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This
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 transistor 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 energised. When an intruder tries to drive the car and takes
it a few metres away from the car porch, the radio link between
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.
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