Headlight Alarm Circuit

  
The base of Ql is connected to the car`s ignition circuit; the easiest point to make that connection is at the ignition switch fuse in the car`s fuse panel. Also, one side of the piezoelectric buzzer is connected to the instrument-panel light fuse; when the headlights or parking lights are on, the instrument panel is lit, too. When the headlights are off, no current reaches the buzzer. Therefore, nothing happens. What happens when the headlights arc on depends on the state of the ignition switch. When the ignition switch is on, transistors Ql and Q2 are biased on, effectively removing the buzzer and the LED from the circuit.
Headlight Alarm Circuit - schematic

When the ignition switch is turned off, but the headlight switch remains on, transistor Ql is turned off, but transistor Q2 continues to be biased on. The result is that the voltage across the piezoelectric buzzer and the LED is sufficient to cause the buzzer to sound loudly and the LED to light.




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