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Light Warning Buzzer

With such a low capacity battery, you really don't want to leave the lights on for long by mistake. You can easily fit a warning buzzer that comes on when the lights are on and the door is left open. To do this you will need to find a diode and a 12V buzzer. The diode is essential to protect the buzzer from reverse voltages. The basic circuit is shown below:

With the door closed, point A is connected to +12V via the interior light bulb since the door switch is a normally open type. With the lights off, point B is connected to GND via the hedlight bulb since the light switch is open. In this situation the buzzer is connected with reverse polarity so it will not sound. The diode protects the buzzer from the reverse voltages being applied.

If the door is opened (and the lights are still off), the door switch closes and point B is taken down to GND. With both sides of the buzzer at GND, it will not sound.

If the lights are switched on (and the door is closed), point B is taken up to +12V but since point A is also at +12V, the buzzer does not sound. If the door is now opened, point A is taken to GND and the buzzer now has the correct polarity voltage applied to it and will sound.

Warning: To safely fit this buzzer, you need to identify the correct wires using a multimeter and need to attach the buzzer in a safe manner, ensuring that no short circuits are possible. Failure to attach the wires securely and to insulate the connections properly may result in a short circuit and risk a fire in your car.

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Copyright © Rob Collingridge 2009 - Last updated 19 Mar 2002