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 energized.
When an intruder tries to drive the car and takes it a few meters
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 circuit 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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