The base of Q1 is connected to the car's ignition circuit. One side of the piezoelectric buzzer is connected to the instrument-panel light fuse. When the headlights are off, no current reaches the buzzer, and therefore nothing happens. What happens when the headlights are on depends on the state of the ignition switch. When the ignition switch is on, transistors Q1 and Q2 are biased on, removing the buzzer and the LED from the circuit. When the ignition switch is turned off, but the headlight switch remains on; transistor Q1 is turned off, but transistor Q2 continues to be biased on. The result is that the voltage is sufficient to sound the buzzer loudly and light the LED. Turning off the headlight switch will end commotion quickly.
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