I don’t know about you, but I’ve found myself with a dead battery a number of times because I like to drive with my headlights on for visibility (especially with the tiny cars) but old cars don’t generally come with a warning buzzer that tells you when you’ve left your lights on. Here’s a very simple circuit that I’ve fitted to all of my cars to do just that:
The relay is a Bosch style automotive 12V SPDT (Single Pole Double Throw) item that is widely available. It looks like this:
The unusual pin numbering is standard for these types of relays and each pin is usually clearly marked with these numbers. Of course, any 12V SPDT will work – just follow the schematic to ensure that you use the correct pins.
The “buzzer” is any 12V device that emits sound when powered. I usually use a “chime” from Radio Shack (part number 273-071). It’s nice and loud and reasonably pleasant sounding. (If you use this chime, it has three wires. Just connect its black and blue wires together and treat these as the ground connection.)
How it Works
It’s very simple. The diagram shows the relay at rest (no power) so 30 and 87a are connected. With the ignition switched on, the coil will be energised, therefore breaking that connection. The result (regardless of the state of the blue wire) is that no power is being applied to the buzzer – so it’s silent. Now, turn off the ignition and the connection from 30 to 87a is re-established. If the lights are on, the blue wire is “hot” so power is supplied to the buzzer. If the lights are off, no buzz.
A couple of notes:
- These relays take very little current to remain energised, so don’t worry about power draw while the ignition is on.
- It’s generally a good idea to connect the blue wire to either the parking/sidelights or the dash lights as these are always on when the light switch is on. If you connect it to the high beam circuit, for example, and your lights are dipped when you turn off the ignition then the buzzer won’t sound.
