 V.

At an input signal of 0 dBm, the distortion is about 1%, which
drops to 0.3% at -6 dBm and 0.03% at -20 dBm. The signal-to-
noise ratio is of the order of 80 dB.

034

GENERAL INTEREST

AUTOMATIC SWITCH

Many battery-operated instruments, such as digital voltmeters,
have a change-over on-off switch. The automatic switch presented
here makes full use of that. When the instrument is off. Cl charges
fairly quickly via R2 and Dl. As soon as the instrument is
switched on. Cl discharges slowly via Rl. As long as the dis-
charge current exceeds a certain level, T1 is switched on by the
voltage across Rl and the supply to the instrument is on. When,
after a few minutes. Cl is almost completely discharged, T1 tog-
gles and the supply to the instrument is switched off. The period
between switch-on and switch-off may, of course, be varied by
different values of Cl.

(Ph. Bosma)

035

POWER SUPPLIES

LOW DISSIPATION REGULATOR

With the advent of the now well-known three-pin voltage reg-
ulators, power supplies are taken for granted. Yet, there are cases
where such a regulator is not wholly satisfactory. This type of reg-
ulator needs a fairly large potential drop across it (typically not
less than 3 V) and draws a relatively high quiescent current (typi-
cally 6 mA for a 78xx). The regulator presented here is particularly
attractive for battery-operated equipments and offers:

• variable, very stable output voltage;

• low potential drop (some tenths of a volt);

• very small quiescent current (20-30 pA).

In principle, the regulator is a normal series type. The voltage
reference is obtained from a common or garden red led that must
draw not less, nor much more, than 5 pA. Even at that low cur-
rent, an LED has a fairly stable voltage drop. To improve that sta-
bility, the current is drawn from the regulated output via Rl.

Regulation is provided by CMOS opamp Type TLC271. This
amplifier operates in the low bias mode, which ensures very low
current consumption, by connecting pin 8 to the positive output
terminal. The output of the opamp is used as base drive for series
regulator T2 via current source Tl. This configuration enables
good control for only a small voltage swing at the output of the
opamp. This is necesary since the slew rate of the opamp in the
low-bias mode is pretty poor. The supply for the opamp is also
taken from the regulator output. Capacitor Cl therefore serves as
decoupling element for the opamp.

To obtain reliable control, a kind of bootstrap resistor, R5, was
found necessary.

The values of Rl and R4 as shown in the diagram provide a
variable output voltage of 3-8 V. Higher output voltages, up to a
maximum of 16 V, are obtained by increasing R4 by 200 k£2/V.
Resistor Rl should also be increased in value, as long as the cur-
rent through Dl does not drop below 5 pA.

In this type of circuit, great care should be taken to avoid para-
sitic capacitances resulting from long connections. These would
cause a deterioration of the regulation.

The maximum output current depends mainly on the permis-
sible dissipation in T2 and, therefore, to some extent on the differ-
ence between the input and output voltage.

elektor India September 1989 9.69

036

TELECOMMUNICATIONS

DUPLEX AUDIO LINK

Duplex communication is, of course, not a new technique: it
has been used, for instance, in telephone systems for many years.
Those systems, however, make use of transformers to achieve
duplex - the circuit presented here does it with the aid of
electronics. The principle is fairly simple. Two senders impose
signals U1 and U2 respectively on to the audio cable. The voltage
across the cable is then (Ul+U2)/2. The receivers at both sides of
the cable deduct their side's sender signal from the cable signal:
the result is the signal sent from the other end of the cable. This
principle is the basis of the circuit shown. Note that a circuit like
that is required at either end of the link.

Opamp A1 is connected as a buffer amplifier and serves as
sender. The send signal is imposed on to the cable via R4.
Terminating the cable by R4 results in the voltage across the cable
being only half the voltage output of Al. This does not detract
from the operation of the circuit, however. At the same time, R4
ensures that signals emanating from the other end of the link can
not get to the output of Al; if they could, they would be short-cir-
cuited by the output.

The receiver is a differential amplifier consisting of opamps
A2-A4. The quality of the differencial amplifier depends largely
on the resistors used in association with the opamps and 1 % types
are, therefore, essential.

The cable signal, (Ul+U2)/2, is applied to one input of the dif-
ferential amplifier and the (halved) output signal of Al to the
other. Since the differential amplifier has a gain of 6 dB, the
received signal applied to K2 has the same level as the original
signal.

In practice, the proposed duplex system is not perfect and it is
for that reason that, for instance, remnants of the sent signal are
detectable in the receiver. Fortunately, these can be removed with
the aid of PI . Furthermore, the cable used will load the output
slightly capacitively, which causes the compensation voltage at
the wiper of PI to be not wholly in phase with the sent signal.

This effect may be virtually removed with the aid of Cl.

The circuit is calibrated by connecting the cable to it and to its
twin circuit and injecting a sinusoidal signal at a frequency of
1 kHz and a level of 5 V rms to its input. The input bus of the
other circuit must be short-circuited during the calibration. Adjust
P2 for minimum signal at K2. Next, increase the frequency of the
input signal to 10 kHz and adjust C5 for minimum signal at K2.
Repeat the procedure with the other circuit.

The signal suppression at 1 kHz is of the order of 80 dB, while
at 20 kHz it is around 60 dB. These are pretty good values for this
kind of circuit.

HEAD / TAIL LIGHTS FOR MODEL RAILWAY

The price of a model railway locomotive is directly proportion-
al to its facilities and finish. There are quite a few inexpensive
ones on the market of which the finish leaves a lot to be desired.

As as rule, manufacturers start their economy with the lights. The
circuit presented here enables a DC locomotive to be provided
with direction-independent head and tail lights. Since it uses LEDs,
a very long life is guaranteed.

The circuit is based on a number of parallel-connected leds in
a bridge network. The fet at the centre of the bridge ensures a
constant current as long as the supply voltage exceeds 4.5 V. The
brightness of the lights will, therefore, be independent of the sup-
ply voltage.

The leds are connected in parallel to keep the minimum oper-
ating voltage as low as possible. To ensure good current distribu-
tion, the pairs of parallel-connected leds should be of the same
type and colour.

a pi ds os p; b 03 o? db 04

(•O o») (om mo)

9.70 elektor india September 1989

038

IrivOs-

COMPUTERS

l/O-FRIENDLY KEYBOARD

Not all computers have a keyboard, yet it is often essential to
have the use of one. Two circuits are presented here that enable a
keyboard facility to be produced with the aid of only six or seven
l/O lines.

Fig. 1 shows a circuit based on a 74HCT148 and a 74HCT138
that can serve 56 or 64 keys. The circuit in Fig. 2, based on a
74HCT147 and a 74HCT138, can address 72 keys via seven i/o
lines. The choice between the two circuits depends on the number
of available i/o lines and the wanted number of keys.

In either circuit, the key rows are selected by the bits on the A,
B or C input of the HCT1 38. The combination of these bits deter-

mines which of the outputs Y0-Y7 goes low. As long as no key is
depressed, the inputs of the HCT148 in Fig. 1 or the HCT147 in
Fig. 2 are high. When a key is pressed, the inverted binary infor-
mation at the output of the tcs show which key it is.

In Fig. 1, the 0 input of the HCT148 is not used, because the
code associated with that input is the same as that generated
when no key is pressed. The output, pin 14, of this ic is used to
detect whether a key has been pressed. It goes low when a key
has been pressed.

In Fig. 2, four Is at the output indicate that no key has been
pressed.

Fig- 1- Fig. 2.

ENERGY CONTROL FOR BATTERY CHARGERS

In most automatic battery chargers, the power transformer
remains connected to the mains even after the the battery (or bat-
teries) has been charged. In many cases, considerable energy sav-
ings can be achieved by disconnecting the transformer from the
mains when the battery is fully charged. This circuit performs this
function for 12 V car battery chargers.

The battery voltage is monitored by an adjustable window
comparator around opamps A1 and A2, which are powered by a
stabilized supply voltage of 8.2 V (R7-D1). The high and low
switching thresholds, Uh and Ul, are set by presets PI and P2 res-
pectively. The reference voltage for the opamp is obtained from
junction R1-R2 and is a function of the battery voltage. With the
given values of R1 and R2, a voltage divide factor, D, is obtained

D = R2 / [R1 + R2] = 0.43.

Taking into account the series resistors connected to the pre-
sets and the use of an 8.2 V supply voltage, the span of PI is

7.2 / D = 16.7 V (max) to 3.8 / D = 8.9 V (min)

elektor indla September 1989 9.71

and that of P2 is

6.3 / D = 14.5 V (max) to 3.3 / D = 7.7 V (min)

In practice, it will be desirable to switch the charger off at a
battery voltage of 14.0 V and on again when the voltage drops
below 12.5 V, corresponding to a window of 1.5 V.

When the battery voltage is lower than Ul., it is, of course, also
lower than Uh. This means that both T1 and T2 conduct, so that
Rel is energized. Contact K2 switches on the mains to the battery
charger, and contact K1 keeps the relay energized even when T2 is
switched off and the battery voltage rises to a value between Ul
and Uh. When the battery voltage reaches Uh, both T1 and T2 are
turned off, so that the relay is de-energized. After a while, how-
ever, it will be switched on again because the battery voltage
will

have dropped a little owing to the drop across its internal resis-
tance.

Assuming that the required switching levels are Ul = 12.5 V
and Uh = 14.0 V, the presets are adjusted as follows. Disconnect
Cl and set PI for Uh (max) and P2 for Ul (min). Power the circuit
from a regulated supply set to 12.5 V and adjust P2 until the relay
is just energized. Then increase the supply voltage to 14.0 V and
adjust PI until the relay is just de-energized. Finally, connect Cl
again and connect the circuit to the battery terminals.

The relay must be rated at 12 V DC and 300 £2. It must have
two make or change-over contacts, of which at least one has a
voltage rating higher than the mains voltage. One suggested type
is the V23037-A2-A101 from Siemens.

The circuit draws 25 mA, which rises to 65 mA when the relay
is energized.

(M.S. Dhingra)

TEST & MEASUREMENT

TTL SUPPLY MONITOR

The Type LTC1042 lc from Linear Technology is a window
comparator that can function with very small currents. This is
made possible by the use of sampling techniques that enable the
disabling of certain parts of the ic in the non-active phase. This
economical behaviour is not so important in the present circuit:
the monitor draws a current of rather more than the minimum
100 pA.

The comparator is set with the aid of a bandgap reference
diode, Dl. The reference voltage of 2.5 V provided by this diode is
connected direct to the window centre pin 2.

The width of the window is also fixed with the aid of the refer-
ence voltage. Since the circuit is intended to monitor a ttl supply
(5 V), the width of the window is arranged at one tenth of this,
which is a convenient sub-multiple of the reference voltage. Po-
tential divider R4-R5 gives a voltage of 0.25 V at half-window-
width pin 5. This arrangement causes the within window output
to go high when the input voltage (Vin at pin 3) exceeds 2.5 V
±10%. The input voltage is held at exactly half the supply voltage
by potential divider R1-R2.

Transistors T1 and T2 drive indicator LEDs. When D2 lights, the
circuit and the supply voltage are all right. When D3 lights, the
supply voltage is too high. If neither of the diodes lights, the sup-
ply voltage is too low or even absent.

If you want a led to indicate that the supply voltage is too low,
interchange the function of pins 2 and 3. The above window out-
put then becomes a below window output. Note that the circuit
then needs a separate power supply, otherwise the below window
led can not be actuated.

LT1009CZ

w

= 2V495...2V505

The relay will then be energized after 1 7 seconds unless switch SI
is opened in the mean time. Since the location of that switch is
known only to the car owner, anyone entering the car illegally
will set off the alarm.

Two timers Type NH555 are connected as monostable multivi-

brators. Transistor Tf and associated components form
the triggering stage, which is sensitive to both positive
and negative triggering signals supplied by the sensors
or switches, mosfets T2 and T3 serve to provide the
'delayed disable' function and to drive the 'alert' led
respectively.

When the unit is powered, D12 lights, but the alarm
remains off for the time defined by time constant R12-C7.
Since the capacitor is initially discharged, T3 conducts
and pulls the reset pins of both timers to ground until the
voltage across R12 has dropped to about 3 V. That level
enables both timers, and causes the 'alert' led to light.
When the power is switched off for a second and then on
again, C 7 is dicharged rapidly via D9: this sequence
resets the alarm to its initial state.

When enabled, the alarm may be actuated by a low
level at input pinss 11 or 12 (door switches) or a high
level applied to inputs 13 or 14 (ignition system power).
The number of inputs may be expanded as required by
adding one diode for each sensor or switch. When only
one of the inputs is used, it is still recommended to use a
diode.

Any trigger pulse or change in the direct voltage at
the inputs triggers timer IC1 via C3 and C5. The output
pin, 3, of IC1 is driven high so that the 'actuated' LED
(Dll) lights. Diodes D6 and D8 form an or function to
decouple the outputs of the timers. Capacitor C6 is dis-
charged in preparation of the triggering of 1C2. After the
delay set by R6-C5, the output of IC1 goes low, thereby
triggering IC2 via R9 and C6. The output relay is energi-
zed and the transponder is powered by the car battery

via the relay contact. The alarm sounds for about 65 seconds
(defined by R9-C10). The unit may be reset at any time, however,
by operating SI for at least a second.

(R. Lalic)

042

GENERAL INTEREST

TWILIGHT SWITCH

This inexpensive unit switches the light on at dusk and off
again at dawn. The circuit has separate time bases for the on and
off delays. The dotted line in the diagram divides the circuit into
two halves: A, the light-dependent switch around gate Nl; and B,
the on-off time base around N2 and led and relay driver T1-T2.

The voltage at junction R2-R9-C1 is inversely proportional to
the light intensity measured by light-dependent resistor (ldr) R9.
Schmitt trigger gate Nl toggles whenever this voltage reaches one

of the input threshold levels. Since the difference between these
levels is large compared with the voltage span produced by the
potential divider, a variable feedback loop is provided to achieve
an effective switching span of 300 to 400 mV. When the output of
Nl is high, the voltage at junction Rl-Pl is almost equal to the
supply voltage. When the output is low, the voltage drops to the
level required for the threshold difference at the input of Nl.

The output of Nl drives two time base circuits: C2-R4-D1 for

the 'on' state and C2-R5-D2 for
the 'off' state. These networks
switch the output of N2 on
and off after the wanted
delays. The lamp relay and an
indication led are driven by
darlington stage T1-T2, which
is controlled by the output of
N2.

Capacitor Cl prevents hf
signals picked up by the cable
between the ldk and the
switching unit causing spurW
ous triggering. Because of the
high output impedance of the
Type 4093, the cable should be
a screened type.

The remaining two gates in

elektor mdia September 1989 9.73

the 4093 package may be used for duplicating the time base (part
B) to obtain a switching sequence. The values shown for R5-C2
and R4-C2 form a good starting point for dimensioning the delays
introduced in this additional time base. The lowest permissible
value of R4 and R5 is 47 kf2: their highest value depends mainly
on the leakage current of C2.

During the testing of prototypes of this circuit it was noted
that the switching threshold and hysteresis depend on the make
of the 4093. Good results were obtained with sgs's HCF4093BE;
devices from other manufacturers may require the value of R3 to

be slightly different. The inputs of the unused gates in the 4093
must be earthed.

The ldr is mounted in a suitable waterproof housing, screened
from direct light sources.

Preset PI serves to adjust the light intensity level at which the
circuit switches. To prevent too slow a response to changes in the
setting, the timer should be aligned with SI open.

The current drawn by the timer is mainly that drawn by the
energized relay.

(R. Lalic)

043

CAR ELECTRONICS

PSYCHOLOGICAL CAR LOCK

The lock circuit is based on elementary psychology rather than
on any recent development in electronics.

The lock consists of a 12-key membrane keypad and an associ-
ated visual indication circuit. The complete unit is installed in an
out-of-the-way position below the dashboard in a car, where it is
not too difficult for a potential car thief to spot.

When the keys on the keypad are pressed, the impression is
given that the lock will enable the car ignition when all four LEDs
light. Because of the special configuration of the lock circuit, this
will never happen. Eventually, the would-be thief gets frustrated
(we hope) and tries another car, not realizing that the lock circuit
is simply not connected to the ignition circuit. Only the rightful
owner knows that the car can be started after a special switch,
marked, say, 'wiper' is operated. This switch, installed as an
accessory on the dashboard, is connected in series with the posi-
tive supply line to the iginition coil.

The membrane board must be a type with a common line, not
one with a matrix configuration. A suitable keyboard may, of
course, also be made from individual keys with numbered caps.
The lines marked K0-K9 in the diagram must go to the associated
key number on the keypad. Keys and # are non-connected dum-
mies. The four leds are fitted in a row near the keypad, and give
an indication that suggests that the combination entered is correct.

As already stated, the circuit makes it impossible for all four

LEDs to light simultaneously. This is because pin 3 of N1 is logic
high when D1 is on. This means that T5 is off, so that D4 can not
light in spite of all other key combinations. It also means that D4
can not light unless D1 is out.

Eight xor gates in two 4070 packages determine which leds are
on for a particular code entered via the the keyboard. RC networks
connected to each keyboard line keep this active for about four
seconds after the key has been released. When, for instance, key 1
is pressed briefly, the voltage on line K1 rises to the supply level.
Since pin 13 of gate N8 is then the only logic high input of all xor
gates, pin 4 of N2 goes high. This causes T2 to conduct and D2 to
light. After about 4 s, the voltage on Cl has dropped to a level that
N8 recognizes as logic low, and D2 goes out. Within the 4 s period
it will, however, go out the instant key 5 or 7 is pressed. Diode D3
then lights, followed by D2, which remains on for a short period.
The two leds then go out simultaneously. Note that this functional
description applies to only one of many possible combinations.

Apart from its use in cars, the circuit may also find application
in games.

The current drawn by the circuit is mainly that drawn by the
leds that are on during the 4 s interval. In stand-by mode, less
than 1 mA is drawn.

(C. Sanjay)

9.74 elektor india September 1989

GENERAL INTEREST

X-Y PLOTTER INTERFACE

This low-cost circuit can drive two stepper motors and a
relay by digital control data supplied from a computer. It can
also detect the position of two microswitches and supplies
logic levels back to the computer as positional information.
This combination makes the interface ideal for use as an X-Y
plotter or for building a buggy-style robot.

Circuit IC1 is configured as two set-reset (S-R) latches to
provide contact debouncing for the two microswitches, SI and
S2, whose position is detected by computer reading port bits
D5 and D6. The relay drive circuit around T1 may be used to
switch a solenoid-operated pen on and off under the control of
port bit 4.

Motor drive is provided by xor gates N5-N12, bistables
FF1-FF4 and integrated motor coil driver IC3. This combina-
tion can drive two unipolar 4-phase stepper motors in half-step
mode by setting the direction of rotation with the aid of bits D1
and D3, and a high-to-low logic pulse transition on bit DO or
D2. The control functions are summarized in the table. The
motor driver, 1C4, is capable of sourcing 500 mA per phase at a
maximum motor voltage of 50 V. The ULN2803A has internal
diodes that protect it against reverse emf generated by the
motor coils when they are deactuated.

To drive the interface from a computer, set the l/o port for
five output bits, D0-D4, and two input bits, D5 and D6, and
send the appropriate control signals to the circuit.

The interface requires two supply voltages: 5 V for the logic
circuits and the relay driver, and 12 V (+Ve) for the stepper
motor coils. Motor 1 is L1-L4 and motor 2 is L5-L8.

Step pulse

Motor 1

Motor 2

LI

L2

L3

L4

L5

L6

L7

L6

0

1

0

1

0

1

0

1

0

1

1

0

0

1

0

1

1

0

2

0

1

0

1

0

1

0

1

3

0

1

1

0

1

0

0

1

0

1

0

1

0

1

0

1

0

:~V • ■

04 *

)

GENERAL INTEREST

TIMER WITH AUDIBLE WARNING

Applications of this little circuit include a portable parking meter timer
and egg timer. The 14-stage binary ripple counter Type 4060, IC1, has an on-
chip oscillator capable of stable operation over a relatively wide frequency
range. In the present circuit, the oscillator frequency is determined by an
external rc network connected to pins 9, 10 and 11.

When the circuit is switched on with SI, the pulse at junction R4-C2 resets
the counter and counting starts. When the count reaches bit 14 (Q13), pin 3
goes high so that the self-oscillating piezo-electric buzzer, a 12 V type, is
turned on via driver Tl.

The time delay is set with the aid of PI. Time delays of between one
minute and two hours are possible by appropriate dimensioning of the timing
components:

1-30 minutes: Cl = 220 nF; PI = 500 k£l

1-60 minutes: Cl = 470 nF; PI = 500 kfi

1-120 minutes: Cl = 470 nF; PI = 1 MQ

elektor india September 1989 9.75

The timer is powered by a 9 V pp3 battery. Light-emitting
diode D1 does not affect the operation of the circuit and is includ-
ed merely to show that the timer works. Diode D1 and resistor R3
are, therefore, optional components. A mercury tilt switch may be
used for SI if the unit is to be used as a kitchen timer. The timer is

then started by inverting it like a sand-glass.

With the buzzer actuated, the timer draws a current of about
10 mA.

(R.G. Evans)

HCMOS SQUARE WAVE GENERATOR

This pulse generator supplies three rectangular
output signals with a duty factor of 0.5. The signals
have a fixed phase relation to one another: output 3
is the reference; output 2 has a phase shift of about
180 and output 1 has a phase shift of about 10 °.

The generator is formed by the four bi-direction-
al iicmos electronic switches contained in a Type
74HC4066 ic. Its operation is based on the fairly
accurately defined switching threshold of an iicmos
input. The toggle point for low and high levels lies
around Ub/2 to ensure a duty factor of 0.5 and thus
a square wave output signal.

When the supply is switched on. Cl is charged
via R3 and the on-resistance of ES3. When the volt-
age of Cl reaches Ub/2, ES4 closes and pulls the
control input of ES3 low, causing Cl to discharge
via ES2 and R3. When the 'low' switching threshold
is reached at the control input of ES4, the generator
starts to oscillate.

Considering the oscillator is basically an rc type,
its stability is pretty good.

The output frequency of the generator is a func-
tion of the control voltage, U c, as shown by the
accompanyhing characteristic.

ES1...ES4 = IC1 = 74HC4066

HIGH-POWER ZENER DIODE

Although its regulation characteristics are not as good as those
of an integrated voltage regulator, a high-power zener diode has,
nevertheless, useful applications, for instance, in shunt regulators.

The circuit shown in the diagram simulates a fairly expensive
and hard-to-come-by high-power zener diode. Basically a two-
stage current amplifier with a low-power (400 mW) zener refer-
ence, it is capable of sinking up to 500 mA at a maximum 25 V.
The effective zener voltage of the circuit is about 0.7 V higher than
that of the reference device, Dl.

Using a preset, PI, instead of a fixed resistor, Rl, enables the
output voltage defined by Dl to be increased slightly. This substi-
tution is useful when, say, a 6V2 or 9V1 zener is to be simulated
and a low-power type of this rating is not available. The next
lower value in the E6 range (here, 5V6 or 8V2) may then be used
for Dl.

When the voltage across the circuit increases beyond the zener
voltage, T1 conducts and provides a base current to T2. This
power transistor then passes virtually all the current the 'zener' is
capable of sinking. The dimensions of the heat sink for T2 depend
on the maximum expected dissipation.

Transistor T1 should be a I’M’ type with a fairly high current

(C. Sanjay)

9.76 olektor india September 1989

VOLTAGE-CONTROLLED OSCILLATOR

The voltage-controlled oscillator (vco) presented here is based
on a Type OP80 operational amplifier. This opamp has an excep-
tionally low bias current of, typically, about 200 fA (femto-ampere
= 10' 5 amp) and 2 pA maximum, so that any off-set caused by this
current is minute. It is, therefore, ideal for use as an integrator,
because the operation of that kind of circuit is affected readily by
off-sets.

The OP80-based integrator in the diagram is used as a vco that
is not affected by the polarity of the control voltage. A direct volt-
age at the input of the circuit will cause Cl to charge. Depending
on the polarity of the input voltage, the potential across Cl, and
thus the output voltage of IC1, will be positive or negative. The
speed with which Cl charges depends on the magnitude of the
input voltage: this characteristic is used to generate a signal at a
voltage-dependent frequency. To that end, the outp ut signal of
IC1 is applied to a window comparator that has a switching

threshold for both the positive and the negative maximum signal.
These maxima are set to ±100 mV by P2. In some cases, it may be
advantageous for symmetry to split R2 or R3 into a fixed resistor
in series with a preset potentiometer.

When one of the comparators toggles, T1 is switched on via
N1-N4 so that Cl discharges. This results in a neat sawtooth sig-
nal at the output of the circuit, whose frequency depends on the
input voltage. Gate N4 ensures that the fet reacts to both com-
parators. The other three gates delay the switching signal slightly
to ensure that the fet is switched on long enough to allow Cl to
dicharge completely.

The Type BSV81 mosfet is provided with a separate substrate
connection that must be linked to the source. Since the substrate is
already connected internally to the housing, the device is very
sensitive to random radiation, so that the oscillator is best fitted in
a small metal enclosure.

If a BSV81 is not obtainable, another mosfet with very low
Roson ar, d very small C rs may be used. If that also is not possible,
a junction fet may be tried, but in that case a diode must be con-
nected in series with the gate and a resistor of about 10 kU
between the gate and the negative supply line. It is important to
ensure that the pinch-off voltage level is reached readily. It may
well be necessary to experiment with the value of Cl.

Correct dimensioning of R1 will enable the relation between
input voltage and frequency to be set at, say, 1 Hz/mV. With the
input short-circuited, adjust PI for the lowest possible frequency
of the output signal (ideally, /= 0). The maximum input voltage is
determined by the peak output current of IC2 (15 mA) and
amounts to 15 x 1CH x Rl.

The output signal of the vco is a clean sawtooth signal at a fre-
quency of up to 10 kHz, although higher frequencies are possible.
The frequency as a function of the input current is given by:

/='in/( u topXCl> [Hz],

With values as shown in the diagram,

/=I in /(3.9xlO-'») [Hz],

Finally, note that the supply voltage to the OP80 must under
no circumstances exceed ±8 V. The circuit draws a current of typi-
cally 4 mA.

MONITORING TEMPERATURE WITH THE C64

Maplin's module Type FE33L provides an inexpensive and
convenient means of monitoring temperature. The module has a
built-in a-d converter and an lc display and works from a single
1.5 V battery. Since it is often impractical to take frequent readings
manually, the module provides a serial data output that can be
used with most microprocessor systems. The combination of
hardware and software given in this article enables a C64 comput-
er to use the serial data, within basic, via the usr function.

The hardware consists of nothing more than a simple ttl level
driver and may be mounted on a small piece of prototyping
board. This may be connected to the module by three short wires,
while the outputs go to a two-by-twelve 0.156" pitch edge connec-

tor for the C64's user port. Pins 5 and 16 of the module should be
short-circuited to obtain the maximum sampling rate of one per
second. Check all connections before switching the computer on.

The listing provided loads a machine-code program into the
small section of ram above the basic rom at location 49152
($C000). Note that some lines are very Similar to others thus
assisting entry. Once this has been run (without errors), and sys
49152 has been entered, the temperature is obtained as follows:

TEMP = USR(0) : PRINT TEMP

This line can be incorporated into any basic program.

elektor india September 1989 9.77

REM *********************************
REM *** Maplin Temperature Module ***
REM *** Interface Software ***
REM *********************************
REM *** SYS 49152 to initialise ***
REM *** Temperature = USR(O) ***

0
1
2

3

4

5

9

10 FOR L = 0 TO ' 178 : READ A : T=T+A

11 POKE 49152+L, A:NEXT L

12 IF TO232 90 THEN ? "Error in DATA "

13 :

14 DATA 162,0,142,3,221,169,16,141

15 DATA 17,3,169,192,141,18,3,96

16 DATA 120,169,2,162,0,44,1,221

17 DATA 208, 251, i72, 1,221, 232, 44,1

18 DATA 221,240,250,224,90,48,236,162

19 DATA 13,152,41,1,157,179,192,202

20 DATA 169,2,44,1,221,208,251,173

21 DATA 1,221,41,1,157,179,192,202

22 DATA 169,2,44,1,221,240,251,224

23 DATA 0,208,229,162,13,189,179,192

24 DATA 73,255,41,1,157,179,192,202

25 DATA 208,243,160,0,32,162,179,32

26 DATA 12,188,162,12,160,4,10,24

27 DATA 125,179,192,202,136,208,247,32

28 DATA 155,192,10,24,125,179,192,202

29 DATA 136,208,247,32,155,192,10,24

30 DATA 125,179,192,202,136,208,247,168

31 DATA 32,162,179,32,106,184,32,254

32 DATA 186,173,192,192,240,3,32,180

33 DATA 191,88,96,142,191,192,168,32

34 DATA 162,179,32,106,184,32,226,186

35 DATA 32,12,188,169,0,160,4,174

36 DATA 191,192,96

The first part of the machine-code program sets the usr vector
and all the port B lines to input, while the remaining code is
called by the usr function itself. When called, the program waits
for the primary clock pulse, which is longer than the others, and
then reads in each subsequent bit from the data line. These bits
are converted from bcd format into a single floating-point

1 2 3

ABC

4 12

D N

J 1

'MINIM

894060 - 12

^ «

' V !n

UUUUUUUUUUUUUUUL

1 2 5 9 10 >6

0

894060-13

that is returned by the usr function. The software will behave cor-
rectly only when the module is in the default °C mode, but this is
not a restriction as readings can be converted readily to another
scale. If the device is to be used for serious control applications, it
must be borne in mind that the software will wait patiently for
the primary clock signal to arrive from the module. If the clock
signal fails for any reason (for instance, a break in the cable), the
control program will be left hanging in an endless loop. It is,
therefore, recommended to use a non-maskable interrupt (nmi)
generated by the timers on CIA #2 to interrupt the program after a
specific duration (for example, greater than the expected sam-
pling time) and return some sort of error condition. For simple
applications, this is not necessary and no further programming is
required.

(J. Pelan)

050

POWER SUPPLIES

SIMPLE VARIABLE POWER SUPPLY

This low-cost power supply has an output voltage range of
1.5-15 V at a maximum current of 500 mA. Regulation is better
than 2% for output currents not exceeding 350 mA. Voltage
adjustment is effected by a potentiometer and an acoustic over-
load indication is provided.

Transistor T4 compares the voltage at the wiper
of PI with the output voltage. When this is 0.65 V
higher than the set voltage, T2 is switched on,
which removes the base current from darlington
power stage T3-T5. In this manner, the output volt-
age of the supply is 0.65 V higher than the refer-
ence potential at the base of T4, which is derived
from a 15 V zener diode, D5.

The voltage at the 18 V, 1 A winding of the
external mains transformer is rectified by bridge
B1 and smoothed by Cl. A simple acoustic over-
load alarm (Bzl) is actuated when the output cur-
rent exceeds around 500 mA. Note that the exact
level of actuation depends on the electrical specifi-
cation of the buzzer, which should be a 24 V, self-
oscillating type.

The power supply is, in principle, not short-circuit proof,
although the use of a generous heat sink for T5 will enable that
transistor to withstand the maximum dissipation of about 20 W
for the few seconds that lapse before the supply is switched off.

©-

9.78 elektor india September 1989

Capacitors C2 and C3 should be tantalum types.

To obviate radiation, the supply must be fitted in a metal
enclosure. Interconnections should be kept as short as possible.

(P. Sicherman)

HEATING TIMER

This timer has temperature and time settings. The temperature
range is about 150 °C and the time delay is around 25 minutes.

The temperature controller, IC2, is driven by sensor IC1, the
familiar Type LM35, which produces an output voltage of
10 mV/°C. This voltage is compared with a reference potential
provided by a high-stability, temperature-compensated zener
diode, Dl. Presets PI and P2 form the fine and coarse controls
respectively for setting the tmperature. The comparator switches
on T2 whenever the temperature measured by IC1 is below the
set value. This causes the relay, Rel, to be energized so that the
heater element is powered via the relay contacts.

The timer function is based on oscillator/divider IC3, whose
clock frequency is determined by the variable rc network
between the Pt and PO pins (9 and 11). The clock signal, divided by
2” and 2 14 , appears at pins 15 and 3 respectively. Toggle switch SI
selects either of these outputs to give time delays of 6 s to 1 .5 min.
and 1.5-25 min. These settings are marked A and B respectively.

When the delay set by P3-P4 has lapsed, the oscillator in IC3 is
disabled by the high level at the pole of the time selector switch.
At the same time, T1 is switched on, T2 is switched off, and the
(active) buzzer sounds to indicate that the set time has lapsed.
The relay is de-energized via T2 and the heater is disconnected
from its supply The timer may be reset while the heater is on by
pressing S2

Some accurate calibration is required in the temperature con-
troller. Connect a digital voltmeter between earth and junction
R3-P1 and adjust P2 to obtain a voltmeter reading of 100 mV (=
10 °C). Set PI as appropriate by actually measuring the tempera-
tures at which the relay is energized. Next, set P3 to minimum
resistance and SI to position A and adjust P4 to obtain a time
delay of 5-6 seconds after S2 has been pressed. The time delays
are set by P3 with the aid of an accurate watch. This procedure is
not required for position B, since in this delays 16 times as long as
in position A are provided automatically. If a simple thermostat
only is required, the timer circuit, Tl, T3 and T4 may be omitted.

The circuit is powered by a regulated 5 V supply and draws a
current of about 30 mA with the relay inoperative.

The coil resistance of the relay must be not less than 400 Q.

The temperature sensor must be fitted, of course, at some dis-
tance from the heating element.

(C. Sanjay)

IMPROVED LOW FUEL INDICATOR

The indicator described here obviates the flickering of the 'low
fuel' warning light on the dashboard caused by vehicle move-
ment. The indicator ensures that the light remains extinguished
until the duty factor of the signal supplied by the fuel sensor is
smaller than 0.5. When that happens, the light comes on and
remains on until the fuel tank has been filled to a sufficient level.
The present circuit tests the light by causing it to flash for about 5

seconds everyt time the ignition key is turned to start the engine.

The signal processor is switched on together with the ignition.-
Initially, Cl is discharged, enabling oscillator N2 via inverter Nl.
One input of N3, pin 12, is connected to R3-C3, the time constant
of which is equal to that of Rl-Cl. If the output of the fuel sensor
is high, pin 12 of N3 is held high via R2-C3. The 1.5 Hz signal
from oscillator N2 is inverted by N3 and passed to darlington

elektor India September 1989 9.79

lamp driver T1-T2. After the delay introduced by Rl-Cl has
lapsed, gate N1 disables oscillator N2 so that the warning light
goes out.

When the fuel sensor inside the fuel tank supplies pulses
owing to vehicle movement, C3 is charged via R3 and discharged
via R2-R3. When the duty factor (on /off ratio) of the level sensor
signal drops below 0.5, the potential across C3 becomes high
enough to enable the lamp driver, so that the 'fuel low' warning
light is on permanently without any flicker.

(R. Lalic)

A SIMPLE VOX

A vox is a voice-operated switch that is often used as a substi-
tute for the press-to-talk switch on a microphone. The one des-
cribed here can be connected to almost any audio equipment that
has a socket for an external loudspeaker. The actuation threshold
is set by the volume control on the af amplifier that drives the
vox.

The (loudspeaker) signal across R2 is capacitively fed to the
base of Tl. Resistor R3 limits the base current of this transistor
when the input voltage exceeds 600 mV. Diode D1 blocks the pos-
itive excursions of the input signal, so that can not become
more negative than about 0.6 V.

The output relay is driven by darlington T2. Resistor R4 keeps
the relay disabled when Tl is off. The value of bipolar capacitor
C2 allows it to serve as a ripple filter in conjunction with T2.
Resistor R5 limits the base current of T2 to a safe level.

The switching threshold of the vox is about 600 mV across R2.
The maximum input voltage is determined by the maximum per-
missible dissipation of R2 and R3. As a general rule, the input
voltage should not exceed 40 V (p-p).

The current drawn by the vox is mainly the sum of the cur-

rents through the relay coil and through R5. The resistor may
carry up to 100 mA when the vox is overdriven.

(S.G. Dimitriou and F.P. Maggana)

LOW-NOISE MICROPHONE PREAMPLIFIER

The microphone preamplifier described is based on the
SSM2015 from Precision Monolithics Inc. (i’Mt), which offers a
very high gain and very low noise (1.3 n V /'If ). It is intended for
use with balanced input signals and is capable of providing an
amplification of 10-2,000, depending on the value of R4. With R5
= R6 = 10 kil, the amplification. A, is calculated from

A = (20,000 / R4) + 3.5

With values as shown in the’cliagram, the amplification is,
therefore, about 1,000.

Resistor R3 sets the operating point of the differential input
amplifier and thus determines the bandwidth and the slew rate. A
value of 33 kQ gives near-optimum values of these characteristics,
but results in a relatively high input bias current of 4.5 p A (with
R3 = 150 ktl , the current is only 1 pA). Moreover, the input noise

9.80 elektor indis September 1989

© +o oji’

gpi o qj|<> qpe p
qb 3 to gpii p ape p
£ 20.(1000000

C3CE [] ooooooo^

i-O o

cHIo

ST

Rl; R2 = 10 k
R3 = 33 k; 1 %

R4 = 20R;1%

R5; R6 = 10 k; 1%
Pi = 250 k preset

list

Capacitors:

Cl =180 p styroflex
C2; C3 = 15 p

C4 = 47 p styroflex or ceramic
C5; C6 = 100 n

IC1 = SSM2015

894063 - 12

Table 1

R3

C3

C2

27-47 kn

15 pF

15 pF

47-68 kn

15 pF

10 pF

68-150 kn

30 pF

5 pF

Table 2

R3

27—47 kn

47-68 kn

68-150 kn

A = 10

PI =

500 kn

250 kn

250 k£2

A = 100

PI =

500 kn

ioo kn

ioo kn

A = 1000

PI =

250 kn

ioo kn

50 kn

level, particularly the current-noise contribution, has increased a
little. Nevertheless, the preamplifier has a signal-to-noise ratio of
95 dB, measured with short-circuited + and - inputs and an out-
put level of 0 dBV. Resistor R3 enables the source impedance to be
matched to the input of the differential amplifier: if Z = 600 n, R3
has an optimum value of 33 k n. With a 600 n resistor across the
input terminals, the signal-to-noise ratio is of the order of 86 dB.
The tables and the curve in Fig. 1 give a number of values for the
bias resistor and compensation capacitors C2 and C3.

The differential inputs of the SSM2015 are of the floating type,
so that external resistors, Rl and R2, are required to give a suit-
able DC setting. In single-ended (unbalanced) applications, care
must be taken to prevent off-sets arising from biasing differences
between the inputs, owing to different impedances (ground for
one input, and the source for the other). Resistors Rl and R2 cause
common-mode noise and must not be given a higher value than

shown in the diagram.

Off-set compensation with the aid of preset FI is required for
the given value of R3, which results in the available amplification
of 1,000. The value of R3 depends on the gain setting - see Table 2.
Capacitor C4 compensates the on-chip input current regulator,
and Cl suppresses HF signals.

Distortion of the preamplifier at 1 kHz and 0 dBV was mea-
sured at less than 0.006% and less than 0.01% at a test frequency
of 10 kHz. The half-power bandwidth is 180 kHz at 3 V across
1 kn. Common-mode rejection at 50 Hz is greater than 100 dB.

PM! states that the output of the SSM2015 is not intended to
drive long lines: capacitive loads greater than 150 pF should be
decoupled by a 100 Q resistor in series with the output (but note
that R5 must remain connected to pin 3).

(Precision Monolithics Inc.)

HIGH-VOLUME ALARM

When this alarm is actuated by a low-level signal at input ~
the (hf) loudspeaker produces a number of 4-tone sequences sep-
arated by quiet intervals. Each sequence sounds louder than the
previous one to give the alarm a very distinctive character. The
peak output is reached after about 28 seconds.

As long as the en input is logic high, counters ICla and IClb

remain reset and interval oscillator N2 and tone generator N3 are-
disabled. The alarm is then off.

When en is actuated, the oscillator and the two counters are
enabled. Counter ICla is clocked with pulses (prf = 8 Hz) from
N2. Gates N1 and N4 at counter outputs QO and Q3 cause T1 to be
turned off during eight consecutive clock cycles from ICla.

elector india September 1989 9.81

During the next eight cycles, the transistor is
alternately switched on and off as illustrated
in the timing diagram. The loudspeaker
sounds only when T1 conducts.

Since out put Q 3 of counter ICla is con-
nected to the ci.k input of counter IClb, the
latter is incremented by the 16th negative
pulse transition. In practice, this means that
counter IClb is clocked after each tone
sequence. The most significant outputs of
counter IClb drive the 3-bit selection inputs
of analogue multiplexer IC3. Since QO is not
used, IClb requires two clock pulses to
enable the multiplexer to connect the next
input, Xn, to the output, X. The seven resist-
ors at the multiplexer inputs cause the base
voltage of T3 to increase after each alternate
tone sequence. As a result, the voltage
across the loudspeaker increases, so that the
alarm sounds louder.

Transistor T1 switches the successive
bleeps on and off. Resistor R16, capacitor C5
and transistor T2 prevent abrupt switching
of the voltage across the loudspeaker when
T1 is turned off, and ensure that the sound
level reverts slowly to the level set by the
resistor at the relevant multiplexer input.

After the alarm is actuated, the output
volume rises sevenfold. Diodes Dl, D2 and
D3 cause counter IClb to stop at state 1110,
so that the multiplexer passes the full posi-
tive supply voltage at input X7 to volume
control T3. The alarm then sounds continu-
ously at maximum volume, requiring the
power supply to provide a peak current of
up to 1.25 A through the loudspeaker. The
resultant sound is ear-piercing.

Prtesets PI and P2 serve to set the inter-
val repetition rate and the sound frequency
respectively.

(C. Sanjay)

MAINS POWERED TIMER

This timer may be inserted in a power
line to provide a controllable delay before
a load is energized. It was developed to
work in conjunction with a passive infra-
red movement detector as part of an
intruder alarm.

The mains voltage is reduced by C3
and rectified to give about 30 V across
Cl. This potential charges C2 slowly via
R4-P1. When Uc2 reaches about 14 V,
electronic switch T1-T2 actuates a solid-
state relay (a Type S202DS from Sharp).
When the mains voltage is removed,C2
discharges rapidly via D6 and RIO. The
delay extends from 15 s (PI set to mini-
mum resistance) to 5 min (PI set to maxi-
mum resistance).

The solid-state relay needs cooling in
accordance with the current drawn by
the load: at up to 1 A no heat sink is re-

9.82 elektor india September 1989

quired; at 1-3 A (max), a 5x5 cm heat sink is advisable.

During the building of the circuit, due consideration must be
given to safety since many parts will be at mains potential. For
instance, fitting the unit in an abs or other man-made-fibre enclo-
sure is a must. If a potentiometer is used for PI, its spindle should
be an insulated type. If a preset is used, it must not be accessible

through a hole in the enclosure.

Switch SI is a dpst type that disconnects the circuit from the
mains. Nevertheless, the only way of working on the circuit in
safety is by taking the plug out of the mains socket and allowing
C3 sufficient time to discharge.

(D. Dwyer)

SINGLE-CHIP MELODY GENERATOR

This melody generator, based on a Type 4093 CMOS
Schmitt trigger, may be used in alarms, doorbells and
cars (audible reverse gear or lights on indicator).

Three of the four nand gates in the 4093 are con-
nected in series by rc networks. Oscillation is effected
by feedback of the output signal of N4 to the input of
N2. The logic high levels produced by the cascaded
gates in the oscillator circuit are used for biasing one
of associated diodes Dl, D2 and D3. The relevant
diode connects one of frequency-determining capaci-
tors C1-C3 to tone oscillator Nl. The audio signal
available when SI is pressed is applied to comple-
mentary transistor pair T1-T2 that drives the loud-
speaker.

The frequency of the emitted tone may be adjusted
to individual taste by preset PI.

INFRA-RED MICROPHONE

The circuit of this microphone was originally designed to mon-
itor a 7-segment display used on the flight deck of a Boeing 737.
That display used filaments, so that an ir detector was an obvious
choice. In the Boeing, it was connected to a small portable

recorder: the sensor therefore acted as a microphone that reacted
to ir light instead of to sound. This idea of an ir microphone is
made more tangible by housing the device in the shell of din plug
as shown in the photograph.

The ir microphone consists of a Type BP104 photodiode con-
nected to the inputs of a Dc-coupled operational amplifier, whose
gain is determined by Rl.

The device may be used to 'listen' to the visual world around
us. It is particularly effective where sources of noise, such as
incandescent light bulbs, are switched off. A gas flame, such as
that of a cigarette lighter, is manifested as a soft breeze. A cosy fire
burning in the grate comes out as a real hurricane. This means
that the microphone may be used as an acoustic fire alarm, but
that is about the only application we can think of. However, the
circuit is intended more to give us an opportunity to see our envi-
ronment from a different angle. If the BP104 is replaced by a
BPW34, the sensitivity of the device is shifted from the infra-red to
the visible spectrum.

The current drawn by the circuit depends to some extent on
the supply voltage and should be 2-5 mA.

elektor india September 1989 9.83

FOUR-QUADRANT DIMMER

This very special mains-operated dim-
mer for domestic or industrial lights is
not available in proprietary form: it
enables brightness control of two groups
of lights in one operation. The possible
combinations of brightness are shown in
the table. It will be clear that it is not pos-
sible to obtain continuous control of
brightness in the two groups. Instead,
the circuit affords the setting of four
states of brightness in either group: full
on, fully dim-med, 1 /3 on and 2/3 on.

Both sections of the circuit operate on
the well-known principle of a triac being
switched from the blocking state to the
conducting state with the aid of an rc
network and a diode. The RC network
provides the necessary phase shift and
determines when the triac is switched.

The rotary switch selects the resistor in a
given network and thus the brightness of
the relevant group of lights. No resistor
means that the group is off; a short-cir-
cuit gives maximum brightness, and
resistors of 10 kQ and 18 kli mean inter-
mediate brightness. The diodes prevent
the groups affecting one another.

The 64 pH choke, LI, and the 150 nF
capacitor across the bridge rectifier pre-
vent the dimmer causing interference in
other equipment connected to the mains.

If the triacs are fitted on a heat sink rated at 12 K/W, up to
500 VV per group may be controlled. It is, of course, essential that
the enclosure in which the dimmer is fitted provides ample cool-
ing: a fair number of slots or holes in it are, therefore, essential:
these should not permit the circuit elements to be touched.

The switch should have a non-metallic spindle: this is not only
safer than a metallic one, but it also enables the easy removal of
the end-notch so that the switch may be rotated continuously in-
stead of having to be returned to the first stop every time it is
operated.

It is recommended that mains on-off switch S2 is fitted with a
built-in 'on' indicator bulb: this shows at a glance whether the cir-
cuit is on even though SI may be in the off position .

Finally, do bear in mind that this circuit carries mains voltage
in many places: good workmanship and insulation are, therefore,
of the utmost importance.

(C.G. Mangold)

Switch position

Brightness

Group A

Group B

1

0

0

2

1/3

0

3

2/3

0

4

1

0

5

1

1/3

6

1

2/3

7

1

1

8

2/3

1

9

1/3

1

10

0

1

11

0

2/3

12

0

1/3

SENSOR SWITCH AND CLOCK

One Type TL084 ic and an old quartz watch enable the con-
struction of a de luxe on-off switch. Two of the four opamps con-
tained in the TL084 (At and A2) are used to amplify the input sig-
nals from the sensors hundredfold (with the component values as
shown in the diagram). Just touching the sensors with a finger
causes a good 50 Hz input signal (hum). Note that the amplifica-

9.84 elektor india September 1989

tion drops rapidly with rising frequency.

Diodes D5 and D6 rectify (single-phase) the 50 Hz signal. Since
the diodes are connected in anti-phase, touching the 'off' sensor
results in a positive potential across CIO, whereas touching the
'on' sensor gives a negative potential across CIO.

Opamp A4 is connected as an inverting bistable, so that a nega

GENERAL INTEREST

MJ2500

IC1

LM317

4700D

35V

894103-11

five potential across CIO causes relay Rel to be energized. Be-
cause of feedback resistor R16, this state is maintained until the
other sensor is touched.

The relay may also be energized at a predetermined time with
the aid of a quartz watch. The 1.2 V supply for the watch is de-
rived from the voltage drop across diodes D9 and DIO: it may be
increased to 1.8 V by adding a third diode.

The piezo buzzer in the watch is connected to the input of A3 via
C5. As soon as the alarm goes off (the hour signal must be off),
the voltage across CIO becomes negative, the relay is energized
and the load is switched on.

The circuit, excluding the relay, draws a current of about
20 mA.

(R. Ochs)

MINI-DRILL CONTROL

u ,N1 °°T \z

The circuit described here is intended as a
revolution control for small DC motors as fitted,
for instance, in small electric drills (such as used
in precision engineering and for drilling print-
ed-circuit boards, among others). The behaviour
of these motors, which are normally permanent
magnet types, is comparable to that of indepen-
dently powered motors. In theory, the rpm of
these motors depends solely on the applied
voltage. The motor adjusts its rpm until the
counter emf generated in its coils is equal to the
applied voltage. There is, unfortunately, a drop
across the internal resistance of the motor and
this causes the rpm to drop in relation to the
load. In other words, the larger the load, the
larger the drop across the internal resistance
and the lower the rpm.

The present circuit provides a kind of com-

elektor india September 1989 9.85

pensation for the infernal resistance of the motor: when the cur-
rent drawn by the motor rises, the supply voltage is increased
automatically to counter the fall in ri>m.

The circuit is based on an enhanced voltage regulator consist-
ing of 1C1 and Tl, which provides a reasonably large output cur-
rent (even small drills draw 2-5 A). The 'onset' supply voltage,
and thus the rrm, is set by 1*2. Because of emitter resistor Rl, the
currents through IC1 and Tl will be related to one another in the
ratio determined by Rl and R2. Owing to this arrangement, the
internal short-circuit protection of IC1 will also, indirectly, pro-
vide some protection to Tl .

As soon as the current drawn exceeds a certain value, T2 will

be switched on. This results in a base current for T3 so that R5 is
in parallel (well, more or less) with R6. This arrangement auto-
matically raises the output voltage to counter a threatened drop
in rpm. The moment at which this action occurs is set by PI, so
that the present circuit can be adapted pretty precisely to the
motor used.

If only very small motors are likely to be used, the power sup-
ply (transformer and bridge rectifier) may be rated rather more
conservatively. As a guide, the current in the transformer sec-
ondary should be about one and a half times the maximum DC
output current.

(G.J. Lammertink)

CALL TONE GENERATOR

Amateur vhf relay stations are normally actuated by a 1750 Hz
call tone. This may give problems when the relevant sending
equipment has no internal call tone generator, or it has one whose
frequency is not sufficiently accurate, or whose tone duration is
not long enough to securely energize the relevant relay.

These problems can be overcome by the stand-alone generator
described here. Simply placed in front of the microphone, it
makes absolutely certain that the relay station is actuated.

The generator consists of a quartz oscillator, a frequency
counter and a buffer-amplifier, all contained in just two CMOS ICS.
It is powered by a 9 V pp3 battery, from which it draws a current

of around 5 mA.

Gates N1 and N2 form an oscillator that is controlled by a
3.27680 MHz crystal and provides clock pulses to IC2 which is
connected as a programmable scaler. Diodes D1-D5 determine
the divide factor of 1872. Counter output Q1 thus provides the
wanted 1750 Hz signal, which is buffered by N3-N6 before being
applied to a piezo electric buzzer. Capacitor C3 suppresses any
harmonics, while R4 determines the volume of the output signal.

(N. Korber)

Parts list

Resistors:

Rl = 1 M
R2 = lk2
R3 = 10 k
R4 = 2k2

Capacitors:

Cl = 60 pF trimmer
C2 = 68 p
C3 = 220 n

^Semiconductors:
:D1-D5 = 1N4148
1C1 = 4049
|IC2 = 4040

parallel

Miscellaneous: Bzl = piezo electric buzzer

XI =3.2768 MHz; 30 p

MAINS FAILURE INDICATOR

When the mains voltage is present at the input terminals, the
transistor in the optocoupler is on, Tl is off and silicon-controlled
rectifier Thl is in the conducting state. Since both terminals of the
piezo electric buzzer are then at the same potential, the buzzer is
inactive. If the mains voltage drops out, transistor Tl conducts
and causes one of the terminals of the buzzer to be connected to
earth; the thyristor remains in the conducting state. In this situa-

tion there is a large enough potential difference across both the
buzzer and D5 to cause these elements indicating the mains fail-
ure both audibly and visibly.

When the mains is restored, the circuit returns to its original
state. A touch on the reset button then interrupts the current
through the SCR so that the thyristor goes into the blocking state,
and the other terminal of the buzzer is connected to ground.

9.86 elcktor india September 1989

BRX16

■ 220n
630V

IC1

OPU264B

BC547B

The unit is powered by a 9 V pp 3 battery and
draws a quiescent current of 1. 7-2.5 mA.

It is important that the enclosure is a well-insulat-
ed type.

Finally two points to note. If by accident the circuit
to the optocoupler and R2 is broken, electrolytic
capacitor C2 may be damaged since it will be charged
well above its 25 V rating. Secondly, where a plug is
used for the mains connection, it is advisable to solder
a 1 MSI resistor across Cl so that this capacitor does
not retain its charge after the plug is removed from
the mains socket.

Resistors:

Semiconductors:

R1 = 15 k; 2 W

D1-D4 = 1N4004

R2; R8 = 1 k

D5 = LED

R3 = 4k7

Tl = BC547B

R4; R7 = 10 k

Thl = BRX46

R5 = 5k6

IC1 = OPI 1264B

R6 = 100 k

Miscellaneous:

Capacitors:

SI = switch with 1 make

Cl =220 n; 630 V

contact

C2 = 4p7; 25 V

Bzl = piezo buzzer 9 V

1 SfH 1 "It- |

9-VPP3 battery

Parts list

J. '

. - . ~ . - ■

I

ELECTROPHONICS 1

GUITAR COMPRESSOR

The control of this compressor is based on the dependence of flowing in Tl.
the dynamic resistance of a diode on the current flowing through The input signal is applied to preamplifier stage A1 via low-
it. The heart of the present circuit is the diode bridge D1-D4, pass filter Rl-Cl that removes any hf noise from the input. Switch
which behaves as a variable resistance controlled by the current SI in the feedback loop of A1 sets the amplification to 1 (position

|33°V| -?3. 6,
BJ, »V S '

nr?

£6 ra C7 6

II CHD ♦ II L

680n 100n

D1...D4 -= AA119

elektor india September 1989 9.87

BRX4b J '

G_/ K.

I 100k I '

D1...D4 = 1N4004

A), 6 (C) or 11 (B). The amplified signal is applied to the diode
bridge direct via R12 and C5, and inverted via inverter A2, capac-
itor C6 and resistor R13. The two signals are summed by the
bridge, amplified (in A3) and then split again into two, one of
which is inverted by A4. The positive half cycles of the two sig-
nals are used to switch on T2 and T3 respectively. Capacitor Cll is
then charged via R12. When the potential across this capacitor
reaches a certain level, T1 is also switched on, after which a con-
trol Current flows through the bridge via R21 . This current lowers
the resistance of the bridge so that the signal is attenuated (com-
pressed). At the same time, the led lights to indicate that the sig-

nal is being compressed. Capacitor C12 prevents any DC voltage
from reaching the output.

The output signal is taken from the wiper of PI. Low-pass sec-
tion R20-C13 limits its bandwidth to 12 kHz.

Switch S2 enables the selection of various decay times of Cll.
The values shown in the diagram have in practice proved to be
the most useful. Nevertheless, these values are subjective and
may be altered to personal taste and requirements.

(W. Teder)

LC SINE WAVE GENERATOR

This compact LC oscillator offers a frequency range of about 1 kHz to almost
9 MHz and a low-distortion sine wave output.

The heart of the circuit is series-resonant circuit L1-C2-C3 in the feedback loop
of amplifiers T1-T2. Transistor T2, which is connected as an emitter follower, serves
as impedance converter, whereas Tl, connected in a common base circuit, is a volt-
age amplifier whose amplification is determined by the impedance of LI in its col-
lector circuit and the emitter current. The feedback loop runs from the collector of
Tl via the junction of capacitive divider C1-C2, source follower BS170 and the
input impedance formed by R1 and C4. The whole is strongly reminiscent of a
Colpitts circuit. The signal is also taken to the output terminal via C5.

Of particular interest is the amplitude control by the current source. The signal
is rectified by two Schottky diodes, smoothed by C9 and then used to control the
current through T3. The gain of amplifier Tl is therefore higher at low input levels
than at higher ones. This arrangement ensures very low distortion, since the ampli-
fier can not be overdriven.

The resonant frequency may be calculated from

/= 1 / 2 it V[L1 Cl C2 / (Cl + C2)]

With values as shown, it extends from 863 Hz (LI = 10 nH) to 8,630 MHz (LI =
100 nH).

The unit may be used to measure the Q of inductors. To that end, a potentiome-
ter is connected in parallel with LI and adjusted so that the current through the
amplifier is doubled. The Q is then calculated from

Q = Rp / 2nfL

066

TEST & MEASUREMENT

SHUNT FOR MULTIMETER

The current range in multimeters, par-
ticularly the more inexpensive ones, is
restricted by the load limits of the internal
shunts to 1-2 A.The photo shows how eas-
ily a precision heavy-duty resistor from
Dale or RCL (0.1 i2; 20 W; 1%) may be used
as an external shunt. These resistors were
not designed for this purpose, but they are
much cheaper than custom made shunt re-
sistors. The 20 W rating applies only, by the
used: without that its rating is only 8 W.

The maximum current through the device on a heat sink is
about 14 A; the larger versions draw up tol7.5 A. When mounting
the shunt, make sure that the test terminals as well as the device
terminals are soldered properly, otherwise the resistance of the terminals is added to the shunt.

0

894043-11

way, if a heat sink is

9.88 elektor India September 1989

REAR WINDOW WIPER COUPLER

From any point of view, it is convenient to
couple the rear window wiper on our cars
to the windscreen wiper. Since the rear
window of a moving car does not get near-
ly as wet as the windscreen, it suffices if
the rear wiper operates only once for ev
ery 8, 16 or 32 wipes of the windscreen.

At terminal 53e (green/black wire),
which is the return of the windscreen
wiper motor, the clock signal for that
motor is present. This signal, which is a
square wave, is applied to the clock input
of counter IC1 via R2-R3. The Q3 output of
the counter goes high every eighth clock
pulse, and the Q4 output once every six-
teenth clock pulse. The output pulse is
applied to monostable IC2 via switch SI.

The monostable may be a Type 4528 (as
drawn) or a Type 4548. At each trailing
edge at pin 5, the monostable output (pin
6) goes high at a frequency of 1.5-16 Hz.

When that happens, T3 is switched on, the
relay is energized and the rear wiper oper-
ates.

The supply for the circuit is taken from terminal 53e also: dur-
ing the intervals between the clock pulses, this terminal is at a
potential of 12 V. In that state, T1 is switched on an Cl charges.
When the clock signal is present at the terminal, T1 is off so that

Cl can not discharge. Diode D1 limits the supply to 5 V.

A regular reset during the windscreen wiper interval is
ensured by R4-R5-C3-T2-D2.

(G. Kleine)

PULSE SKIPPER

Noise and other interfering puls-
es on RS232 lines are the bane of
a data processor's life. It is diffi-
cult to get rid of these pulses,
because how does any relevant
circuit differentiate between the
real and the spurious? That is
where the present circuit comes
in: it skips those pulses at the
input that are not constant for a
given period or are shorter than
normal.

The wanted pulses are de-
layed by about 55 ns plus the du-
ration of the pulse. If parallel
data lines are to be 'sterilized', it
is imperative that the various
pulse durations (delays) are
equalized.

In Fig. 1, the input pulses are
passed to an edge detector via
buffer amplifier Nl. The signal is
passed both direct and delayed

Fig. 1. Circuit diagram of the pulse skipper.

to, pulse shaper N10 via xor gates N7, N8 and N9. For every
pulse change at the output of Nl, xor gate N10 outputs a logic 0

for about 30 ns. Moreover, each pulse variation at the input trig-
gers monostable 1C3. After a time t=R 1C1, the monostable toggles

elektor india September 1989 9.89

and returns to its original state.

At the same time, the toggling of the
monostable generates an enable pulse for
latch IC4. Each low-level pulse at the clock
input of IC4 results in the storage of the
momentary level at input D. The end of the
clock pulse clears output Q, which afterwards
only carries the stored level.

It is, of course, possible that during this
process the input level has changed, which
could lead to storage of the spurious pulse. To
prevent this, the wanted signal is delayed by
gates N2-N8 by about 65 ns before it is
applied to input D of 1C4 and stored. At the
same time, it is ascertained that the level
change at the input triggers the monostable
before it is passed to the latch. Fifl- 2. Normal pulses are longer than the mono Fig. 3. Signals recorded from an oscillo-

The action of the circuit is shown in the time (a), while spurious ones are shorter (b) scope. From the top: (1) data signal; (2) noise

pulse diagram of Fig. 2. A normal pulse, that pulses; (3) distorted signal; (4) “sterilized”

is, one that lasts longer than the time constant signal at the output,

of the monostable, is present, aftrer a short

delay, at the Q4 output of latch IC4 in its original state (Fig. 2a). A lk5 potentiometer or by a number of fixed resistors brought into

spurious pulse, which is shorter than the time constant of the circuit by a rotary switch.

monostable, is absorbed in the circuit and does not lead to a level The time of the monostable is computed from

change at the Q4 output of the latch (Fig. 2b).

The circuit consists of four ICs, one resistor and one capacitor. T = 0.7 R1 Cl
It is easily fitted in a small enclosure or, better still, in the equip-
ment that needs "sterilizing". In that case, the necessary suppl;y Practical values for R1 range from lk4 to 40 kf2, while Cl may
voltage (5 V) will also be readily available. have values from 10 pF up to 1000 pF.

If different time constants are needed, R1 may be replaced by a (fq Korber)

FLASHING-LIGHT CONTROL

Life without flashing lights has become unthinkable. The circuit
described here is intended primarily for model constructors, but
may also be used in warning or alarm systems in the 'real' world.

The circuit is conspicuous by its simplicity; yet, it can deliver a
current of ujp to 3 A. It is thus possible for it to power two 21-W
car bulbs and these emit quite a lot of light!

Model constructors may connect several bulbs in parallel: Lai
lights when La2 does not and vice versa. It is also possible to use
only Lai and omit La2.

The LI 65 from STM (formerly sgs) is a common or garden oper-
ational amplifier with a powerful output stage. If it is intended
that the output current goes ujp to the full 3 A, the device must be
mounted on a heat sink of 2.5 K/W. Note that the enclosure of the

£l

Resistors: Semiconductors:

R1-R4 = 10 k IC1 = LI 65 (L465)

PI = 100 k preset

Capacitors: Miscellaneous:

.

C1-C3 = lOgF, 25 V Lai; La2 = 12 V, 21 W (max)

Heat sink 2.5 K/W (see text)

9.90 elektor india September 1989

opamp is connected to pin 3. If the output current is not likely to
exceed 0.5 A, a heat sink is not needed.

The LI 65 is internally protected against short-circuits and high
temperatures.

If the brightness of the two bulbs is not the same, this may be
corrected by adding a Type 1N5401 diode in one of the supply
rails of the opamp. The relevant rail (pin 3 or pin 5) must then be
decoupled by a 100 pF, 16 V, electrolytic capacitor to earth.

The LI 65 may be replaced by a Type L465.

Excluding the bulbs, the circuit draws a current of about 15
mA. The power supply must therefore be rated at the maximum
current through the bulbs.

DAYLIGHT-RESISTANT OPTOCOUPLER

Many X-Y plotters, particularly the DIY types, have, for all sorts of
reason, no protection against incident light, so that the phototran-
sistor in the optocoupler can not differentiate between the light
from the associated LEDand daylight. The circuit proposed here
offers a solution to this problem.

A Type 555 timer pulses the LED in the optocoupler at a rate of
10 kHz. If, at the receiver end, only the signal at that frequency is
amplified, neither daylight nor bright artificial light can disturb

the operation of the light barrier.

At a pulse rate -of 10 kHz, the pulse spacing (or pulse repetition
period) is 100 jis. If the duty factor is 6:4, the pulse width is 60 (.is.
At that rate, the led can be pulsed at a fairly high current: about
45 mA. The pulsating current is fully compensated in relation to
the voltage by C3.

The on-time of the pulse signal at the Q output of IC1 is deter-
mined by R1-R2-C1 and the off- time by R2-C1:

pulse width tl = 0.693C1(R1+R2) = 60 us;

pulse spacing t2 = 0.6930 R2 = 40 |is.

The receiver in the optocoupler, that is, the
phototransistor, is actuated by the reflected
light from the light barrier and applies the
consequent 10 kHz signal to a classical ampli-
fier (x80) via C5. This capacitor and the input
resistance of the amplifier form a high-pass fil-
ter. The collector of Tl has a d.c. potential of
3 V. Capacitor C8 and diode D1 cause a d.c.
shift, so that at the anode of D2 positive pulses
with a width of 60 gs are present. These pulses
charge capacitor C9 via D2 and R9. If sufficient
pulsating light is reflected, that is, for instance,
when the paper is less than 15 mm from the
light barrier, the potential across C9 is suffi-
cient to switch on T2. The output signal is then
taken direct from the collector or via the relay
(do not forget D3D.

The circuit draws a quiescent current of
about 30 mA and an operational one of around
80 mA - in both cases, the relay current is not
included.

(A. Schaffert)

elektor India September 1989 9.91

IMPROVED ECONOMICAL PORCH LIGHT

The practical application of the original
circuit (Ref. 1) required a number of
enhancements, resulting in the modified
versions presented here.

The aim of the original design was to
provide timed operation of a dusk-
switched porch light. Presets PI and P2
control the timing period and dusk
threshold respectively. The circuit pro-
vides zero-voltage switching of the triac.
The original design has been modified to
make it suitable for installation in an
enclosed porch immediately above a
wall-mounted light.

Transistor T3 has been added as a
shunt across C4 to allow the photocell to
be disab led when the light is on. The
sensor can thus be mounted close to, or
even as part of, the light fitting without
any problems.

The sensor con tinues to be disabled
by D6 after the light is switched off. This
prevents spurious retriggering by late-
night visitors, when light from the hall
may fall into the porch.

Diode D4 causes the light to be
switched off when Q12 goes high, but the
counter continues until Q13 goes high
also: D2 then inhibits further counting.
Diode D5 keeps the light off during the
time the circuit is on standby for its day-
light reset.

Capacitor C5 and resistor R14 prevent
the circuit from locking with Q12 and
Q13 high. The prototype of the circuit is
reliably controlling a 60-W light.

Component changes around the 4060
slow the clock to restore the light's timing
to the original range. For reliable opera-
tion, C3 must be a low-leakage polycar-
bonate or polyester capacitor: multilayer
mkm or mkt types are preferred.

Reference "Energy-saving porch light”
Elecktor India Aug/Sept 1984 p. 8-67

(S.C. Dellow)

RINGING PRUNER

If, like most of us, you get annoyed at times by the incessant ring-
ing of the telephone, this circuit will keep your blood pressure
down since it allows the telephone to ring only a few, predeter-
mined, times.

Every time the telephone rings, the counter in the 4516 is
increased by 1. The counter position at which the bell circuit is
broken with the aid of a triac (Tril) may be set at 1, 2, 4, 6, or 8.

The bell signal on the A or B lines of the telephone is used as the

9.92 elektor irrdia September 1989

input for the circuit. It is split into two: a clock signal that raises
the counter by one every time the bell rings, and a reset signal that
puts the counter back to zero after the caller has hung up.

The bell sign al is buffered by T1 and T2, each of which drives
an rc network. When the bell rings, T1 is on and capacitor C2 is
discharged via R6. At the onset of the interval between consecu-
tive rings, the potential across C2 rises again and this causes IC1
to be clocked. The reset signal is produced in a similar manner,

Fig. 1 . Circuit diagram of the ringing pruner. Fig. 2. See text.

but C3 takes much longer to recharge than C2. The recharge peri-
od is longer than the intervals between consecutive rings, so that
the counter is reset only when the rings cease.

When the counter reaches the predetermined number, the CE
(count erable) input goes high, so that further counting is not pos-
sible: T4 then switches off. This in turn switches off the triac,
resulting in the bell circuit being broken.

Transistor T3 has been added to keep the current in the bat-
tery-powered circuit low: it removes the gate current from Tril as
soon as the reset signal is high. In this state, the circuit draws a
mere 10 pA.

It is, of course, necessary to power the circuit from a battery in
view of the connexion to the telephone network. A PP3 (9 V) bat-
tery is ideal.

In older types of telephone with a separate beel terminal, the

circuit is connected as shown in Fig. 2.

Note that the connexion F.B-B in the telephone, plug or wall-
mounted box must be removed to ensure that the triac is in series
with the bell.

Electronic telephones with two-wire connexions may not
allow the addition of the present circuit without some alterations
inside them. With these telephones, it is better not to use a triac to
prevent short-circuits. A miniature relay and freewheeling diode
between the collector of T4 and earth are then used in place of
Rll, C6 and Tril. A make contact of the relay is then con nected in
series with the bell.

It should be noted that in somne countries the use of this type
of circuit is not allowed. If in doubt, consult your local telephone
or nr manager.

SMALL STEP-UP CONVERTER

A draw-back of battery-based power supplies is their relatively
low output voltage. The present circuit is intended to convert this
5-12 V output into a variable, higher one of 15-30 V.

The converter is based on an L497A from Texas Instruments,
which is ideal for the construction of a fly-back circuit. Each
switching period of a fly-back type of converter consists of two
phases. In the first phase, the magnetic energy is stored in an
inductor by connecting this to the input voltage via a conducting
transistor. The current through the inductor rises linearly until a
predetermined maximum (here set by R1 at 500 mA) is reached. In
the second phase, the input voltage is disconnected from the
indictor by the toggling of the transistor. The inductor then trans-
fers its magnetic energy to an electrolytic capacitor by charging
this via a diode.

The L497A contains an oscillator, a current limiting circuit to
prevent the inductor from saturating, an error detector that com
pares the output voltage with an internal reference of 1.2 V, and a
power transistor with freewheeling diode.

The oscillator has an on-time that is determined by C4. With

elektor india September 1989 9.93

varying loads, the oscillator frequency shifts to adjust the duty
factor.

The inductor may be a standard 40 pH, 2 A, choke (the value
is not critical), which is readily available. Electrolytic capacitor C2
is shunted by a good-quality decoupling capacitor. Cl, to ensure
removal of unwanted voltage peaks from the output. If the output
is used to power an analogue circuit, it may be necessary to con-
nect a small choke in series with it.

Capacitor C3 is necessary if the circuit is powered by a battery,
since that invariably has a high internal resistance.

The layout of this type of circuit, in which relatively high peak
currents may occur, is fairly critical. Start the construction with a
star junction of Cl, C2 and the output voltage divider.

From this junction, take an earth line straight to pins 4 and 5 of
the IC. The emitter of the internal switching transistor must be
connected direct to the negative terminal of C3. It is particularly
important that the emitter current does not flow through the earth
return used by the internal voltage reference (pin 4).

The peak output current depends, among others, on the dif-
ference between input and output voltages: it will normally be of
the order of a few hundred milliamperes. The ripple on the out-
put voltage is about 100 mV (this is always worse in switch-mode
converters than in linear types). The stability, even with varying
input voltages, is excellent, however.

The circuit draws a quiescent current of about 8 mA; its effi-
ciency is around 70%.

GENERAL INTEREST

EEPOT

An EEPOT is an electronic potentiometer with built-in eeprom that at the circuit shows that CE is always low. The eeprom is thus not
memorizes the last-set potentiometer position without needing a programmed, so that the potentiometer is always at zero at
supply voltage. It is a linear device available in three standard power-on.

values: 10 k (X9103); 50 k (X9503); and 100 k (X9104). To render its If it is required that the last set potentiometer position be avail-
linear characteristic into a logarithmic one, a resistor with a value able at power-on, the input signals of Nl, N2 and the output of

of one tenth the nominal value of the potentiometer must be con- N9 must be connected to the trigger inputs of a retriggerable

nected between pins 5 and 6. monostable with a mono tim e of about 2 s. The Q output of the

The eepot has three control terminals: an up/down input monostable then serves as the CE signal. The monostable prevents
(U/D) where the direction of travel is determined; a clock input the unnecessary programming of the eeprom when the poten-

(INC) that enables the wiper to be moved one step at a time; and a tiometer is being adjusted; only after nothing has changed for two

chip ena ble ( CE) input that enables the potentiometer to be selec- seconds is the position memorized.

ted. The CE line serves a second function in the programming of Since the selection of the hundred positions of the potentiome-
the eeprom: the contents of the counter are not written into the ter by switches alone would be a lengthy procedure, the circuit
memory until the leading cdgAof the CE signal arrives. A glance provides an automatic repeat facility. Tire cascaded network N3-

9.94 etektor india September 1989

-N8 ensures that the circuit reacts instantly to a key being pressed,
To that end, the switching pulse is delayed slightly by R3-C3 to
enable the U/D input to stabilize. The delayed pulse is then
compressed to 20 ps by N5-R4-C4-N8 to enable repeat pulses also
reaching the INC input.

In the network shunting N3-N5-N6-N8, capacitor C5 is

charged sufficiently after one second to cause pin 9 of N9 to go
high, which results in this gate generating square waves, whose
frequency may be set between 5 Hz and 30 Hz by Pi .

The voltage at the potentiometer terminals should not exceed
±8 V and should preferable be kept to ±5 V. The current through
the wiper terminal should not exceed 1 mA.

LED VOLTMETER IN SMT

The voltmeter described has been designed primarily tor monitor-
ing the charging and discharging of a car battery. Because if its
very small dimensions, it may be fitted anywhere on the dash-
board.

The unit is based on quad opamp Type LM324, which is inex-
pensive and can work direct from the car battery. The voltage
indication is obtained from a comparison of the battery voltage
and a reference voltage at each of the four opamp inputs. The ref-
erence voltage is obtained from a zener diode that is operated via
bias resistor Rl, which allows a current of about 6 mA.

rhe zener voltage of 5V6 was chosen deliberately, since zener
diodes operating between 5 V and 6 V have the best temperature
stability. Moreover, even if the battery voltage is low, the drop
across Rl is sufficient for correct operation.

The reference voltage is applied direct to the negative input of
A1 and to the other opamps via potential divider R2-R4-P1. The
potentiometer provides fine setting of the reference voltage (zener
tolerance!).

The ratio R5:R6 has been chosen to ensure that the levels of the
battery voltage stated alongside the leds are directly proportional
to the reference voltage steps.

(M. Thurn)

Parts list

Resistors:

Rl = lk2
R2-R4 = 680R
R5 = 15 k

R6 = 10 R7-R10 = 1 k
PI = 10 k preset

Semiconductors:

D1 = zener diode 5V6, 400 mW
D2-D5 = LED
1C1 = LM324D

RS232 INTERFACE FOR C64

Although the Commodore C64 at the time of its introduction cre-
ated a sensation, it has a number of drawbacks. One of these is the
non-standardization of the connexions. Plugs, sockets and signals
are not always compatible with a host of other equipment, such as
printers, modems and other computers. The interface presented
here transforms the serial output to a standard RS232 connexion.

The quasi-RS232 connexion is found at the rear of the C64.
Some lines of the user I/O connector are used for the present pur-
poses. Unfortunately, just a few lines to a standard 25-way D-type

connector as used for RS232 connexions are not enough. What is
also needed is the conversion of the m. signal levels to RS232 lev-
els, that is, to between +5 V and +25 V for a logic zero, and to
between -5 V and -25 V for a logic one. To this end, the 9 V a.c.
available on the user I/O conmnector is converted to a symmetri-
cal d.c. supply. The positive supply is obtained by simple, single-
phase rectification by D1 and Cl,

Obtaining the negative rail is slightly more complicated, since
the a.c. line is internally connected to the logic earth. Therefore,

elektor indis September 1989 9.95

during each half cycle at pin 11 of the I/O connector, C2 is
charged via D2. During the negative half cycles, C2 will trickle-
charge C3 via D3. Note that the 'negative' is hardly negative with
respect to logic earth, although it is relative to pin 10.

Although the symmetrical supply so obtained is not very sta-
ble, it is more than adequate for powering RS232 line driver 1C1.

The four most frequently used RS232 signal lines, that is: TxD -
transmitted data; RxD - received data; RTS - request to send; and
CTS - clear to send, are provided at the standard RS232 connector
via gates N1-N4.

REAR WASH-WIPE CONTROL

Although many cars, where
necessary, are now fitted with
a rear wash-wipe facility, few
provide intermittent opera-
tion of it. The control descri-
bed here provides switch-on
delay for the rear wash and a
switch-off delay for the rear
wipe operation. To that end,
the existing connexion
between the on-off switch
and the motor, and that
between the wash and wipe
motors, must be broken.
Normally, this can be done in
the relevant compartment in
the boot (or rear of an estate
car).

The circuit is connected between these three wiring breaks. The
positive motor terminals are connected to the relay contacts (rated
at not less than 16 A), while the other terminals are connected to
the +12 V line. Some extra cable may be needed here. The motor
earth is normally connected direct to the car body. Some cars have
a positive-earth system: that makes no difference as far as the
relay contacts are concerned. The polarity does not matter to the
remainder of the circuit either (but be careful not to cause a short-
circuit), although SI must be arranged to switch in the positive
line onlv-

Most of the noise and other interference pulses are removed
from the 12 V vehicle supply by D1-R1-C1: the resulting voltage is
perfectly suitable for cmos devices.

Standard relays (12 V, 16 A) with one make contact may be
used: special car relays draw rather more current.

Input filter R2-C2 debounces push-button switch SI and at the
same time provides appropriate time constants for Schmitt trigger
1C1. When SI is closed, C3 charges rapidly via D2, but discharges
only slowly via R4 when the switch is released. Only when the
signal drops below a certain level does the Schmitt trigger switch

off relay Rel via T1 (switch-off delay). With values as shown, the
delay is about 2 seconds.

The delay for the wash motor is obtained in an identical, but
reversed, manner. For this, electrolytic capacitor C4 is charged
slowly via R5 until the required threshold level is reached, which
takes about 0.5 s with values as shown in the diagram. It is then
discharged rapidly via D4.

When SI is pressed briefly, the rear wiper operates once or
twice. When it is pressed for more than around 0.5 s, the wash
motor causes a little water to be squirted on to the rear window
while the wiper operates.

It is common experience that the rear wash-wipe facility is not
needed nearly as often as that of the windscreen. Nevertheless,
the delay times in the present circuit may be altered by changing
the resistor values as appropriate. Combining the control with the
windscreen intermittent switch is only feasible if relatively short
clock pulses are available at that switch.

To prevent condensation on the circuit, it is not advisable to fit
it in a closed box.

(A. Schaffert)

9.96 elektor india September 1989

r

GENERAL INTEREST

DIFFERENTIAL AMPLIFIER

Differential amplifiers are normally designed with the aid of one
or three operational amplifiers. A good-quality type (instrumen-
tation amplifier) may also be constructed from two opamps as
described here. The quality of the amplifier is largely dependent
on the stability of the components used.

To ensure adequate suppression of common-mode signals, the
following relation must be true:

R2R4 / (R1 +P1)R3 = 1.

Preset PI makes it possible for this condition to be met, what-
ever the tolerance of the other components. The circuit may be
simplified by choosing the same value for R2, R3, and R4. In that
case, the amplification of the circuit is:

2|1 + Rx / (R5 + P2)I

in which Rx = R1 + PI = R2 = R3 = R4.

A Type OP227 was chosen because of the very' good and virtu-
ally identical specifications of the two opamps it contains.

The circuit draws a current of about 7 mA.

"

. * *• ” m

Xliea&i ■■•x-ii isSssSaSsfai

TEST & MEASUREMENT

HF PROBE FOR OSCILLOSCOPE

This active probe will allow the measurement of signals up to at
least 100 MHz. It has the benefits of loading the metering point to
only a negligible degree and not being loaded by the cable that
connects it to the oscilloscope.

The circuit is basically nothing more than a voltage follower.
The negligible loading of the metering point is achieved by the
use of a dual-gate mosfet. Since that device makes the input
impedance of the circuit too high, the impedance is brought down
to the standard value of 1 Mil by R1 .

The probe is constructed on the small printed-circuit board in
Fig. 2. Regrettably, that board is not available ready made, but
drawings of it on paper or film may be obtained at low cost - see
the Readers' services section towards the end of this magazine.

Cl =100 p

C2-C4 = 1 n (ceramic)
C5 = 10 n (oeramic)

Semiconductors:

T1 = BF981
■ ferrite bead T2 = BFR91

elektor India September 1989 9.97

POWER SUPPLY WITH EEPOT

The eepot control described elsewhere in this supplement (Circuit
No. 074) is eminently suitable for use in a variable power supply.
The advantage of this is that the eepot is then fitted where it
should: in the circuit and not on the front panel or some other
remote place. That arrangement reduces the likelihood of hum,
noise and other interference on the power lines.

The power supply proper is fairly simple to build. A stable ref-
erence voltage of 6.9 V is provided by an LM329. This potential is
used for setting the output voltage via preset potential divider P2-
P4-R2, where P4 is the EEPOT.

The power section of the circuit is based on 1C1 and T1 which
together behave as an operational amplifier as far as positive volt-
ages are concerned (negative ones do not occur here, of course).
This combination opamp, together with PI, R5 and R6, is config-
ured as a non-inverting amplifier. This means that the potential
across the output terminals is proportional to the voltage at the
wiper of P4. Potentiometer PI serves to set the maximum output
voltage, and P2, the minimum output voltage. Moreover, at
power-on the output voltage will be a minimum because no use is
made of the integral memory of the eepot.

To protect the power supply, current limiting is provided. To

that end, the output current is converted to a voltage by Rll. As
soon as that voltage (presettable by P3) is high enough to switch
on T2, the voltage regulation of the circuit is replaced by current
regulation via the strobe input of IC1. Depending on the setting of
the controls, the maximum current lies between 0.8 A and 3 A. To
ensure that T1 is not damaged at maximum dissipation, the cur-
rent must not exceed 1.5 A when the output terminals are short-
circuited.

Alignment of the circuit is not difficult. First, set P4 at maxi-
mum resistance and wait a minute or so to let Z1 and IC1 reach
their normal operating temperature. Next, adjust PI to give an
output voltage of 25 V. Finally, set P4 to minimum resistance and
adjust P2 to give an output voltage of 250 mV.

The minimum output voltage is chosen deliberately at 250 mV,
because that ensures that the various components operate on a
linear section of their characteristics at all times. It also ensures
that each of the 100 steps of the eepot is equivalent to 0.25 V.

A separate small power supply for the EEPOT is provided by
R12, Z2 and C4. Two points need to be borne in mind here: the
earth lines must be run as shown in the diagram, and T1 must be
mounted on a heat sink of about 1.5 K/W.

ONE OR TWO MBIT EPROM PROGRAMMER

More and more computers are fitted with a 1
Mbit or 2 Mbit eprom. The programming of
these devices is rather more complicated than
that of the well-known 27128 types. Moreover,
there are only a few programmers on the mar-
ket that can process these large eproms, and
those are fairly expensive. The holder
described here enables the large eproms to be

programmed by a standard 27128 program-
mer. The only proviso is that the large eprom
has an 8-bit wide data bus and a non-segment-
ed address range, that is, continuous - not
split into banks: in other words, a 8x128 Kbyte
or 8x256 Kbyte type.

It is seen from the circuit diagram that part
of the address decoding is effected with the

9.98 etektpr india September 1989

aid of four dip switches. This is possible because externally the
only difference between a 27128 and the larger types lies in
address lines A14, A15, A16, and possibly A17. These dip switches
arrange the splitting of a 1 Mbit eprom into eight 128 Kbyte blocks
(note that A17 is absent from the 1 Mbit type), while a 2 Mbit type
is split into sixteen 128 Kbyte blocks.

The proposed holder therefore enables the larger EPROMS to be

programmed in eight or sixteen blocks, which makes it much
simpler.

At the onset, when all dip switches are in position on, insert
the master (donor) eprom into the zif (zero insertion force) socket
and start the programmer's reading of the contents. Replace the
donor eprom by the 1 Mbit or 2 Mbit eprom, as the case may be,
and commence its programming. This procedure must be repeat-
ed eight (1 Mbit) or sixteen (2 Mbit) times, each with a different
setting of the dip switches.

Construction is fairly simple. Use a zif socket for the holder:
this enables the insertion and removal of eproms many times
without damage to the pins. Unfortunately, these sockets are
available only with 40 pins. That is why the lower eight pins in
the diagram are not connected.

The connections between the circuit and the programmer may
be made, for instance, with the aid of a wire-wrap socket as
shown in the photograph.

MIDI INTERFACE FOR AMIGA

Although the Amiga is not available with a midi interface, it is for-
tunately not too difficult to convert its RS232 output to a MIDI out-
put. The circuit to do that is inexpensive and fully compatible
with ready-made interfaces, so that existing software can be readi-
ly used with it.

A midi interface is a serial interface that makes use of a current

loop. The speed on this serial channel is pretty high: 31,250 baud.
The design of the Amiga allows operation of the computer at that
speed, which makes the construction of the interface fairly simple.
In fact, all that is required is a convberter to translate the serial
data on the RS232 interface (voltage) into a midi signal (current).
As is evident from the circuit diagram, only few components are
needed for this: three ICs, an optocoupler and some resistors and
capacitors.

The midi input, Kl, converts the input current into a voltage
via optocoupler IC2. Part of the serial data is taken to midi
throughput K2 via buffers N1 and N2. The remainder of the data
is taken to the RS232 interface via RS232 driver IC1.

Resistors:

R1-R9 = 220R
RIO = lk

Capacitors:

Cl = 10 p; 16 V
C2-C5 = 1 p; 16 V (radial)
C6 = 100 n

Parts list

Semiconductors:
D1 = 1N4148
IC1 = LT1081
IC2 = 6N136
IC3 = 74LS14
1C4 = 7805

Miscellaneous:
Kl-K5= 5-pin DIN

-

elektor india September 1989 9.99

At the same time, serial data
from the computer is taken to three
midi outputs via driver gates.

Because of IC1, the circuit is pro-
tected against voltages that occur on
an RS232 interface and it can, there-
fore, be used in combination with
other computers.

The wiring from the pcb to the
various din buses must be made
manually. Populating the board
itself should not present undue dif-
ficulties. Unfortunately, the I’CB is
not available ready made, but draw-
ings on paper or film may be
ordered at low cost - see the Rea-
ders' Services section towards the
end of this magazine.

Where a regulated 5-V supply is
available, IC4 may be omitted. In
that case, a wire link must be placed
between connexions 1 and 2 for IC4.

There is not much that needs to
be said about the use of the inter-
face. Connect it to the RS232 output,
after which any of the special MIDI
programs, like Aegis Sonix, Studio
Magic, Musi Mouse, and many
more, may be used for many hours
of pleasure.

(E. Ponsen)

AUTOMATIC CHARGER ADD-ON UNIT

Lead-acid-battery chargers are fairly inexpensive these days, but
unfortunately most of them are also made on the cheap. For
example, although it is important for charging to cease once the
battery is fully charged, there are few, if any, chargers in the low to
medium price bracket that offer this facility. The proposed add-on
unit switches the charger off when the battery voltage rises above
13.8 V (this may be set somewhat higher when the unit is used
with a booster charger) and switches it off again when the battery
voltage drops below 12.6 V.

The circuit has its own power supply, a 12-V mains adaptor,

since otherwise flat batteries could not be charged.

Opamp A4 and zener diode D7 provide a stable reference volt-
age. When the charger terminals are open-circuited, the input cur-
rent of the opamp produces a small drop across R4. This voltage
causes the output of opamp A2 to go low. The relay is not ener-
gized and the charger may be short-circuited with impunity. Only
when a potential of not less than some tens of millivolts exists
across the charger terminals is the relay energized and does the
charging of the battery begin. If the battery has been connected
with incorrect polarity, nothing can happen because the relay does

Parts list

Resistors:

Semiconductors:

R1 = 470 R

D1-D5 = 1N4148

R2, R3 = 100 k; 1%

D6 = LED (green)

:R4 = 1 M

D7 = 5V6; 400 mW zener

R5 = 100 k

D8 = 1N4001

R6 = 8k2

T1 = BD679

R7 = 10 M

IC1 = LM324

R8 = 10 k

:R9 = 270 R

Miscellanmeous:

R10 = 3k3

PI, P2 = 5 k preset

Rll = 22 k

Mains adaptor 12 V, 250 mA

R12 = 270 k

Rel = relay, 12 V, 250 mA

Enclosure (e.g. OKW2492)

Capacitors:

Cl = ln5

C2 = 100 |i; 16 V

9.100 elektor India September 1989

not become energized.

When a battery is connected to the
charging terminals, the output of A2
goes high. As long as the battery voltage
is lower than 13.8 V, the output of A1
remains high. The relay is then energized
and charging begins. The relay should be
of a type that can switch currents of at
least 5 A: a vehicle type is ideal. As soon
as the battery voltage gets too high, the
output of A1 goes low and the relay is
deactuated. Since the charging current
into a fully charged battery is small, the
switching of the relay creates no prob-
lems.

The add-on unit may be built on the
PCB shown, which is, however, not avail-
able ready made. Drawings on paper or
film may be obtained at low cost - see
the Readers' Services section towards the
end of this magazine.

Alignment of the circuit is fairly sim-
ple. The value of the reference voltage is
set by PI and is correct when the output
of A4 is exactly 6.9 V. Since resistors R2
and R3 have the same value, the add-on
unit switches off when the battery volt-
age is exactly 13.8 V. Resistor R5 and
diode D3 provide some hysteresis whose
magnitude may be set by P2.

After the reference voltage has been
set, the battery may be charged until the
relay is de-energized. Then, adjust P2
until the voltage at pin 3 of A1 is exactly
6.3 V. This ensures that charging begins
when the battery voltage drops below
12.6 V.

(H. Huynen)

—

GENERAL INTEREST

SMOKE DETECTOR

One of the better ways of detecting smoke is the use of an ioniza-
tion chamber. Unfortunately, these devices are not without danger
because they contain a small amount of radioactive material. They
should, therefore, never be opened for whatever reason. If they
are defect, DO NOT put them in the dustbin, but take them to one of

v DO

Fig. 1. Circuit diagram of the MCI 4467

the special council depots for dangerous waste materials.When
you buy one of these chambers, make sure that is of an approved
standard.

Apart from the small amount of radioactive material, the
chamber contains two electrodes, one of which is normally

formed by the enclo-
sure - see photograph.
When the air is not
contaminated by
smoke the resistance
between the electrodes
is high; when smoke
particles enter the
chamber, the resistance
drops. The same parti-
cles cause a charge
between the elec-
trodes, so that a cur-
rent begins to flow
between the elec-
trodes. This current is
minute so that the
electrical connection
between the chamber

elektor india September 1989 9.101

and the circuit is critical. Normally, the ionization chamber is
quite separate from the printed-circuit board or the board is pro-
vided with a track around its perimeter that is at the same poten-
tial as the chamber. That arrangement prevents leakage currents
to other tracks on the board. The potential may be measured at
low-ohmic pins 14 or 16, but not at high-ohmic pin 15,

The necessary electronics is usually housed on one chip, for
instance. Motorola's Type MCI 4467. The internal oscillator of this
IC provides the timing, which may be altered by changing the
values of R2 and C3. Resistor R1 is the normal bias resistor for D1
through which a current of about 10 mA flows at every 24th clock
pulse. This current is also used for monitoring the state of charge
of the battery. This is the reason that the current must not be low-
ered, because then the battery test becomes unreliable.

Testing the 1C is rather tricky, because, in order to economize
on current, it switches itself on for only 10 ms every 1.67 s: during
that period it draws about 50 pA plus the led current. However,
the IC may be made to work continuously by connecting pin 12
temporarily to the 0 V rail.

When the alarm sounds (smoke!), the voltage at pin 13 drops
to about 0.1 V. The alarm may be tested by pressing switch SI.

(Source: Motorola)

VFO STABILIZER FOR UP TO 100 MHZ

Tire stabilizer presented here enables the precise tuning of h.f. os-
cillators if these have a control input. That input is normally used
for varying the capacitance of a varactor.

The signal at the input is amplified by a fast operational ampli-
fier, IC1. Tire output of this opamp is a rectangular signal that is
applied to the D input of bistable FF1. The clock input of the
bistable is provided by generatolr IC3. The two outputs of the
bistable are the product of the dock and the input signal. The fre-

quency of this composite signal lies between 0 Hz and half the
dock frequency. To ensure thg best possible control characteristic,
the output signal of the bistable is compared with a reference sig-
nal that has a frequency one quarter that of the clock. To that end,
a second bistable, FF2, is connected as a binary scaler; its input is
provided with a signal whose frequency is half that of the clock
applied to FF1.

The differentiating network at the output of FF1 uses only the

9.102 elektor india September 1989

Parts list

Resistors:

Cll = 68 p

RlyRll, R13 = 1 k

02, 03 = 1 p (MKT)

R2 = 10 k

04, 05 = 47 p; 16 V

R3 = 2k2

07 = 100 n (MKT)

R4-R7, R10, R12 = 1 M

R8 = 10 M

Semiconductors:

R9 = 220 k

D1-D5 = 1N4148

R14, R15 = 680 R

D6 = led (two colours)

Capacitors:

Cl = 22 n (ceramic)

1C1 = pA 733

IC2 = 74F74 or 74S74 or 74AS74

IC3 = 4060

C2 = 4p7; 16 V

IC4 = LF411

C3 = 100 n (ceramic)

IC5 = 78L05

C4 = 10 n (ceramic)

C5-C8 = 470 p (polyester)

Miscellaneous:

C9 = 60 p trimmer

SI = single-pole switch

negative pulses of the output signal, whereas that at the output
of FF2 uses only the positive pulses. All these pulses are com-
bined in an integrator, resulting in a stable voltage. Since both
the Q and the Q output are used, the ripple is halved.

If the frequency of the input signal is not stable, the ampli-
tude of the integrated signal varies. The variations are used 'to
control the oscillator in a manner where the deviations are
negated.

The clock is constructed around a 4060 and an inexpensive
watch crystal. That crystal may, of course, be replaced by a dif-
ferent type, as long as this has the required stability.

The clock frequency, and thus the required grid, is set with
the aid of jump leads. The frequency on row B must always be
half that on row A.

The construction and alignment should not present any
undue problems if the circuit is built on the ready-made i*cb.

The oscillator is set to exactly its centre frequency by C9:
this can be verified at test point Tp, which carries the buffered
clock frequency.

The circuit is powered by a 12-V supply that is brought
down to 5 V and stabilized by regulator IC5.

Indicator D6 remains out as long as the oscillator frequency
is stable. If the frequency drifts, the led lights: its colour and
intensity indicate in what direction the drift occurs and how
serious the drift is.

The integrating action may be disabled by SI, which
enables the circuit to settle down more rapidly than with it on.

—

TEST & MEASUREMENT

CRYSTAL TESTER

In this easy-to-build tester, an led indicates whether a crystal
oscillates or not. The unit is best constructed on the printed-circuit
board, which is, however, not available ready made. Drawings
can be supplied on paper or film at low cost, however - see the
Readers Services section towards the end of this magazine. As
always in h.f. circuits, all wiring, terminals and pins should be
kept as short as feasible.

Although die tester was designed primarily for use with fun-
damental-frequency crystals between 1 MHz and 30 MHz, the
prototypes worked satisfactorily with overtone crystals also.

Many crystals in the range 1-4 MHz oscillate more readily

when switch SI is closed.

The current drawn by the tester is typically not greater than 30
mA at 9 V.

Parts list

Resistors:

R1 = 10 k
R2 = 390 R

CapacitorsCl, C2 = 100 p
C3, C4 = 1 n
C5 = 470 n

Semiconductors:

D1 = led (red) 3 mm
Tl = BF199

Miscellaneous:

Ll = lmH
SI = 2-pole switch
Crystal socket

elektor india September 1989 9.103

N1...N6 = IC1 = 74HCT04

N7 = IC2 = 74HCT30

VOLTAGE TRACER IN SMT

A voltage tracer is a handy tool at any time and when it is made since it is then easily carried around in one's pocket. Basically, a
in surface-mount technology - smt - it becomes even more useful, voltage tracer is a small voltmeter whose indicator is an led. Both

9.104 elektor india September 1989

087

'

SMALL I/O CARD

COMPUTERS

Although the I/O card presented here is
small, it offers no fewer than 24 i/o lines for
a number of applications. If the computer
has enough slots, up to eight of the cards
may be installed.

The circuit is fairly simple: an address
decoder and an i/o chip. The addresses are
set out in the table. The circuit is fitted on a
double-sided printed-circuit board that is
just a little wider than connector Kl. The
amount of unused space on the board is
filled with soldering pads for use by those
who are keen on experimenting. Make sure
when fitting the board that connector Kl
faces outwards through the opening in the
PC enclosure.

To keep costs down, the I’CB, although
double-sided, is not through-plated and it
is, therefore, advisable to solder IC1-IC3
direct on to the board. A holder may be used
for IC4, but even then the pins have to be
soldered at both sides of the board.

Owing to lack of space, it is not possible
to give the programming information for the
Type 8255 l/o chip here, but that is, of
course, given on the data sheets for the
device. It is also contained in, among others,
our Data Sheet Book 2

(S. Mitra)

EUS Table

Address range
(hexadecimal)

Jump leads

i 300-303

B, D, F

304-307

A, D, F

308-30B

B, C, F

30C-30F

A.C.F

\k 310-313

B,D,E,

\% 314-317

A, D, E

318-31B

B, G, E

h 31C-31F

A,|E X

L— ii

-:L'. iwuij

Parts list

;C1. C2 = 100 n

flCI = 74HCT04
i IC2 = 74HCT30
IC3 = 74HCT32
104 = 8255

Kl * 34-pin right-angled
male header
PCB 894005

TEST

& MEASUREMENT

Vi'::' >'»

the value and the polarity of the voltage under test are indicated.
If the main prong, that is, the one connected to the sharp end of
the pcb, is negative with respect to the auxiliary prong, only the
-led lights. If, however, a positive voltage is measured, one of the
five +i.EDs lights. Which one depends on the magnitude of the
voltage. If an alternating voltage is measured, the -led and +leds
will light simultaneously.

The actual measurement is carried out by an hcmos ic. As soon
as the input voltage of one of the gates exceeds half the supply
voltage - which is regulated by Tl, T2 and D1 - its output goes
low. Note that this does not happen with hct or ls types: these
are, therefore, not suitable for this circuit.

The LEDs are connected to the output in a manner that ensures
that only one of them lights at any one time. The low supply volt-
age for the ic and the current-source characteristic of the outputs
render current-limiting resistors for the LEDs superfluous. The
switching thresholds have been chosen to coincide with frequent-
ly encountered supply voltages.

The printed-circuit board, which is not available ready made,
is designed for surface-mount components, although it will
accommodate a number of traditional components. Drawings of
the board are available on paper or film at low cost - see the
Readers Services section towards the end of this magazine.

If the Type BFR101 B SMT FET is not obtainable, a (conventional)
BF256A may be used. The smt type is placed alongside Dl l with
its gate (the slightly broader terminal) opposite the cathode of the
diode. The BFA256 is placed on its flat side alongside Dll: its pins
should be soldered to the pads between Tl and D3.

The board itself forms the main prong and may, for instance,
be potted in exposy resin to give a sturdy, well-insulated unit. The
auxiliary prong is connected to the round pad at the square end of
the board.

The circuit is designed for measuring only voltages that do not
exceed 30 V.

Resistors
R1-R6 = 100 k
R7 = 220 k
R8 = 390 k
R10 = 680 k
Rll = 47 k
R12 = 100 R

Parts list

Semiconductors;

D1-D4 = BAV100
D5-D10 = LED (red)

Dll = 3V9, 500 mW zener
Tl = BC847B
T2 = BFR101B (see text)
IC1 = 74HC04 (not HCT)

LIQUID-LEVEL MONITOR

The monitor is based on an L4620 from sgs. The internal oscillator
of that device, in conjunction with C osc and R osc , generates a rec-
tangular signal at 1.6 kHz. This is divided by 32 and the resulting
50 Hz signal is applied to pin 2 of the ic via the sensor driver.

The sensor proper is a humidity-dependent resistor. This may,
for instance, consist of two wires suspended in a vessel in a man-

ner that when liquid being poured into the vessel exceeds a cer-
tain level, or liquid being taken from the vessel drops below a cer- '
tain level, pin 3 is short-circuited to earth. The rc network
between pins 2 and 3 is a simple band-pass filter with a centre fre-
quency of 50 Hz. This filter is needed only where exact level indi-
cations are wanted: in most other cases, Ca and Cb may be

elektor india September 1989 9. 1 05

replaced by a wire link.

The sensor interface receives a 200 Hz signal from the oscilla-
tor and compares the level at pin 3 with a reference voltage
whose value depends on the logic state at pin 2. When that pin is
low, the reference voltage is O. 2 U 2 ; when it is high, the reference
voltage is O. 4 U 2 . When these levels are exceeded, the interface
passes a signal to the sensor polarity stage.

The alarm is actuated by this signal in two ways: (a) when pin
8 is high and the level at pin 3 is higher than the reference voltage;

(b) when pin 8 is low and the level at pin 3 drops below the refer-
ence level. All this happens only when the alarm state signal from
the sensor interface is constant for not less than ten seconds with
pin 7 low, or than 20 seconds with pin 7 high.

In the alarm state, the output at pin 6 can provide a current of
up to 300 mA.

The current drawn by the circuit depends on the supply volt-
age, which may be 5-28 V: at 5 V, it is typically 6.4 mA.

‘ON’ INDICATOR FOR GAS-OPERATED FRIDGES

This indicator is intended primarily for refrigerators used by
campers or caravan owners. Whether the glas flame is on may be
indicated with the aid of a thermocouple, a device that reacts to
variations in temperature. It normally consists of two dissimilar
metals - here iron and constantan - soldered or welded together
at one end.

The e.m.f. produced across terminals A and B when the tem-
perature at the weld varies is applied to the non-inverting input of
comparator IC1. Here, the e.m.f. is compared with a reference
voltage of 0.7 V applied to the inverting input of the opamp.

When the temperature at the weld of the thermocouple is
about 150 °C, the e.m.f. across terminals A and B is also 0.7 V. The
comparator then toggles and causes the led to light, indicating
that the gas flame is on.

The iron terminal must be soldered to A and the constantan
terminal to B.

(U. Miinch)

9.106 elektor india September 1989

AUTOMATIC FOG HORN

An interesting aspect of the fog horn described here is that it may
be constructed for real-size as well as for model boats.

The circuit can emit automatic warning signals in the form — ,

> — • *, and • • • • • at 2-minute intervals. Other signals may

be given manually.

The circuit is based on a combined CMOS oscillator-binary
counter Type 4060. Apart from the clock for the warning signals,
this ic also provides the intervals between successive warning sig-
nals.The rc oscillator is configured to generate low frequencies.
Counter output Q8 is reset to zero every 256 cycles.

The signal generator, 1C1, is driven by both the clock and the
reset signal from IC2. Basically, it is a l-in-10 counter running syn-
chronous with the clock, so that one of its outputs as always 1. A
reset signal makes Q0 logic high.

The clock enable input, when high, that is, when Q9 is high,
stops the counter instantaneously.

When the power is switched on by SI, a reset signal is given
simultaneously to both lcs to ensure synchronicity. The 4017 then

begins to pass its signal sequence to the diode matrix. Each single
pulse lasts for one second (= dot). The dash in the signals consists
of three fused dots. If, for instance, the switch is set to — • •, Q0,
Q1 and Q2 each output a single pulse: the three pulses are com-
bined to a dash. The pulse output by Q1 to the diode matrix also
becomes available via Q4 and Q6 as two dots. Outputs Q3, Q5, Q7
and Q8 are not used with this switch position. Output Q9 holds
the circuit in this state until 120 seconds later a reset from IC2
starts the foregoing sequence afresh.

Transistor T1 amplifies the signal from Sla to a level suitable
to operate a buzzer or the horn relay via switch S2. The relay con-
tacts must be able to switch currents of a few amperes.

The power supply should be between 12 V and 30 V, which is
reduced to 10 V and stabilized by 1C3. The quiescent current
drawn by the circuit is very small. In fog, however, it is essential
that the battery is charged continuously

(P. W. Rutters)

HEADPHONE AMPLIFIER WITH SCART PLUG

The headphone socket of most television receivers is coupled
direct to the loudspeaker, so that the headphone volume can not
be adjusted separately. This is a serious drawback for the many
viewers who are hard of hearing or who want to listen to a pro-
gramme in high ambient noise (family gatherings!).

The amplifier described here, although designed primarily for
use with a television receiver, is also very well suited to use with a
hi-fi installation. It provides independent controls for volume, bal-
ance, and bass and treble for each (stereo) channel.

The amplifier is de gned around two tone-control lcs Type

elektor india September 1989 9. 1 07

TDA4290-2 and a stereo amplifier 1C
Type TDA2004.

The TDA4290-2 ICs have d.c.-operat-
ed volume and frequency controls. They
need few external components and offer
low-distortion and low-noise operation.
Capacitors C2 and Cll determine the tre-
ble cut-off frequency, and C5 and Cl 4,
the bass cut-off frequency. The volume
control voltage is applied to pin 5.
Furthermore, at output pin 2 a reference
voltage is available for modifying the
potential at control inputs pins 5, 8 and
14. The output signal is available at pin 6,
from where it is applied to the output
stage via an rc network.

Like the TDA4290-2 lcs, the TDA2004
needs only a few external components to
determine the stage gain (R13-R14 and
R20-R21: about 40 dB); the bandwidth
(R12 and R19: about 22 kHz); and protec-
tion against short-circuits and inductive
loads.

Because of the asymmetrical power
supply, electrolytic capacitors C25 and
C27 are essential.

Resistors R23 and R24 serve to match
the input sensitivity and headphone
impedance to the amplifier: some experi-
mentation with their values may be nec-
essary.

Resistors R17 and R18 enable the elec-
trolytic capacitors to charge, even with-
out load, which prevents clicks in the
headphones when these are plugged in.

Some technical data (measured with
an output voltage of 1 V at 1 kHz): distor-
tion <0.03%; -3 dB bandwidth 10 Hz to
22 kHz; signal-to-noise ratio 70 dB. The
bass and treble controls have a range of
17 dB and 20 dB respectively.

The tone control ics are provided with
a physiological volume control: to bring
this into operation, simply interconnect
pins 2 and 4.

A power supply of 11-18 V is re-
quired. The current drawn by the ampli-
fier with a supply of 15 V is 150-200 mA.

Semiconductors:

IC1, IC2 = TDA4290-2

IC3 = TDA2004 (with heat sink)

(H. Burchardt)

Parts list

Resistors:

Rl, R6 = 100 k
R2, R7 = 2k7
R3, R8, R17, R18 = 1 k
R4, R9 = 22 k
R5, R10 = 820 a
Rll = 47fi
R12, R19 = 390 Q
R13, R20 = 2k2
R14, R21 = 22 Q
R16 = 120 k
R23,R24 = 33Q

P1-P6 = 10 k linear potentiometer
P7, P8 = 22 k linear potentiometer

Capacitors:

Cl, CIO, C32, C33 = 1 pF, 16 V
C2, Cll = 6n8
C3, C12 = 330 pF

C4, C9, Cl 3, Cl 8, C28 = 10 pF, 16 V

9.108 elektor india September 1969

C5, C6, C7, Cl4,

C15, C16, C23, C31 = 100 nF
C8, C17, C21, C30 = 3n3
C19, C22, C24, C26,

C29 = 100 pF 16 V
C20 = 1000 pF, 16 V radial

DIGITAL TRIGGER FOR OSCILLOSCOPES

The circuit described here enables an oscillo-
scope to be triggered when a pre-determined
binary code word is applied to one of the cir-
cuit's inputs.

Integrated circuits IC1 and IC2 compare the
sixteen inputs with the code set by switches SI
and S2. If one of the inputs has a data word that
is equal for not less than 100 ns to that set by SI
and S2, pin 19 of IC1 goes high. Note that,
because of the pull-up resistors, open inputs are
treated as high.

When pin 19 of IC1 is high, monostable

MMV2 is triggered and outputs a negative
pulse from its pin 4 that, depending on the set-
ting of PI, is 0.1-1.5 |is long. If during that time
the pre-determined trigger value disappears, no
triggering takes place. Potentiometer PI is a
logarithmic type to enable very short times to
be set accurately.

The output pulse from MMV2 triggers a sec-
ond monostable, MMV1, whose mono time has
been set to 1 (is by R23-C3.

Either the positive signal from the Q output
or the negative signal from the Q output.

n rpi X s f

6

5

4

3

2

10k

0

14

6

Ifi

4

IB

^ '0» 1

J

-1

P7

G

07

P6

06

P3

05

P4

IC1

04

74HCT

P3

688

03

P2

Q2

PI

Q1

P0

P»Q

OO

I ’ 9

S cJ_ P

iC3 IC2 IC1 =

( “)©(«)

MMV1.MMV2 - IC3 = 74HCT123

depending on the setting of switch
S3, may be applied to the oscillo-
scope.

Parts list

Resistors:

R1-R16 = 10 k
R17-R20 =100 k
R21, R23 = 2k2
R22 = 470fl

PI = 100 k log potentiometer

Capacitors:

Cl = 10 pF
C2 = 47 pF
C3 = 1 nF
C4 = 100 nF

Semiconductors:

IC1, IC2 = 74HCT688
IC3 = 74HCT123

Miscellaneous:

SI, S2 = octal dip switch
S3 = SPDT switch
K1 = bnc socket
18 crocodile (test) clips
Enclosure, e.g. OKW A9010 065

SYMMETRICAL POWER SUPPLY

Integrated circuit L16o makes it possible for a 6-40 V asymmetri- of ±3 V to ±20 V. Only a few external components, like some elec-
cal voltage to be converted into a stabilized, symmetrical supply trolytic capacitors for smoothing purposes, are needed. Capacitors

elektor India September 1989 9.109

6...40V 3...20V

Cl and C2 should preferably be fitted as dose to the tc as possi-
ble, while C4 and C5 should be soldered to the output terminals.

Since the circuit can deliver currents of up to 3 A, heavy-duty
wiring and a heat sink for the ic are essential.

The LI 65 may be replaced by a TCA1365, but that does not fit
on the printed-circuit board. If that chip is used, its pins 3 and 4
must be strapped together, and a 220 pF capacitor must be fitted
between its pins 5 and 6.

(Source: Siemens)

Parts list

Resistors:

Rl, R2 = 22 k
R3 = 1 £2
R4 = 22 k
R5 = 1 k

Semiconductors:

IC1 = L165 or TCA1365

Capacitors:

Cl =100 pF, 40 V
C2 = 100 nF
C3 = 220 nF
C4, C5 =10 pF, 25 V

Miscellaneous:

Heat sink for L165 or TCA1365

AUTOMATIC PRINTER SHARING

The title of this article is a slight misnomer, because the circuit
described here does not only allow two computers to share one
printer, but it also enables two printers to be connected to one
computer.

Basic operation of the circuit is illustrated in Fig. 1 . If it is used

Fig. 1. Block schematic of the printer sharing unit

9.110 elektor india September 1989

Parts list
Resistors:

Rl, R17-R21, R37-R40,
R56-R59 = 1 k
R2-R16, R22-R36, ,

R41-R55, R60, R65 = 10 k
R61, R66 = 100 £2
R62, R63, R67 = 1 M

Capacitors:

Cl, C4 = 22pF, 16 V
C2, C3, C5-C9=100nF

Semiconductors:

!C1— 1C4 s 74HC645
IC5 = TLC556
T1,T3= BC557B
T2, T4 = BS170
D1,D2 = UED

Miscellaneous:

51 = press-button switch
(1 make contact)

52 = change-over switch
K1-K3 = 36-pole male

header

PCB8S4082

Fig. 3. Circuit diagram of the printer sharing unit

to have two computers share one printer, switch S2 may be omit-
ted since that serves to choose between two printers. As soon as
one of the computers starts printing, the associated retriggerable
multivibrator is triggered. At the same instant, the electronic
switch via which the data must proceed is closed and the busy
input of the second computer is made high. Because the mmvs are
retriggerable, this state will persist for as long as the selected com-
puter continues printing plus 30 seconds (that is, the mono time).
Which printer is selected is indicated by an led.

Unfortunately, the automatic selection may give rise to prob-
lems if you use a program that, during printing, needs more than
30 seconds to compute new printer data. The simplest solution to
this is to come to an understanding with the user of the second
computer. It is also possible to lengthen the mono time by increas-
ing the values of Cl and C4, but that has the disadvantage of
making the switch-over very slow.

If two printers are to work with one computer, the mmvs are

replaced by change-over switch S2.

The circuit diagram in Fig. 3 looks more complex than it is,
mainly because of the large number of pull-up and pull-down
resistors.

Wire links A or B are present at various places: they determine
into which direction data are passed. When links A are used, the
direction is from two computers to one printer, whereas if links B
are fitted, the direction is from one computer to two printers.

The circuit is powered by a printer: depending on the setting,
this is the printer connected to K2 or K3. Note that if only one
printer is used, this can not be connected to Kl.

Also dependent on the setting are wire links C to H, which are
placed only if links A are fitted. Otherwise, S2 is fitted and the cir-
cuit to the left of links C to H is omitted.

Reset switch SI is provided to stop the printer in an emergen-
cy.

Because of the use of hcmos ics, the circuit draws only 50 mA.

elektor india September 1989 9. 1 1 1

VOICE-BAND FILTER

Using only four opamps, this
filter provides a sharp cut-off
profile: -3 dB at 300 Hz and
2,800 Hz and -40 dB at 100 Hz
and 4,000 Hz. Since the attenu-
ation outside the pass band is
at least 50 dB, the filter is emi-
nently suitable for use in, say,
direct-conversion receivers or
as an anti-aliasing network.

Fundamentally, the circuit is
an LC filter of which the induc-
tances are simulated by opera-
tional amplifiers (for a more
detailed discussion of this type
of filter, see Ref. 1 and Ref. 2).
Note that the opamps affect the
filter characteristics: the res-
ponses mentioned earlier were
achieved with a TL084.

Amplifier IC1 serves merely
to compensate losses in the fil-
ter.

Ref. 1. “Filters Theory & Prac-
tice- 3“ Elecktor India.
Novemebr 1987 p. 11-30

Ref. 2. “The positive Impe-
dance converter” Elektor
India November 1987 p. 11-50

SLIDE FADER UPDATE

The "Computer-controlled slide fader" we published last year
(Ref. 1 ) was based on dimmer chip Type TCA280. In spite of this
device being a Philips Components preferred type at the time, we
learned from a number of readers that this device was virtually
unobtainable. When approached. Philips Components admitted
that they had taken the TCA280 out of production without prior
warning, and that no pin-compatible replacement was available.

It has taken us some time to find a suitable replacement and
have found that the TCA785 from Siemens is a good, but not pin-
compatible, substitute.

The vsync input is provided with a 50 Hz square wave, which
is used internally for mains synchronization. The ic is powered
via R2, D3, Cl and zener diode D4.

An internal current source, set by PI and R5, causes a linearly
rising voltage across C3. At each mains zero crossing, C3 is dis-

9.112 elektor india September 1989

charged rapidly, so that the potential across it has a sawtooth
waveform. The amplitude depends on the setting of PI.

The sawtooth voltage is compared with a control voltage that
is applied to pin 11 of IC1 via filter R3-C2. If the sawtooth voltage
rises above the control voltage, a pulse is generated at pin 14 or
15, depending on at which part of the cycle the mains voltage is.
The two outputs are connected to a triac via diodes D5 and D6,
and resistor R6, which enable the triac to be triggered. The instant
that the triac begins to conduct is, therefore, dependent on the
control voltage at the input, resulting in a voltage-controlled fader.
The control voltage may be provided by the slide projector or a
potentiometer. In the latter case only, it is also possible to fade 12-
V halogen lamps. Zener diode D4 then needs to be replaced by an
8.2 V type.

The fader is aligned by adjusting PI in the off condition, when

Parts Hat
Resistors:

R1 = 27k
R2 = 330£2
R3 = 2k2
R4 = 10 k
R5, R7 = 4k7
R6 = 150ii
PI » 100 k preset
P2 = 10 k

Capacitors:

Cl = 470 pF, 25 V
C2,C3 = 100nF
C4= 150pF

Semiconductors:

IC1 * TCA785

Tril = TIC236 or TIC 246

D1, D2, D5, D6 = 1N4148

03 = 1N4001

D4 = zener 15 V, 1 W

Miscellaneous:

SI =spst switch
PCS 894078

the control voltage is at a maximum, till the lamp just glows.

The slide projector is aligned by setting the relevant poten-
tiometer on the projector pcb to the centre of its travel, when
the lamp(s) should be out.

When that is done, the lamp(s) should be switched on and
off a couple of times to make sure that the two potentiometers
(PI and that on the projector board) are adjusted correctly.

Since the control characteristic of the TCA785 is different
from that of the TCA280, it is not advisable to mix the two
devices.

When SI in the present fader is closed, the projector lamps
are off: in other words, for normal operation, SI must remain
open and it may, therefore, in some cases be omitted.

Ref. 1 . Elcktor India May & dune 1988

REFERENCE-VOLTAGE SOURCE WITH INDICATOR

A novel use for a Type LM3914 display driver is described in this
article. Since the ic is normally used as an indicator driver for
analogue circuits (vu meter, current indicator, and so on), it has an
internal, very stable 1.25-V reference source. The reference voltage
is available at pin 7 and may be set
anywhere between 1.25 V and 16 V
by multi-turn potentiometer PI. It is
assumed here that the supply voltage
is at least 18 V. The reference voltage
is calculated from:

U ref = 1.25(1 + Pi; R5] + 75P1x10' 6 (V)

Tire circuit is calibrated by setting
PI to give a reference voltage of
exactly 15.0 V, and then P2 to make

LED

UrelOO

1

1. 5-3.0

2

3.0-4.5

3

4.5-6.0

4

6.0-7.5

5

7.5-9.0

6

9.0-10.5

7

10.5-12.0

8

12.0-13.5

9

13.5-15.0 j

10

15.0...... 1

elektor india September 1989 9,1 1 3

GENERAL INTEREST

LED10 just light. The other leds light in accordance with the set
reference voltages shown in the table.

' The circuit can deliver a current of up to 3 mA: if higher cur-
rents are wanted, it should be followed by a buffer opamp. The

circuit can then be used as a simple variable current source with
voltage indication.

As shown, the circuit draws a current of about 30 mA with a
supply voltage of 20 V.

TIME-DELAYED FLASH

The photographing of falling water drops, bullets that tear
through balloons or playing cards, and other happenings for
which our human reaction powers are too slow, remain a fascinat-
ing aspect for many photographers. That the means therefor need
not be expensive is explained in this article.

To start with, a light barrier is needed where the fast-moving
object can be registered. It will also tell us when the object arrives
at the place where it is to be photographed. The time needed
therefor is bridged by a time delay circuit.

The circuit of the light barrier is shown in Fig. 1, where D1 and
T1 form the light barrier proper. Transistor T2, resistors R3-R5
and capacitor Cl serve to minimize the effects of ambient light
and mains hum. Opamp A1 buffers the signal from the light bar-
rier before it is applied to pulse shaper A2. The circuit is very sen-
sitive, although this may largely be overcome by earthing the
common supply rail.

The pulse from the light barrier is delayed by the circuit shown
in Fig. 2. First, the pulse is used to drive an l ED , which facilitates

calibrating the circuit. The pulse also

clocks a data bistable, FF1, which is
configured as a monostable and

determines the time delay. To enable i7) j .

the delay to be set very precisely, svV,

two potentiometers are used: PI T

(fine) and I’2 (coarse). The setting i f

can be made even more precise by D FF

using a multi-turn potentiometer for r\pv_a_ 3 clk

I’l. I _

After the time delay has lapsed,
the Q output of FF1 clocks bistable PH HM

FF2, which is also configured as a ^ ,

monostable. Immediately on being Gf/

clocked, FF2 generates the ignition 0- g I** 05478

pulse for the flash gun. This pulse is
applied to the flash gun via T2 and

BC547B»5|

BP103

Fig. 1. Circuit diagram of the light barrier

TIC106

894033 • 12

Fig. 2. Circuit diagram of the delay unit

Thl = T1C106
01. LED

Semiconductors:

IC1 = TLC272
T1 . BP103
T2 = BC547B
01- LED, red

Miscellaneous:

K1 « phono socket

Light barrier
Resistors:

R1 -330£l
R2 = 10k
R3.10M
R4-R7 =220k
Pi = 10 k preset

Semiconductors:

IC1.7805

102-4013

T1.T2-BC547B

Capacitors:

Cl = 10 pF, 16 V
C2 = 47 nF

9.114 elektor indie September 1989

Opr

A1.A2:

thyristor Thl.

Both circuits are housed on a printed-cir-
cuit board that, if desired, may be cut into
two. The long part of the board is intended for
the light barrier circuit and the almost square
part for the delay circuit. Note that since the
design started as two independent projects,
there are two resistors Rl, two capacitors Cl,
and two circuits 1C1 .

A problem with electronics for photogra-
phy remains the flash gun connexion, for
which the parts are difficult, and in most
places impossible, to obtain. For our proto-
types, we used high-quality phono connectors and audio cable,
but for the connexion to the flash gun itself, we fabricated a diy
adaptor from a phono socket, as shown in Fig 4, and a flash gun
socket that was cut off an extension cable.

CALIBRATION (Electronics)

The light barrier must be calibrated in total darkness wit the aid
of the led on the delay board. Adjust PI until the LED just goes out
(= no object in the light barrier). At that point, sensitivity is at its
highest. If the circuit proves very susceptible to interference, con-
nect the common power rail to a really good earthing point.

Once that is done, check with PI and P2 set at minimum resis-
tance (=time) whether the flash gun will work when a water drop
falls through the light barrier. If yes, the drop will be seen sus-

pended in the barrier (it is, of course, still
pitch dark in your work room). If no, the flash
gun connexions on K1 must be reversed.

Once the water drop is suspended in the
light barrier, its position can be shifted, if
desired, by PI and P2 to where the photo-
graph is to be taken.

CALIBRATION (Photographic)

The shutter of the camera must be set to the B
position before photographing the drop of
water, since no camera can react fast enough at
normal settings. At the flash gun, check
whether the automatic exposure facility covers the correct expo-
sure for the photograph: this is, of course, dependent on the back-
ground and on the shortest distance at which the flash computer
still works. Place the flash gun as close as possible to the object,
because the flash duration is then as short as possible.

If you do not trust the automatic exposure, set the flash gun to
manual and ascertain from the handbook how long the flash lasts
- this is often longer than you may expect.

Next, measure the distance in metres between the flash gun
and the object and divide that by the guide number of the flash.
Set the stop so obtained on your camera (rounded off upwards for
slides and downwards for negatives).

The current drawn by the entire circuit amounts to 20-30 mA.

R-2R RESISTANCE NETWORK IN SMT

This article describes an interesting sil R-2R network for use in,
say, a digital-to-analogue converter. The network is constructed in
surface-mount technology (smt) and is inexpensive, precise,
space-saving and allows unusual values to be obtained. It is
described here in a digital-to-analogue converter that uses a cmos
latch Type 4042.

The latch and the associated R-2R network are connected in the

sil

feedback loop of an LF356. The network uses 100 k!2 resistors (the
2R resistors are made up of two series-connected 100 kS2 resistors).
The output voltage (in V) of the converter is calculated from:

U Q = U r (R-i + P : ) / 6R(2°Q3 + 2 _1 Q2 + 2" 2 Q1 + 2- 3 Q0).

The Qs in the formula have a value of 0 or 1, depending on
the state of the latch output. The factor (Rl + PI) / 6R is the
amplification A, which can be set between 0.4 and 2.0 by PI.

The supply voltage for the latch, which is also the refrence
voltage for the network, U p may be between 5 V and 15 V: the
logic levels at latch inputs D0-D3 and at clk must be in accord
with that voltage. The reference voltage must be decoupled direct
at the ic input.

The circuit is calibrated by making the inputs of the latch 0 and
adjusting P2 for an output of 0 V from the opamp. Then, load the

elektor indta September 1989 9.1 15

data inputs-with Fj lex and adjust PI to obtain maximum output
voltage.

If a larger number of bits is to be processed, two or more PCBs
may be connected back-to-back.

Moreover, the pcb may be used with other components, such

as diodes, capacitors, combinations of resistors and diodes or sim-
ple potential dividers for measuring instruments.

The current drawn from the Ur supply is 75 pA; that from the
5 V supply is about 7.5 mA.

(H. Bierwith)

BREAK-JACK ADAPTOR

A break-jack adaptor is a means of connecting, say, effects units to
the 'send' and 'return' sockets of an output amplifier. In fact, the
adaptor is vital there, because the levels of the effects unit(s) and
amplifier are normally not compatible. Without its use, overdriv-
ing and noise would be the consequences.

The adaptor converts the connexion from a passive into an
active one, which is possible with any output amplifier. For that
purpose, it uses two voltage amplifiers, A1 and A2. The gain or
attenuation of these is set complementary to one another by
stereo potentiometer PI.

If we assume that the wiper of PI is at the centre of its travel,
the opamps have neither gain nor attenuation. In other words, in
that situation an effects unit at the input would behave exactly as
if there were no adaptor. If then PI is turned clockwise, the input
signal is amplified. If PI is turned anti-clockwise, A1 attenuates
the signal, whereas A2 amplifies it.

Effects units with levels ranging from 0.5 V to 1.5 V may be
used. Lower levels can be accommodated by increasing the value
of R2 and R6 to 33 kU maximum. The 'neutral' position of PI is
then no longer at the centre of travel of the wiper, but at about
three quarters of the way. The level range is then limited to about
±3 dB (=x2).

(W. Teder)

15V

POWER SUPPLIES

'■

MAXIMUM/MINIMUM VOLTAGE INDICATOR

Circuits Type TL430 and TL43I from Texas Instruments are active
zener diodes with an integral 2.5-V reference source, comparator
and output stage. The maximum supply voltage is 30 V at
100 mA. This sort of device makes it fairly simple to construct a
variable-voltage indicator.

Resistor R1 has a value of (Uj n - 4.5) / 10: the factor 10 indi-
cates an led current of 10 mA. Depending on the setting of PI, the
led lights when the input voltage becomes too high or goes out
when that voltage becomes too low.

If you build two identical circuits with differently coloured
i.Lds, you obtain a very effective voltage monitor. The combina-
tion is calibrated for given maximum and minimum levels: within

9.116 elektor india September 1989

the correct range of levels only one led lights; when the voltage low, both leds are out.
becomes too high, both LEDs light; and when the voltage is too

(F. Roth)

DECOUPLING POWER RAILS

Adequate decoupling of the power rails of most circuits is a seri-
ously underestimated necessity. Particularly in the design of print-
ed-circuit boards, it happens all too often that at the last moment
the thought occurs that there is no or insufficient space left for
decoupling capacitors, small though these normally are. It is, of

TTL

CMOS

Fig. 1. Because of the internal 50 £2 resistor, TTL circuits have some inher-
ent short-circuit protection that CMOS circuits have not.

self - inductance too high

lower sell - inductance

Fig. 2. The self-inductance of power rails may be reduced by using two or
three rails in parallel.

course, not surprising that such negligence often results in sponta-
neous oscillations in analogue circuits and unreliable operation of
digital circuits. Especially sequential digital circuits, such as
dividers, counters and bistables are prone to these problems, the
causes of which are normally very difficult to find.

Power rails should be decoupled by a capacitor close to the rel-
evant pins of the 1C, since the rail has a certain amount of induc-
tance. Variations in the current through this inductance cause a
potential drop that manifests itself as a short pulse or spike. The
capacitor serves to buffer (that is, to minimize) the current tran-
sients.

Current transients arise, for example, during the switching of
logic circuits, since all sorts of parasitic capacitance are charged or
discharged during the change in output level. Also, just at the
instant the change is taking place, the transistor in the output
stage that switches to earth and the transistor that switches to the
positive rail are conducting simultaneously. This means that for a
very short time the power supply is short-circuited. The 50-ohm
resistors in TTL logic circuits limit the consequent short-circuit cur-
rent, but 4000 and hc(t) series CMOS circuits have no such protec-
tion. It is for that reason that cmos circuits need to be decoupled
even more effectively than TTL circuits. Note that the static current,
which is many times greater in ttl circuits than in CMOS circuits,
has no bearing whatsoever on the degree of decoupling needed.

Decoupling capacitors must be connected with their terminals
cut as short as feasible direct to the supply pins of the relevant ic.

The effectiveness of a number of standard types of capacitor is
discussed below.

• The wet aluminium electrolytic capacitor has a fairly large
self-inductance owing to its construction (rolled foil). Neverthe-
less, it performs very well as a decoupling device. Its value does
not matter much: from 1 ,uF to 10 pF are suitable values.
Disadvantages are a relatively short life and fairly high leakage
currents.

• The dry aluminium electrolytic capacitor is in the same league
as the tantalum capacitor. Its life is considerably longer than that
of the wet electrolytic type. Like tantalum capacitors, they are
excellent for decoupling but, again like tantalum types, they are
relatively expensive.

• The tantalum capacitor, although relatively expensive, is
widely used for decoupling pur-
poses, because of its excellent all-
round properties.

• The ceramic capacitor is the
decoupling capacitor par excel-
lence. It is inexpensive, has excel-
lent h.f. properties, so that relative-
ly low values (22 nF to 100 nF) may
be used, while its large tolerance
and non-linear temperature beha-
viour do not matter for decoupling.

• The metallized film capacitor
(mkt, mks, and so on) is, perhaps,
too good for decoupling purposes.
This is because the capacitor and
the self-inductance of the power
rail form an oscillatory circuit. The

Fig. 3. It is better to place the low losses of metallized capacitors
rails close together than to sep- c * use underdamping of the occa-
aratethem. sional oscillations. It is interesting

ongunstig

tT 3 sooeoo

9 OOOOOOQ

gunstlg; lage
zelfinductie

O^JflOOflOO

or 009000)

894113-13

elektor india September 1989 9.1 17

to note that the much higher losses of ceramic capacitors are, in
this respect, a definite advantage.

Some guide lines for effective decoupling are given below.

• Provide each and every PCB with its own 47 u F to 100 pF elec-
trolytic buffer capacitor.

• Both input and output of voltage regulators should be decou-
•pled by a capacitor of at least 100 nF (positive regulators) or

220 nF (negative regulators).

• Simple logic gates should be decoupled by one capacitor of
not less than 22 nF per four ics if these are close together. More
complex circuits like bistables need one capacitor for every two
ICS, while counters and dividers should have one capacitor for
each ic. Individual tcs should have a separate decoupling capaci-
tor.

In addition to the use of decoupling capacitors, the self-induc-
tance of the power rails can be reduced by two or three rails in
parallel as shown in Fig. 2. Research has shown that increasing
the diameter of the rails merely reduces the resistance, but not the
self-inductance.

Another aspect is that the self-inductance of the rails is directly
proportional to the enclosed surface. It is, therefore, better to
place them close together than to separate them — see Fig. 3.

In hybrid circuits, the analogue part may be separated from
the digital part by a choke in series with its power rail, but only if
the linear part of the circuit does not experience regular variations
in the current, because then things may get worse instead of bet-
ter.

LIGHT(S) OUT?

Described here is a useful indica-
tor to warn of lights that have
not been switched off when they
should have. It is small enough
to be fitted inside most switch
housings.

When the switch is open, the
thyristor is off, so that the bridge
rectifier is fed with only the positive half cycles of the mains via
D3. Consequently, there is no d.c, voltage across C3.

When the light is switched on, and the load current exceeds
some 2 mA, there is a sufficiently high voltage drop across D1-D2
to cause the thyristor to conduct. The full mains voltage is then
present across the bridge rectifier, C3 is charged and relay Rel is
energized. The relay may be used to switch on a visual indication
as shown here or it may be used to set off an audible alarm.

The relay should be a 24 V, 1200 Q type for pcb mounting. It
may be omitted and the led with its bias resistor connected direct
across terminals A and B.

(R. Lambach)

D3 . . . 07 = 1N4004

01/D2= 1N4004/1N5404 8M067-12

MODULAR GUITAR AMPLIFIER

A guitar amplifier may be constructed in a simple w T ay with the
aid of a Type HY83 module from ILP. The module comes complete
with an instruction manual. The complete amplifier fits very nicely
in a 2-unit high 19-inch cabinet as show’n in the adjacent photo-

graph. In the back of the cabinet can be seen the mains trans-
former, the main pcb and the power amplifier - here a Type
HY124 from ILP. At the centre of the cabinet are the reverberation
springs. The guitar amplifier module is located direct behind the

9.118 elelcor india September 1989

front panel and is mostly hidden from view by wiring. To sup-
press mains hum, it is recommended to fit a stout earth rail close-
ly along the module and the potentiometer: all earth connexions
may then be made to this rail. Note that screens must be earthed
at one side only.

The use of light, screened cable in most positions, except for
the power supply and power amplifier output, is recommended
since the connector terminals are very close to one another.

The HY83 comes complete with a front panel foil that may be
used with a 1-unit high as well as with a 2-unit high 19-inch cabi-
net.

(ILP Application)

DONALD DUCK GENERATOR

To speak like Donald Duck, you need not inhale helium: you can input signal at the relevant rectifier. This explains why the fre-
leave it to the circuit described here. quency range has to be divided into a number of ranges: the more

The voice signal from the microphone is first amplified in A1 ranges, the smaller the intermodulation distortion,

and then divided into four frequency ranges by band-pass filters Tire rectifier stages are followed by another set of band-pass

A2-A5. The four signals are passed through half-wave rectifiers filters that are tuned to twice the frequency of the filters preceding
A6-A9. During the negative half cycles, the opamps invert the sig- the rectifiers.

nals with unity gain, since the diodes then conduct. During the The four signals are then recombined to make Donald Duck
positive half cycles, the diodes are in the blocking state. speech available at the output of IC5.

The result of this is that the frequency of the signal at the junc- The circuit draws rather less than 50 mA from the 5-V supply,

tion of the diodes and the feedback resistors is twice that of the A small 5-V mains adaptor is, therefore, more than adequate.

elektor india September 1989 9.1 19

NEW PRODUCTS

Timer cum Transistor Tester
KP-982

KP-982 is a mains operated tester which
can check Timer 555 for normal opera-
tion such as specified drain, symmetrical
waveform output, dc offset etc. It can
test op. amp 741 or other similar op-
amps for input for input output faults.
Transistors can also be tested alongwith
measurement of their beta gains with the
help of coloured LEDs.

Electronic Instrument Laboratories •
B69/004, Anand Nagar • Chhatrapati
Shivaji Road • Dahisar (East) • Bombay-
400 068 •

Up-Down/Stroke Counter
(U.D.C. 500)

UDC-500 sold by BMP Marketing is a 5
digit presettable Up-Down counter with/
without memory. Memory retains cur-
rent COUNTER & RELAY status in
case of power failure. UP or DOWN
counting mode is selected automatically
attributed to forward or reverse rotation
of the spindle if used as a revolution
counter.

It can also be used as a single stroke or
double stroke counter as needed in offset
machines (textiles etc.)

UDC-500 can accept sensing inputs from
variety of sensors such as microswitch,
proximity switch, opto-sensors etc. It
finds application in winding machines -
offset machines textiles etc.

BMP Marketing Pvt. Ltd. • Lai Bunglow
• Jyoti Studio Compound • K.B.A. Irani
Bridge (Kennedy Bridge) • Bombay-400
007*

Programmable Clock Timer

ION Electricals have developed a mic-
roprocessor based real time clock with
programmable timer. Programs can be
either set for either are time operation,
daily except the weekly holidays or the
working week. The timer can store 16
programs and with external relays logic
four different electrical devices can be
automatically switched ON/OFF.

A LED display shows the time in 3 diffe-
rent modes. A six button key board on
the front panel facilitated the function of
setting the clock, changing the display
mode, entering a program can stop the
relay operation whenever desired. The
timer can be used for switching on/off
lights, sirens and operation of appliances
like heaters/air conditioners.

&

- BBS „

1 in

1 18 18

RTC-P ^ ,

SCR’s and two diodes alongwith a free-
wheeling diode. The application of half
controlled connection is that with a con-
trollable circuit when invertor operation
is not required, only two of the four arms
need be controllable and in the other tw'o
positions rectifier diodes can be used.
The advantage of a half controlled single
phase module is that the cathodes of
both the thyristors have the same poten-
tial and this simplifies the triggering cir-
cuit. Function of the free wheeling diode
by-passes the current from the bridge
components and protects them from
over-currents due to the loss of bridge
control. The full-controlled thyristor
single phase module consists of four
SCR’s designed for controlled panel op-
eration. Active components are isolated
from the body for the isolation voltage of
250C volts a.c. Number of modules can
be mounted on one heat-sink. With a
suitable heat-sink this module can draw
60 Amps current easily. These modules
find application for control of power
supplies for electronic equipment, d.c.
motors, UPS systems, controllable bat-
tery chargers, a.c. and d.c. drives etc.

SPFC 45 PD

16 A. 25 A. 35 A

FULL CONTROLLED

SCR SINGLE PHASE BRIDGE

Doshi Enterprises • 304, Goradia Houses
• 104, Kazi Saved Street • Vadgadi •
Bombay-400 003 • Tel: 330280 •

ION Electrical • 1/1 Mahalaxmi Engg.
Estate • 571 L.J. Cross Rd 1 • Bombay-
400 016 Bombay-400 016 • Tel: 467735,
468157 •

Thyristor Modules

Silicon Power Electronics has intro-
duced half-controlled and full-controlled
isolated single phase thyristor modules
with current capacity ranging from 25 to
63 amps and repeatitive peak-off-state
and reverse voltage upto 1600 volts. The
half-controlled module consists of two

9.120 elektor india September 1989

NEW PRODUCTS

• Digital Auto/Manual Multimeter

PLA introduces DM-20AR Digital Auto
Manual Multimeter. Features include: 4-
1/2 digit display for high resolution; auto/
manual modes; swift auto-ranging; fre-
quency measurement facility in manual
mode; audible and visual continuity
tones for quick checks for opens and
shorts; touch-hold gives the user a spare
pair of hands input protection alone;
auto battery test and auto polarity; and a
one year warranty.

r - V-V

B

1 HI

B

Pla Electro Appliances (P) Ltd. • Thakor
Estate • Kurla Kirol Rd •
Vidyavihar (W) • Bombay- 400 086.

External & Internal Circlip Pliers

Marvel Products introduces External &
Internal Circlip Pliers with replaceable
tips used for electronic, Xerographic,
automobile and various industrial as-
sembly lines.

Non slip handle made out of glass filled
nylon moulded. The plier body is fitted
with a return spring.

Replacable tips; bent, straight, angular
tips can be replaced by tunscrewing the 3
mm gnub screw. Allen key for replacing
tips and spare tips are packed in one
pouch.

Marvel Products • 208, Allied Ind. Est. •
Mahim • Bombay-400 016 • Tel: 468346/
466846

SINGLE PHASE SILICON
BRIDGE RECTIFIERS

Silicon Power Electronics has intro-
duced miniaturised, 1.6 cm x 1.6 cm
square isolated silicon single phase
bridge rectifier in the current range of 4,
6 and 10 amps with peak inverse voltage
upto 1600 volts. These bridges have
metal base and adjustable lead connec-
tion. Electronically isolated active com-
ponents and terminals have a maximum
high-pot-test voltage of 2500 volts a.c.
These bridges have operational/storage
temperature rating of -55 C to 150 C and
blocking voltage from 100 volts to 1600
PIV. RC network can be used only on
DC side for voltage surge protection.
These are suitable for chassis and PCB
mounting. These silicon bridge rectifiers
find applications in general power
supplies, input rectifiers for variable fre-
quency drives, rectifier for dc motor field
supplies, battery chargers, UPS systems,
emergency light, low power controlled
panels, stabilizers, laboratory equip-
ments, process control equipments etc.

BSPR 6 PB

_St

SIZE : 1.6 x 1.6 c.m.

4 A 6A & 10 A UP TO 1600 VOLTS
BSPR 4 BSPR 6 & BSPR 10
RECTIFIER BRIDGE

Railton Electronics • 3/C, Madan Street
• 2nd Floor • Calcutta-700 072 • Tel:
275488 •

3 DIGITAL LINE FREQUENCY METER

"MECO has introduced a high accuracy,
3 digit Frequency Meter for measuring
line frequency in Control panels, Sub-
stations, Power Plants, Generating sets.
Distribution centres, etc.

The instrument is housed in standard
ABS 1/2 DIN case 96 x 48 mm with depth
of 150 mm. Ranges available are 0-99.9
Hz with accuracy of 0.1 Hz and 999 Hz
with accuracy of 1 Hz. High accuracy and
long term stability is achieved by incor-
porating a crystal controlled clock
generator. The operating voltages and

input signal voltages are 110 V, 220 V
and 440 V and option for input signal vol-
tages to vary from 10 V to 500 V AC.
Single Phase. LED display clearly indi-
cates the frequency.

MECO INSTRUMENTS PVT. LTD. •
Bharat Industrial Estate • T.J. Road •
Sewrce • Bombay-400 015 • Tel:
4137423/4132435*

POWER CONNECTORS

G.H. INDUSTRIES introduces Power
Connectors Pitch 3.96 mm, 5.08 mm,
5.0-7. 5 mm, 5.0 mm Range : 2 Way to'22
Way.

Male Square Pin Headers are also availa-
ble with or without Friction Lock,
Straight or Right Angle. Current - Rat-
ing 7.0 Amps. Voltage 250 Volts.

G. H. INDUSTRIES 6 84-B Government
Industrial Estate • Kandivli 6 Bombay-
400 067 6

9.122 elektor India September 1989

new Products

Photoelectric Switch

Electronic Switches have introduced a
miniature a miniature photoelectric set
with infrared beam consisting of sepa-
rate transmitter and receiver with built-
in amplifier and prewired cable. Solid
state design provides positive detection
of opaque objects and does not get af-
fected by vibration or ambient light con-
ditions.

Working on 10 to 24 V DC supply these
are available in light and dark switching
models. These switches have w'ide appli-
cations in non contact sensing, switch-
ing, controlling and regulating various
processes etc. in Packaging, film, paper,
chemical, pharmaceuticals, automobiles
and other industries.

Electronic Switches (Nasik) P. Ltd. •
Nahush • Gangapur Road • Nasik-422
005 • Tel: 0253-78452 •

Portable Data Entry Unit

Oriole Services have developed a Porta-
ble Data Terminal PDT-1. This comes
with 84 keys IBM-PC typefull travel
mechanical key-board and has a Two
line Liquid Crystal Display, each line of
40 characters. Standard data memory 48
KB Operational features; Standard In-
sert/Dclete Function and UP/Down
scrolling available directly through keys,
Ni-Cd battery back up, Automatic bat-
tery Charging. Power requirement is
230V AC, 100 mA. The form filling
firmware allows formatted data entry in
a d Base user programmable structure.
Optionally cassette interface available
with the terminal.

Sinbros Distributors • 9. Sahakar Bha-
van • 2nd Floor • Narayan Nagar •
Ghatkopar (W) • Bombay-400 086 •

PCB Storage Rack

Circuit Aids Inc introduces Model CR-
21-2 EC, cost effective PCB Storage Sys-
tem suitable for horizontal storage of
PCBs prior to and after soldering. Fea-
tures easily adjustable Antistatic
grooves to suit various sizes of PCB upto
a maximum of 42 numbers.

The bell works on 3 volts cell by pressing
the push button switch once, 5 to 7 sec-
ond music is available then it stops au-
tomatically. This can be used as a door
bell or call bell in homes, offices. Banks,
hospitals, factories etc.

Electronic Hobby Centre • F-32, Band
Dham Industrial Estate • Marol Maroshi
Road • Andheri (E) • Bombay 400 059 •
Tel: 6366123 •

Circuit Aids Inc. • No. 451, II Floor •
64th Cross • V Blk • Rajajinagar •
Bangalore- 560 010 • Tel: 359694 •

UHF/VHF Converter

Elicon is manufacturing UHF & VHF
Convertor meant for converting the full
range of UFIF Brand IV (channel 21 to
39) so that they can be viewed on an ordi-
nary B/W TV (Channel 4) . Having a high
gain and low noise figure having a
stabilised power supply and operating
from 220 V AC supply. It is also useful
for computers and video games if their
output is in the U.H.F. range.

16/12 Tunes Bell or Kit

Electronics Hobby Centre has de-
veloped 16, 12 & 1 Bell Kits as well com-
plete assembled bells. The bell kit is in-
corporated with programmed IC which
gives famous time of Jingle Bell etc. The
kit is available with cabinet component
and circuit diagram for easy assembling.
The cabinet is available in different at-
tractive plastic moulded colors. This is
most useful practical project in Elec-
tronic Certificate or diploma courses.

Electronic Instruments & Controls •
4319, 3-Ansari Road • Daryaganj • New
Delhi-110 002 •

9.124 elektor india September 1989

NEW PRODUCTS

TIMERS

Vectrol Engineer has introduced solid
state timers. “ON delay" and “Instan-
taneous pick up & delay drop out” timer
relays are provided with dual opera-
tional voltages so as to reduce the stock
inventory. Available in the time ranges
0-30. 0-60, 0-120, 0-180 sec maximum,
the timer have two sets of of delayed out-
put change over contacts for customers
control wiring. These timers are used in
electrical & instrument control panels
for a member of applications to achieve a
definite functional requirement related
to time. Vectrol also offers ON-OFF se-
quential (Cyclic) & complex sequential
timers having number having number of
events. Timers can be manufactured to
suit the customers specific requirements.

Vectrol Engineers • 4A/32, Versova View
Co-Op Hsg. Soc. • Four Bunglow Road •
Andheri (YV) • Bombay-400 058 •

Telephone Call Accounting
System

Teleguard is a microprocessor based
Telephone Call Accounting system,
which provides automatic print out of
number dialled and duration of calls
made from a telephone or its extensions.
Red print out distinguishes STD, ISD
calls. Serial number is also recorded au-
tomatically toensure completeness of in-
formation. Red and green led indication
shows line usage and dialling activity. It
consumes low cost standard paper roll
and typewriter’s ribbon. Its heavy duty
printer ensures continuous operation.

The system is connected parallel to athe
telephone line and operates on 220 volts-
main and has optional battery back up.
Ideal for Offices, Industries, Shops,
Hotels and Restaurants etc.

Five Star Commercial Centre • 5-A,
Madhya • Sector 7-C •

Chandigarh- 160 019.

Digital Wattmeters/
Megawattmeters

“DELTA” Control Engg. Corp. offer
Digital Wattmeters for power measure-
ments in DC, Single-phase & Three-
phase AC system. Ranges are from mic-
rowatts to megawatts to suit various vol-
tage and current inputs. Accuracy of
+ 0.5%, + 1 digit or better is assured
over very wide power factor of 0.05 -
unity

+ 0.05 by the use of high precision
analog multiplier IC. 31/2, 4'/> digits, 12.5
mm LED display is convenient to read.

Official calibration certificate from
IDEMI, Bombay- 400 022 can be pro-
vided. Application range from lamp in-
dustry, sodium vapour lamp ballast in-
dustry, electrical heaters manufacatur-
ers, motor & switchgear industry and
power utilisers.

Delta Control Engineering Corporation •
C-224, Shreyas Industrial Estate • West-
ern Express Highway • Goregaon (E) •
Bombay- 400 063.

Printed Circuit Board Terminals
For 2.5 Sq. mm

CHEMITRON1CS INDIA offers indi-
vidual connectors that can be stacked to-
gether for the required number to form a
multiway, suitable for international stan-
dard module dimensions of printed cir-
cuit boards mountings. Connection is by
soldering of pins and screw clamping the
wire termination.

These PCB terminals are housed in in-
dustrial polyamide or which are polyp-
ropylene housings. They have brass con-
ductors, which are tin plated after nickel
coated.

These PCB terminals can accept conduc-
tors of 2.5 Sq. mm with rating of 500
Volts-15 Amps.

Useful to electronic Instruments, equip-
ments manufacturers. Computer man-
ufacturers, Television & consumer Elec-
tronics product manufacturers, Defence
& Dept, of Atomic Energy etc.

CHEMITRONICS INDIA • B-4, Plot-10
• Above I.C.B • H.F.S. Road • Bombay-
400 060 < Tel: 6329536 •

9.126 elektor india September 1989

NEW PRODUCTS

Digital V-A-F Meter

Jivan has introduced Digital V-A-F
meter. This measure voltage, Ampere &
Line Frequency. It has a compact size of
96 (H) x 96 (W) x 170 (D) mm. It directly
measures measures voltage upto 600 V,
with P.T., it can measure any desired
voltage. The current range is upto 10 A,
but with C.T. , it can measure any desired
current. The frequency range is from
20.000 to 99.99 Hz.

* Dion At v - a

-F METER*

ii n

1 "1

-1 u.

1 II

<3'

* O * "

A^PCRr

•

JIVAN

.Jivan Electro Instruments • 394, GIDC
Estate • Makarpura • Baroda-390 010 •

Sequential Timer Controller

S.S. Controls have developed a fully
solid state MC-10 Sequential Timer con-
troller. Having 10 A.C output channels
which cover ON sequentially for a fixed
duration of time. Both ON/OFF timer
period are continuously variable by
means of potentiometers. ON condition
is indicated by red LED. Each channel
has an opto isolator for achieving total
isolation from the mains for safety. The
unit can be used for less than 10 outputs.

The operation cycle does not stop if one
of the channels develops a fault but goes
on to the next channel at the set time in-
terval. Applications includes power re-
covery in powder coating, cement indus-
try etc.

S.S. Controls • 60, A-6/116 Jolly Jeevan
• L.I.C. Colony • Borivli (W) • Bombay-
400 103 •

H.R.C. Fuse Fittings

Kaycee Industries Ltd., have introduced
H.R.C. Fuse Fittings, type ‘KF\ Availa-
ble in 20/32/63 AMP, 500 volts, suitable
for front wiring at both ends and tested
as per IS 9224 for High Voltage Test, In-
sulation Resistance Test and Tempera-
ture Rise. Salient features include. Base
& carrier moulded from High Grade
Phenolic Compound which is non-in-
flammable and non-hygroscopic with
black hard gloss finish surface. Phosphor
Bronze contact clips. Block made from
brass. Visible fault indication through
window on carrier. Monlithic brass base
contact block with adequate hole, suita-
ble for aluminium cable.

Kaycee Industries Ltd. • 32, Rain jibtjai
Kamai Road • Ballard Estate •
Bombay- 400 038.

Shaft Encoder

HENGSTLER of Japan offer optical
Shaft Encoder in three Models 613/614/
616 (DS Series). Pulses per revolution
(P/R) are from 4 to 10(X). Operating on
5V DC 12V DC with ripple tolerance
uptop 5%, these have a frequency re-
sponse of 30 KHz with output rising fal-
ling time of less than 2 micro seconds.
These encoders are tested to withstand
severe environmental conditions. Life
expectancy is upto 7200 million revolu-
tions and they find applications in
Machine Tools, Packing Machines, Test-
ing Equipments etc.

Universal Automation (Agencies) • 47,
Mitramandal • Punc-411 009.

Annunciator Windows

Metal king manufacture Annuciator
windows of full Fibre-Glass body. These
windows are heat-resistance and are av-
ailable in four sizes 70 x 52, 65 x 50, 70 x
35, and 55 x 30. The snap-fit window
frame in provided in ABS. The facia
plate of Acrylic are of Imported and av-
ailable in Clear, White translucent -
(Opal) and other transparent colours.

Metal King • 2/9, Meghal Industrial
Estate • Devidaval Road •
Bombay- 400 080 • Tel:'5618978/5604769

9.128 etektor India September 1989

Every feature of the 8085 CPU and its peripheral
chips can now be studied in complete detail

The ONLY
Microprocessor
Trainer
With highly
advanced features

■ 24 highly reliable
dual function keys.

■ Opto isolator.

■ 8251 for serial
communications.
Upload/Download
from Host.

It's like being
right inside
the 8085 chip.

■ 8255 for parallel I/O.

■ Intelligent EPROM/
EEPROM
programmer.

■ Full memory

expansion possible.

■ Onboard DAC.

■ Onboard ADC.

■ Miniature Speaker.

■ 8279 Keyboard &
Display Controller.

6 digit display.

Real Time Clock.

Cassette interface.

Dynalog

Micro-Systems

For further information, write or call.

Central Sales Office:

112, Hill View Industrial Estate, Amrut Nagar, Off L.B. Shastri Marg
Ghatkopar (West), Bombay 400 086.

Tel: 583908, 586514 Tlx. 01 1 -71 801 DYNA IN FAX-(9 1 >22-358171

Branches: ■ PUNE-Tel: 336907» BANGALORE-Tel: 579757 ■ IN D O R E-T el:79B
Distributors: ■ NEW DELHI-Tel: 270832 ■ MADRAS-Tel: 422705
■ CALCUTTA-Tel: 26 1 8 70 ■ SECUNDERABAD-Tel: 825775

■ Machine single
stepping.

• Small size
convenient for
embedded system
applications.

Battery back U[
for RAM.

System monitor
with Operating
System like feature
and CALL function
facility for calling
system services
through user
programs.

■ Programmable
RS 232 port.

■ Logic Probe.

elektor india September 1989 9.121

mp

MAKE THE RIGHT
CHOICE

High reliability from Micro-pack,
the pioneer manufacturers of
* Fine line double sided PCBs

" I •> mi*. uuuun jiutu r V.UO

* Multi layer PCBs upto 10 layers ' •)> ’

‘PS _

* Multi level — RTF

* Surface Mount Assembly

* Functional Gold plated PCBs

* Tin-lead fused or
SMOBC finish

* State of the Art.

Micropack limited,

Sri Raghavendra Complex,

2805 4 2807, 10th Mam, 4th Block,
iayanagar, Bangalore-560 Oil
Phone Off 643004, 643664,

Cable MICROPACK TELEX 0845-8764 PAC IN

R. N. No. 39881/83

Allowed to post without prepayment. LIC No. 91

MH BV WEST-228
LIC No. 91

GROWEL ushers m a new era

in the INDIAN PCB scenario

TYPf PC BRIDGE

I YPt: PC BU

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In collaboration with

m

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U. K.

Grauer & Weil (India) Ltd., trail blazers in the
electroplating field, offer complete printed circuit
boar 1 plating plants manufactured in India.

Standard plants with thruputs of WOO to SO, 000 sq.
meters per year (single shift basis) can be offered for
panel, pattern, desmear and nickel gold plating.
Optional configurations are available upon request.

Plants incorporate fully automatic microprocessor
based material handling systems. These are capable of
catering to simple as well as to the most complex

process cycles calling for random selection for multiple
product mixes. Supporting features such as CRT
terminals, data-logging, production reports and self
diagnostics are available for on-line process chemistry
analysis and auto-dosing facilities.

Complete range of PCB plating chemicals manufactured
under license from the world - renowned ENTHONE INC. of
U.S.A. is available to support our process chemistry .

Why not let us supply your requirement of PCB
Plating Plants?

GRAUER & WEIL (INDIA) LTD.

Sukh Sagar, N. S. Patkar Marg. Chowpatty. Bombay-400 007.

PHONE : 811 1981. 811 1481, TELEX : 011 75791 CRAMS r-WISHWELL
Branches: BANGALORE:, CALCUTTA. DELHI, LI DHIANA, MADRAS AND SECUNDERABAD.

Where the science of plating is transformed to a fine art.

EXPRESS 713