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This alarm circuit is an anti- theft wireless alarm can be used with any vehicle having 6- to 12-volt DC supply system. The mini VHF FM radio-controlled, FM transmitter is fitted in the vehicle at night when it is parked in the car porch or car park. The receiver unit of the wireless alarm uses an 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 energised. The following is a schematic drawing: 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 res...

This simple and inexpensive anti-theft circuit for vehicles sounds an alarm simulating a police siren whenever someone attempts theft of your vehicle. The alarm sounds continuously for a few seconds even when the intruder switches off the ignition key. The circuit uses only a few components and can be easily assembled and installed on a car with negative grounding. Anti-Theft Alarm for Vehicles Circuit Diagram The circuit consists of an SCR-based trigger circuit and audio alarm circuit. When the ignition key of the vehicle is switched off, base voltage of transistor T1 is low and it remains turned off. When the ignition key is switched on for starting the vehicle, a positive voltage is applied to the base of transistor T1 through diode D1, switch S2, and resistor R1, which slowly charges capacitor C1. As a result, the base voltage of T1 rises. As soon as the biasing voltage crosses cut-in voltage, T1 turns on and SCR fires, giving 12V DC to the alarm circuit. The alarm circuit is built...