These two headlight reminder circuits are easy to install and operate on the KISS (Keep It Simple Stupid) principle. The simple circuit involves adding just a 12V piezo buzzer between the lights circuit and a door switch. The buzzer sounds if the lights are left on and you open a door. The disadvantage of this simple circuit is that it's annoying to have the buzzer sound continuously if you want to leave the door open while the lights are on. The improved circuit overcomes that problem by adding a 1000µF capacitor and a parallel 100 K Ohm resistor in series with the buzzer. Now, when a door is opened, the buzzer gives a brief burst of sound only, while the 1000µF capacitor charges. The 100 K Ohm resistor discharges the capacitor when the lights are switched off.
Circuit Diagram:
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