Here is a simple Anti theft car alarm device for your Car. It generates a loud alarm when there is an attempt of theft. When the intruder opens the door, the circuit senses the attempt of theft and after 2 minutes, the alarm will be activated. The time delay is provided to help the user to leave the car after arming with the device
The anti theft alarm circuit taps power supply from the car battery. Switch S1 is the on/ off switch of the alarm circuit. When the circuit is activated through S1, the flashing LED blinks indicating that the car is armed. But the alarm generator works only if the Dome lamp of the car is switched on through the door switch. This switch turns on the dome lamp, if any one of the door is opened. The user can put S1 in the on position before leaving the car.
There will be a delay of 2 minutes so that the alarm will not be generated. When the intruder opens the door, Diode D1 forward biases and capacitor C1 charges through R1. It takes around 1 minute to charge C1. When C1 fully charges, the 12 volt Zener conducts to trigger the NPN transistor T1.Zener diode switch is provided to avoid false triggering and T1 conducts only when C1 charges fully. When T1 conducts C2 charges and the voltage in C2 triggers the gate of SCR. The Horn connected to the Anode of the SCR sounds indicating the theft. The horn can be either a spare car horn or a Hooter that gives loud alarm.
Note: Before leaving the car, switch on S1 and after entering the car switch off S1. Hide S1 in a place that cannot be detected.