Low cost siren
for burglar alarms
Want a really loud siren which will wake the
dead? Well, maybe this one won't wake the
dead but it will certainly disturb the living.
It's built on a tiny printed board that takes
up hardly any space at all.
By LEO SIMPSON
Since it runs from 12V DC, this
siren could be used in a car or home
burglar alarm system. Its current
requirements are fairly modest, at
1.25 amps, so it could be switched
into circuit with a relay or a power
transistor.
It does not have any time out circuitry so that will have to be added,
if required.
What it does do is make a very
loud sound with a minimum of components. It is a classic siren sound,
heavily modulated in frequency at
about a one second rate. It uses just
one CMOS hex Schmitt trigger IC
and six low cost transistors. The
way in which these components are
hooked together to produce this
alarm circuit is quite interesting.
First, there are two oscillator circuits. ICla is the low frequency
oscillator. Its operating frequency
of about lHz is set by the lMO
resistor and lµF capacitor.
The second oscillator is based
around IClb and its basic operating
frequency is set by the 2.2MO
resistor and .OOlµF capacitor. It is
modulated up and down in frequency by the RC network connected to
the output of ICla. Its range of fre-
quency modulation is from about
600Hz to 3.6kHz; ie, about 6 to 1.
What happens is that ICla
oscillates at about lHz and produces a square wave at its output.
This is filtered to a rough sinewave
by the 220kn resistor and its
associated lµF capacitor. This
signal is then fed to a 2.2MO
resistor and clamped to 4. 7V by
zener diode ZDl, after which it is
fed to IClb via a .OOlµF capacitor.
Thus ICla frequency modulates
IClb.
Hence, the output of ICl b (pin 12)
is a square wave of varying frequency. Its amplitude is almost exactly equal to the input supply
voltage minus the voltage drop
across diode Dl. Dl is there merely
to prevent damage if you connect
the supply around the wrong way.
The square wave output of ICl b
is inverted by IClc and fed to the
base of transistor Ql and to
paralleled inverters ICld and ICle.
These then drive the base of transistor Q2.
H-pack output
Just one CMOS IC and 6 low-cost transistors make up t~is nifty little siren ,
circuit. Don't bolt the tabs of the TIP31/32 power transistors together - you II
create a short circuit if you do.
48
SILICON CHIP
Four transistors, Q3, Q4, Q5 and
Q6, drive the horn speaker which is
what makes all the noise for this
project. The four transistors are
connected in an "H-pack" arrangement which effectively doubles the
drive voltage fed to the horn
speaker. It works in the following
way.
When the output of IClc goes
high, it turns on Ql. Ql 's collector
is connected to the base of Q4 while
its emitter is connected to the base
of Q5, via a 1000 resistor. So when
Ql turns on, so do Q4 and Q5.
The same arrangement goes for
Q2. Its collector is connected to the
base of Q3 while its emitter is connected to the base of Q6, via a 1000
resistor. So when Q2 turns on, so do
Q3 and Q6.
D1
1N5402
r
100{)
+12V
1.2A
02
BC337
B
1M
2.2M
+
-
ZD1 , J
1N7504A
.,.
~
B
EOc
VIEWED FROM
BELOW
SUPER SIREN
Fig.1: ICla is a lHz oscillator & this is used to frequency modulate IClb from
600Hz to 3.6kHz. IClc-lCle then produce complementary signals which drive
Qt, Q2 & the H-pack output transistors.
Because of the way in which the
bases of Ql and Q2 are connected
to complementary square wave
signals (derived from inverters
IClc, ICld and ICle), they turn on
alternately for respective half
cycles of the square wave signal.
So Q4 and Q5 turn on together to
pass current in one direction
through the horn speaker. Then Q3
and Q6 turn on to pass current
through the speaker in the opposite
direction.
At any one time then, the speaker
has virtually the full supply voltage
across it, continually being switched in one direction and then the
other. This maximises the power
output and therefore gives maximum scream for minimum component count.
By the way, while the power in
the horn speaker is maximised, the
SPEAKER
Fig.2: here's how to mount the parts
on the PC board. Be sure to use the
correct transistor type at each
location.
power dissipated in the four output
transistors is very low since they
spend their time either fully switched on (and saturated) or switched
off. Either way, their power dissipation is minimal so they can be
operated without any heatsink.
Neat, huh?
Another neat feature of this
economy circuit is the use of just
one current limiting resistor (1000)
each for Ql and Q2. Other "Hpack" drive circuits we have seen
usually use more transistors and
need more current limiting resistors.
Construction
As we noted before, the printed
board for this neat little number is
teensy weensy. It measures just 32
x 45mm - about twice the size of
an average postage stamp. It comes
with green solder mask on the
underside and a screen printed
component overlay on the topside.
Assembling it is hardly a major job
and probably won't take much more
than about 15 minutes for most
people.
There are a couple of points to
watch though. First, the oscillator
has been designed to suit Philips
40106 ICs. If you use 40106s or the
equivalent 74C14s from another
manufacturer, the chances are that
their different hysteresis levels (ie,
continued on page 69
BCIE
PARTS LIST
1 PC board, 32 x 45mm, with
screen-printed overlay
1 80 horn speaker
Semiconductors
1 40106 (IC1 - Philips, see
text)
2 BC337 NPN transistors
(Q1,Q2)
2 TIP32 PNP transistors
(Q3,Q4)
2 TIP31 NPN transistors
(Q5,Q6)
1 1N5402 silicon diode (01)
1 1N750A 4. 7V zener diode
(ZD1)
Capacitors
1 4 7 µ,F 16VW PC elelectrolyic
2 1µ,F 16VW PC electrolytic
1 .001 µ,F ceramic
Resistors (0 .25W, 5%)
2 2.2MO
1 1MO
1 220k0
3 1000
Where To Get The Kit
Kits for this project are
available from Oatley Electronics.
The basic kit, containing a PC
board and all parts except the
horn speaker, is $10.40 plus
$3.00 packing and postage. The
80 1 OW horn speaker is an extra
$13.50.
You can contact Oatley Electronics by phone on (02) 579
4985 or by post at PO Box 89,
Oatley NSW 2223.
OCTOBER 1990
49,.
The PC board'is housed in a small
plastic case measuring 82 x 54 x
31mm. As shown in the photographs, the voltage adjust potentiometer (VRl) and the on/off switch
are mounted on the lid, while the
output banana terminals are
mounted on one side.
To install the hardware in the
case, first drill the holes in the lid
using the front panel artwork as a
guide, then drill holes in the side for
the output sockets. These sockets
should be 19.5mm apart and should
sit as close to the bottom of the case
as possible.
This done, attach the front panel
artwork, install the potentiometer
and switch, and complete the wiring as shown in Fig.4. When installing the wiring, sit the PC board on
the back of the lid next to the switch
and pot as shown in the wiring
diagram. The PC board is then installed upside down in the case
when the lid is screwed down and
can be held in position using a small
piece of foam rubber.
Testing is straightforward - just
connect the output to your DMM,
switch on and check that the output
voltage can be varied from 0-ZOV
using the 10-turn pot. If you strike
trouble, check for 25.8V between
the cathode of DZ and pin 5 of ICl.
This will tell you whether the fault
lies around ZDl or around ICl and
the voltage doubler.
~
Burglar Alarm Siren upper and low threshold voltages)
will result in the oscillator frequencies being different - you may
have to change some resistor
values.
Second, watch the component
polarities, both for the electrolytic
capa citors and the semiconductors,
particularly the TIP31s and TIP32s.
It is all too easy to put these in the
wrong way around and then you
have a very dud project.
Testing
Don't be an idiot when you hook
this up to your power supply. At the
very least, put the horn speaker
face down on your workbench
when testing it - it is extremely
loud and it will just about blow your
F•
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I-
en
=>
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C
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• Complete Tx kit and PCB with components for
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Because of demand we will hold th is price until
December 1990
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Fig.5: these are the full-size artworks
for the front panel & the PCB.
ctd from page 49
head off if you cop the full blast.
Better still, do your initial testing
with a fair sized resistor connected
in series with the speaker. For example, we used an 8200 5 watt
resistor when we tested the unit on
the bench. However, any value
from a few hundred ohms up to say,
2k0 will do the job and protect your
ears.
What a bout different supply
voltages? Yes, you can increase the
supply up to 15V which is the limit
for the 40106. And the circuit will
operate, with reduced power, down
to about 9 or 10 volts. Below that,
it's not worth bothering and you
would have to change resistor
values to make the oscillators work
correctly.
~
MASTER SLAVE SWITCH
EA JANUARY 1990
Remotely switch your non remote TV off an d on via
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One switch operation, mains filter and overvoltage protection incl uded .Gom_plete PCB an d all on
board components.
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This is not a kit , complete unit includes laser
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$309.00
We also have laser (mains and battery operated) kits, laser tubes, laser diodes and night
viewer kits still available.
OATLEY ELECTRONICS
PO BOX 89, OATLEY, NSW 2223
Telephone: (02) 579 4985
Fax No:
(02) 570 791 0
Certified p&p $4-$6Aust. NZ add $2
OCT0 BER1 990
69
