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. |