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Headlight Warning Chime

The E28's from the mid-80's were not equipped with a device to warn you if you accidentally leave your headlights on.  After accidentally draining my battery (more than once), I decided to build a circuit that controls the chime module to make it operate if the lights are on without having the key in the ignition..   The chime module is the device that goes "bong" if you leave the keys in the ignition when you open the door, or when the temperature gets down to 36 degrees F.   I wanted to use the existing chime module (as opposed to using an after-market buzzer of some type) to maintain the stock sound of the car.  My thanks to Jeffrey Smethers for giving me a hint on how to do this - he had a copy of an article by Paul Reitz from aniX Registry newsletter on a similar procedure.  The method in that article relied on the fact that he had previously disconnected the chime module, but I wanted to keep its basic functionality intact, so I made a few changes.

The figure below shows the circuit that I installed in my car.  Materials consist of the following:

  • Two 12-volt DC relays from Radio Shack ($3.49 each).  You need one "normally-open" relay and one "normally-closed" relay.   I purchased relays of the "single-pole, double-throw" (SPDT) type, which have both "normally-open" and "normally-closed" terminal positions.  The Radio Shack part number is 275-248A.   These are marketed as PC Relays, and are rated to carry 10 amps at 120VAC/24VDC. You can also use regular automotive relays, although the current draw on this design is so small that a smaller, less expensive relays like these from Radio Shack are fine.
  • Two diodes, again from Radio Shack, part number 276-1102 ($0.59 for the pair).  These only need to handle 12 volts and a few hundred milliamps.  Most cheap diodes are rated way beyond that; the ones I bought are rated at 200 volts, 1 amp.
  • Radio Shack general-purpose PC board (part number 276-148a), used to hold the relays and make soldering easier ($1.49).  This board has pre-drilled holes that line up nicely with the relay terminals.
  • A project box, which in the end I didn't use, but you may want to. I simply wrapped the completed relay assembly in generous amounts of electrical tape to insulate the terminals.
  • Miscellaneous bits of wire.  I used the color-coded wire from some old telephone cord (red, green, yellow, black) to help me keep the wiring straight.
  • Splice connectors - to make tapping into the existing wires easier (these were sitting around my house from a previous project).

One of the functions of the chime module is to sound a "bong" if you leave the key in the ignition when the door opens.  The way this works is that the door switch and the ignition switch are wired in series so that ground is applied to one of the terminals on the chime module if both the door is open and the key is in the ignition.     The terminal that does this has a red/brown wire connected to it, through the two-position connector on the rear of the chime module. The objective of my circuit is to take advantage of this design, by applying ground to this terminal if both of the following conditions are met: 1) the headlight switch is set to either headlights or parking lights on, and 2) the key is not in the "run" position.  The chime module has several other connections to it, using the other terminal of the 2-position connector and a separate 3-position connector, but we don't need to be concerned with these other connections, hence they are not shown in the figure.

The circuit is basically two relays wired in series.  Referring to the diagram, the upper relay is normally-open (“NO” in the diagram), meaning that the connection is made only when power is applied to the control terminals (labeled as "coil" on the relay and in the figure).  One coil lead is connected to a wire on the rear of the headlight switch that is energized if the lights are turned on, and the other coil lead is connected to chassis ground.  When the headlights or parking lights are off this relay remains open, and when the lights are on the relay closes.   The input to the relay switch (COM) is wired to the chime module, and the output (NO) is wired to the COM of the second relay.

The second relay is of the normally-closed (NC) variety, and it has its coil connected between ground and a wire that powers an accessory on the car - this is a lead that is energized only with the key in the ignition.  I connected this second relay to the lead that powers the bulb in the foglight switch, but I could have used one of the leads to the radio. You will find that most leads that are red/white are switched 12 volt leads.  The COM terminal of this relay is connected to the "NO" terminal of the first relay, and the NC terminal is connected to ground. If the key is not in the ignition, this relay has no power and the switch remains closed.  Whenever power is applied (i.e., the key is turned to the "run" or accessory position in the ignition) the switch opens.

With this arrangement the chime module is grounded (and hence goes "bong") only if power is applied to the first relay (headlights are on) and there is no power applied to the second relay (no key in the ignition).

I included two diodes in the circuit, which prevent current from flowing the wrong way from other elements in the car.  I'm not absolutely sure that the diodes are needed, but I felt it was the prudent thing to do.  I was concerned that since the relays apply ground to the chime module terminal, there is a possibility that current could be drawn from existing circuitry through the relays to ground, in a manner not originally intended.  The diode on the right in the drawing prevents that from happening.  I cut the existing red/brown wire and soldered this diode into the circuit at that point. The other diode (the one that leads to the relays) prevents any current from flowing through the relays if the chime module is grounded by some other means.   This second diode is probably totally superfluous, but better to be safe than sorry.

Remember that diodes must be wired with correct polarity - since their function is to stop current flow in one direction while allowing current flow in the other, it's important to get the direction right.  Most diodes have a band imprinted at one end or the other - this band represents the cathode of the diode, and should be oriented as shown in the figure.

The chime module is located behind the trim panel below the dash, just above the driver's left knee.  It is mounted to a couple of stud bolts above the hood release handle.  Undo the nuts on the mounting bolts, and you'll be able to pull the chime module forward to get access to the rear where the wiring is.  The headlight and foglight switches are mounted just above this area.

Be sure to solder all connections, and insulate any bare wires with generous amounts of electrical tape.