Did you know electronics can help you to protect your house from the thieves
and unwanted person?
Take a look to my Burglar Alarm Project. This simple tool alone can guard
your doors and windows with a little power consumption. This burglar alarm can
be used to protect windows or glass areas by using window foil that "breaks" a
circuit as the glass is broken. It is an alarm that is triggered when the
protective circuit is opened.
Circuit Operation:- The circuit consist of a n-p-n transistor, an alarm
working on 6V DC and a SCR (Silicon Controlled Rectifier). The transistor is
biased in voltage divider mode. the out put is taken from the emitter which is
fed to the SCR gate. The transistor biasing circuit acts as a voltage follower
to the SCR. When the protective circuit is closed, the transistor works on near
It acts as a open circuit. When the protective circuit is opened or cut
because of breaking of security, all the input voltage falls on the base of the
transistor. Now transistor conducts. The output voltage at emitter triggers the
SCR from the forward blocking state to forward conduction state and the SCR
latches. The alarm is in series with the SCR. It starts ringing. There is no
other way to stop the alarm even if connecting the broken window foil as the
before. The commutation of SCR is possible only if there is an additional
commutation circuitry. So the alarm is turned of if and only if the master
switch S1 is turned off manually.
List for Burglar Alarm
C1- 50µF, 6V DC
transistor, GE-20 or equiv
R1- 1000 Ω, ½
S1- SPDT switch
Controlled rectifier rated 12 PIV or higher (General Electronic C106
V1- 6V DC alarm
The recommended power supply is a AC powered regulated 6V DC
source. As the transistor normally stays in cut-off, the standby current
consumption is low about 100µA. To adjust, connect a voltmeter (10V DC range)
across resistor resistor R1, open the protective circuit and adjust
potentiometer R2 so that meter indicates a voltage rising towards ! volts. The
alarm bell should ring at the time of latching of SCR and the meter will show
approximate 1 Volt. If it does not, there is a wiring error. Finally, set R2 for
the 1Volt mater reading, remove the meter and restore the protective circuit.
We can use the alarm in window and doors during nignt.