KIT 31. 9V SIREN
A surprisingly powerful siren powered by just a 9V battery.
The Kit may provide the final building block in an alarm
circuit using a relay to activate it.
The kit is constructed on a single-sided printed circuit
board (PCB). Protel Autotrax and Schmatic were used.
ASSEMBLY INSTRUCTIONS
The printed overlay shows where to place the components.
Use some wire to connect the switch and the speaker to the
pads on the PCB at the distance you want. Both speaker
and switch can be connected either way around. Make sure
to get the electrolytic capacitor, diodes and the IC the
correct way around. The cathode on the diodes is marked
with a black band. This black band must match the bar on
the diode on the overlay. To save space some of the
components stand up on their ends on the PCB.
CIRCUIT DESCRIPTION
Operational amplifiers are a big subject in electronics. To
understand this circuit first read about opamps in a
reference book then try to follow this condensed
explanation of the circuit.
When the switch is pressed C3 charges up through R4 with
a time constant of 0.47 seconds. When the switch is
released C3 begins a slower discharge through R7 and R3
with a time constant of about 5 seconds. The opamp is set
up as a voltage controlled oscillator. The control voltage in
this Kit is the exponential rise and fall in the voltage of C3
as it charges and discharges.
When the output of the oscillator (pin 7) switches low there
is a charge remaining in C1 which holds pin 5 below the
switching point. Current through R7 is proportional to the
control voltage on C3. This current discharges C1 causing
the voltage on pin 5 to rise toeards the switching point at a
rate proportional to the voltage on C3. When the switching
point is reached pin 7 switches high and initially pulls pin 6
high via C1. This causes the opamp to temporarily turn on
hard. But C3 quickly recharges through D2 causing the
voltage on pin 5 to fall below the switching point and
causing the opamp to switch off again.
The positive pulse output from the opampputs a fixed
amount of charge into C2 slightly raising the potential of
pin 6. This causes the potential on pin 6 to rise and assist
the sharp switch off of the opamp. Also R5 & C2 delay the
rise on pin 6 long enough to get a good output pulse.
The cycle then repeats. However, during the C3 discharge
cycle the rate of charge of C1 is lower with each repetition
of the oscillator (because the control voltage is lower) and
the output frequency is correspondingly lower. During the
C3 charge cycle the reverse applies.
The output pulses are buffered by a second opamp then the
current is applied to a driver transistor. The output
waveform has a low duty cycle but gives a surprisingly
loud sound.
Download the data sheet for the LM358 from the National
Semiconductor website at
http://www.national.com
WHAT TO DO IF IT DOES NOT WORK
Poor soldering is always the most likely reason that a Kit
circuit does not work. Check all solder joints carefully
under a good light. Next check that all components are in
their correct position on the PCB especially the diodes,
electrolytic capacitors, and IC's. If by chance the siren does
not completely turn off but keeps going
click...click...click the increase the value of R6 to 220R.
COMPONENTS
Resistors 5%, 1/4W:
180R brown grey brown
R6
1
47K yellow violet orange
R4
1
100K brown black yellow
R2 R5
2
1M brown black green
R3
2
10M brown black blue
R1
1
Capacitors:
1nF 102 ceramic
C2
1
10nF 103 ceramic
C1
1
10uF
electrolytic C3
1
BC639 transistor
1
LM358 IC
1
8 pin IC socket
1
8 ohm speaker
1
Push on/off switch
1
1N4148 diode
2
9V battery snap
1
K31 PCB
1
Add-on circuit to try. Components not supplied. Try to
predict what will happen before you connect it. Just uses
220K & 270K resistor, LM555 and a 1N4148 diode.
See our website for our full range of kits
http://kitsrus.com KIT 31. 9V SIREN