Posted on Jul 22, 2012 5854
Because power for the circuit is obtained from the car`s side lights, the circuit can`t oscillate unless the lights are o
n. The reset pin on the 555 connects to transistor Ql. The base of Ql is connected through R1 to the ignition auxiliary terminal on the car"s fuse box. When the ignition is turned on, power is supplied to the base of Ql, which turns it on. With Ql turned on, pin 4 of Ul is tied low, which disables the oscillator and inhibits the alarm. If the ignition is turned off while the lights are on, power is applied to the 555 and Ql is turned off, and the alarm starts. Switch Si is an optional override.