@▷ Headlight Alarm | Diagram for Schematic

Headlight Alarm

Normally, when the ignition is on, Q1 is turned on and clamps the base of Q2. When the headlights are off, no power is supplied to the collectors of the transistors. When the headlights are on and the ignition is off, Q1 is also off and Q2 is biased on by R2, activating the buzzer. When both headlights and ignition are on, there is current flowing through R2 and Q1. At 12V, this current is around 21mA.

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Lighting, alarm system