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This anti theft alarm for vehicles is a security system for vehicles that generates a loud and continuous alarm in case any unauthorized person tries to gain access to your vehicle. As soon as a thief tries to start the ignition, the vehicle alarm is activated, leaving no option other than to flee as the alarm can be reset and turned OFF by the owner alone. Build this kit to prevent theft of your vehicle.
Learning from this project
- Application of music generator IC UM3561
- Application of NPN transistors for switching purposes
- Application of SCRs for sensing and switching
- Application of capacitors for temporary storage
The anti theft alarm for vehicles simulates a police siren whenever someone attempts to commit theft of your vehicle. The alarm sounds continuously for a few seconds even when the intruder switches off the ignition key.
The circuit consists of an SCR based trigger circuit and an audio alarm circuit. When the ignition key of the vehicle is switched off, the base voltage of the NPN transistor SL100 (T1) is low and it remains 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, toggle switch (S2) and a resistor (R1) which slowly charges the 100 microfarad 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 the SCR fires, giving 12V DC voltage to the alarm circuit. The alarm circuit is built around the music generator UM3561 (IC1) which has an inbuilt oscillator. The output from IC1 is fed to a transistor amplifier built around transistors BC548 (T2) and SL100 (T3). The circuit gives sufficient time delay to switch ON the alarm and leave the vehicle. The alarm once triggered, sounds until the toggle switch S1 is pressed to switch off the power supply.
A 220 microfarad capacitor (C2) is provided to sound the alarm even when the intruder switches off the ignition key. When the ignition key is switched off immediately, C2 discharges through a resistor R4 and keeps the alarm activated for half a minute. Reset switch (S3) is used to reset the alarm if needed.