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Car
Anti-Theft Wireless Alarm
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FM radio-controlled anti- theft alarm can be used with
any vehicle having 6- to 12-volt DC supply system. The
mini VHF, FM transmitter is fitted in the vehicle at night
when it is parked in the car porch or car park. The receiver
unit with CXA1019, a single IC-based FM radio module,
which is freely available in the market at reasonable
rate, is kept inside. Receiver is tuned to the transmitter's
frequency. When the transmitter is on and the signals
are being received by FM radio receiver, no hissing noise
is available at the output of receiver. Thus transis-
tor T2 (BC548) does not conduct. This results in the relay
driver transistor T3 getting its forward base bias via
10k resistor R5 and the relay gets ener- gised. When an
intruder tries to drive the car and takes it a few metres
away from the car porch, the radio link betw- een the
car (transmitter) and alarm (receiver) is broken. As a
result FM radio module gene-rates hissing noise. Hissing
AC signals are coupled to relay switching circ- uit via
audio transformer. These AC signals are rectified and
filtered by diode D1 and capacitor C8, and the resulting
positive DC voltage provides a forward bias to transistor
T2. Thus transistor T2 conducts, and it pulls the base
of relay driver transistor T3 to ground level. The relay
thus gets de-activated and the alarm connected via N/C
contacts of relay is switched on. If, by chance, the intruder
finds out about the wireless alarm and disconnects the
transmitter from battery, still remote alarm remains activated
because in the absence of signal, the receiver continues
to produce hissing noise at its output. So the burglar
alarm is fool-proof and highly reliable. (Ed: You may
have some problem catching the thief, though, if he decides
to run away with your vehicle_in spite of the alarm!)
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