Because power for the circuit is obtained from the car's side lights, the circuit can't oscillate unless the lights are on. The reset pin on the 555 connects to transistor Q1. The base of Q1 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 Q1, which turns it on. With Q1 turned on, pin 4 of U1 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 Q1 is turned off, and the alarm starts. Switch S1 is an optional override.
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