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. |
