English: Shutdown alarm - headlights on and wipers on - headlights off warning example. Drawn and uploaded by en:User:Leonard G.

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The headlight circuit shown is simplified, not showing the high/low beam circuits. If it is not convenient to tap the common power, use a wire from each of the high and low beam circuits with additional diodes for isolating the two circuits.

Note that an acoustic alarm can be augmented by a warning light wired in parallel.

Theory of operation

If the lights are on and the run buss is grounded (not running) the alarm sounds. With lights off and running the non-iluminated lights provide a path to ground but current flow into the low resistance lighting is blocked by the diode AD1 (preventing alarm operation). With the ights on and running, both ends of alarm device circuit are high and so no alarm.

Wiper and no lights alarm

Late 2005 motor vehicle code regulations in California require that the vehicle lights be on when the windshield wipers are in operation. The circuit shown will sound a reminder tone (a chime, rather than a beep or buzz) when the wipers are on and the lights are off. The positive side of the reminder chime is attached to the wiper motor power. This uses the headlights as a low resistance to ground to determine the condition. When both the lights and the wiper are on both sides of the alarm will be at + 12 volts and no alarm will sound. If the lights are on and the wipers are off current will not flow through the low resistance of the wiper motor as it is blocked by diode AD2.

Additional alarms

By adding a switch that opens only when the parking brake is firmly pulled then a similar alarm signal can be applied via clock or dome light power (on at all times), with the alarm sounding if not running and the parking brake is not well pulled.

If a single acoustic alarm is used with multiple individual warning lights then each light must be isolated from the others by being placed behind appropriate diodes (not shown).