In Reply to: Add your own headlight warning posted by RonW on January 08, 2001 at 22:30:09:
I left my lights on once too often, so I came up with a circuit that would use the e28's built in chime unit to warn me if the headlights or side markers are on, and the driver's door is open. Here's how I made it, and how it works.
I used a normally open automotive relay (and socket) and a diode.
One side of the relay switch (call it "the high side") is connected to the BR/RD wire on the two-wire connector on the chime unit. (The other wire on the chime connector is always at 12V.) The other side of the relay switch ("the low side") is connected to the wire that runs between the "key is in the ignition lock" switch and the switch in the driver's door jamb.
One side of the relay coil is connected to pin 4 on the headlight switch (12V when the headlights or parking lights are on). The other side of the coil is connected to the diode. The diode is oriented so that current can flow from the coil through the diode, but not the other way around. The other side of the diode is connected to the low side of the relay switch.
HOW IT WORKS:
In order for the relay to actuate, the headlights or side markers must be on (supplying 12V to one side of the relay coil) and the driver's door must be open (grounding the other side of the relay coil through the diode.) When the relay actuates, the relay switch closes, grounding the BR/RD wire of the two-wire connector on the chime unit. This turns on the chime.