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Do you want to
know what 110dB
of noise sounds
like? Well now you
can easily find out.
This circuit puts
out between 108dB
and 111dB at a
distance of about
one metre.
Build a 6-12V alarm
screamer module
There are many applications where
a low-cost alarm siren is required. This
very effective unit from DIY Electronics certainly makes a racket and could
serve as the siren in a house alarm
system, in a car, or in many industrial
applications. For example, you could
R1
1.5M
1
D1
1N4148
R2
1.5M
2
4
14
IC1a
7556
6
C1
0.1
RED
R3
15k
5
10
7
C2
.01
T1
C3
10
9
C
1
13
12 R5
27k
E
C
VIEWED FROM
BELOW
D2
1N4148
8
C4
.01
T2
V+
Q1
BC639 R6
B 1k
R7
1k
Q2
BC639
B
E
B
C5
0.1
R4
27k
V+
2
PIEZO
1
IC1b
11
3
3
BLACK
wire it to a door switch in your car via
an external on/off switch to serve as
an intruder alarm.
As shown in the photographs, the
unit is housed in a specially-designed
plastic case fitted with a mounting
bracket (which also forms the rear
RED
3
2
PIEZO
2
C
1
BLACK
panel). Its overall dimensions are 84
x 55 x 33m (L x W x D), not including
the mounting bracket.
What’s so special about the case?
Well, to make the unit as effective
as possible, it features two integral
resonant cavities for the two piezo
transducers that are used to
V+
generate the noise. The unit is
+5-12V
supplied with these two piezo
transducers pre-glued to the
resonant cavities – all you have
0V
to do is assem
ble a small PC
board, connect a few leads and
a power supply, and stand back
to avoid being deafened.
It is interesting to note that
without the resonant cavi
ties,
the sound generated by the
piezo transducers in open air is
barely audible. It’s a completely
different story with the resonant
cavities, though.
E
T1 AND T2: WINDING 1-2 1500T, 44SWG ENCU
WINDING 2-3 220T, 44SWG ENCU
"SCREAMER" ALARM
Fig.1: the circuit employs two oscillator stages based on IC1a & IC1b. IC1a frequency
modulates IC1b which in turn drives two piezo elements via transistors Q1 & Q2 and
autotransformers T1 & T2.
How it works
Refer now to Fig.1 for the
circuit details. In addition to the
piezo transducers, it’s mainly
based on a dual 7556 timer IC,
two transistors and a couple of
autotransformers.
IC1b is wired in astable
August 1995 75
Electronic
Projects
For Cars
FROM
NEW N CHIP
O
SILIC
On sale now
at selected
newsagents
Or order your copy from Silicon Chip. Price: $8.95 (plus $3
for postage). Order by phoning
(02) 9979 5644 & quoting your
credit card number; or fax the
details to (02) 9979 6503; or
mail your order with cheque
or credit card details to Silicon
Chip Publications, PO Box 139,
Collaroy, NSW 2097.
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76 Silicon Chip
The case is supplied with the two transducers glued to two internal resonant
cavities. All you have to do is assemble the PC board, connect a few leads and
apply power.
configuration and oscillates at about
the resonant frequency of the piezo
transducers. This frequency is set by
R4, R5, D2 & C4 and is about 2.7kHz.
In order to produce a realistic siren sound, IC1a is used to frequency
modulate IC1b at a low rate. It does
this by applying a triangular waveform voltage to IC1b’s control pin
(pin 11).
As with IC1b, IC1a functions as an
astable oscillator but in this case its
frequency of oscillation is only about
5Hz. Note that diodes D1 and D2 ensure that IC1a and IC1b both function
with a 50% duty cycle.
The output from IC1a appears at pin
5 and is a square wave. This is then
converted to a triangular wave by R3
& C3 before being applied to pin 11
of IC1b. As a result, IC1b produces a
modulated output at its pin 9 which
constantly sweeps back and forth
through the resonant frequency of the
piezo elements. As well as modulating the frequency to produce a siren
sound, this technique means that
no trimpot is required for frequency
calibration.
The pin 9 output from IC1b drives
transistors Q1 and Q2 with the modulated 2.7kHz signal. Note that these
two transistors are driven in phase.
Q1 in turn drives piezo 1 via auto
transformer T1, while Q2 drives piezo
2 via autotransformer T2.
In greater detail, each time the
transistors turn on, current flows in
the 220-turn winding of each auto
transformer. Conversely, each time the
transistors turn off, the magnetic field
in these windings collapses and this
induces a much higher signal in the
1500-turn windings. As a result, the
autotransformers significantly step-up
the signal voltage that’s used to drive
the piezo transducers.
In fact, a potential of over 200V is
PIEZO 1
PIEZO 2
3 2 1
0V V+
T1
0.1
D1
1 2 3
.01
0.1
1k
27k
27k
1
IC1
7556
1.5M
1.5M
.01
15k
1k
Q1
D2
10uF
T2
Q2
Fig.2: follow this wiring diagram when installing the
parts on the PC board and take care to ensure that the
two autotransformers are correctly oriented (see text).
The photograph at right shows how the PC board is
installed in the case.
induced which can give you quite a
shock if you are careless enough to
touch the autotransformer leads or the
transducer terminals.
This also means that the piezo elements are overdriven and this has been
done deliberately to give maximum
noise output. This causes no harm to
the piezo elements and test circuits
have been run for several hours at a
time without component failure.
Power for the circuit can come from
any 5-12V DC source; eg, batteries
or a 9V DC plugpack supply. Do not
use a 12V DC plugpack as this could
deliver more than 16V when lightly
loaded.
Assembly
The parts for the Screamer Alarm
are all installed on a small PC board
measuring 78 x 48mm. This board
features screened lettering to show
where all the parts go and should
only take about 10 minutes to assemble.
Fig.2 shows the assembly details.
Install the resistors first, followed by
the two diodes and all the capacitors.
Take care to ensure that the diodes
and the 10µF electrolytic capacitor are
installed with the correct polarity. The
remaining capacitors can be installed
either way around.
Next, install the two transistors
and the IC socket. The IC can then be
plugged into the socket, taking care to
ensure that the notch in the IC body
goes towards the 0V & V+ supply
terminals (ie, pin 1 must be adjacent
to the two 1.5MΩ resistors). Pin 1
will also generally be indicated by an
adjacent dot in the IC body.
Now complete the board assembly
PARTS LIST
1 case with two piezo elements
plus 4 screws
1 PC board (DIY Kit 15)
1 16-pin IC socket
2 autotransformers
2 150mm-lengths of hook-up
wire (red, green)
Semiconductors
1 GLC556, 7556 dual CMOS
timer (IC1)
2 BC639 NPN transistors
(Q1,Q2)
2 1N4148 diodes (D1,D2)
Capacitors
1 10µF 16VW electrolytic
2 0.1µF monolithic
2 .01µF greencap
Resistors (0.25W, 5%)
2 1.5MΩ
1 15kΩ
2 27kΩ
2 1kΩ
Where to buy the kit
A complete kit of parts for the 12V
Screamer Alarm (DIY Kit 15) is
available from: DIY Electronics,
22 MacGregor St, Numurkah, Vic
3636. Phone (058) 62 1915. The
price is $23.50 plus $3.50 p&p.
by installing the two autotransformers
(T1 & T2). These are oriented in opposite directions to each other and
must have their leads bent through 90
degrees so that they lie flat against the
PC board. Note that in each case, the
centre terminal must be towards the
top of the device (see photo).
Two “tie-down” pads have been
provided next to the body of each
autotransformer and you can loop
wire links over the autotransformers at
these locations. In practice, the leads
on the autotransformers will usually
be strong enough to stop them from
moving.
Finally, solder the six leads to the
PC board at the designated locations.
There are two each for the piezo
transducers (red to positive, black to
negative), plus two more for the power
supply connections. This done, the
board can be mounted upside down
inside the case, with the supply leads
exiting from the notch, and the cover
secured using the screws supplied.
Testing
Before applying power, wrap the
unit in a towel to muffle the sound
level (so that you won’t be deafened). After that, all you have to do
is connect the power supply and the
unit should immediately start. It’s
best to start with a supply of about
5V and then test the unit at higher
supply voltages – up to 12V. Exercise
caution, though – this unit puts out
ear-splitting sound, so keep it well
wrapped up.
If the unit doesn’t work, the most
likely reason is poor soldering. Check
all solder joints carefully under a good
light and reheat any that appear suspect (disconnect the power supply before starting work). Next, check that all
the parts are in their correct locations
and that the IC, electrolytic capacitor,
transistors and autotransformers are
all correctly oriented.
If only one piezo transducer works,
then check the transistor and auto
trans
f ormer associated with the
SC
non-functioning transducer.
August 1995 77
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