November 2016
I sometimes forget to turn the headlights off on my car, so I wake up to a dead battery and a late start to work. I would like to build a DIY auto headlight on/off switch. Anyone have a schematic or design?
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After a couple trips across town to jump start my wife’s car, I installed a relay to automatically turn the lights off when the ignition was turned off. All you need is one of the generic automotive 30A relays. If you can find the power wire to the headlamp switch, cut it in a convenient place and place the contacts of the relay in series with the headlamp switch. Then connect the relay coil between a switched 12V source (turns on/off with ignition) and ground. The lights will only work when the ignition is on.
If the power wire for the headlamp switch is inaccessible, then remove the headlight fuse, get a relay and an inline fuse of the same value as your headlamp fuse. Then connect the fuse to the hot side of your original fuse holder, the other side of the fuse to one of the relay contacts (usually 30) and the other relay contact (87) to the other side of your original fuse holder. The coil connections will be as above.
Auto-headlights is moderately difficult because of the length of the delays and the quiesient current. Probably best done with a microcontroller; Light level sensing is generally time based and then it stays on until the ignition is turned off. I don’t know what the times are. But take going under a bridge a mile long or so. I think it takes about 20 seconds for the lights to turn on, Never paid attention to the off threshold/time.
I can, however, offer you a simple design that I used in 1982 on my vehicle. In that particular vehicle, there was +12 available when the tail lights were on and ground via the driver’s door switch, when the driver’s door was opened. There was already an isolation diode there anyway, so I took advantage of it. I had an EASY way of “buzzer on if lights were on and driver’s door was open.” So (to +12 when parking lights on), a piezo buzzer and a diode to the driver’s door switch (ground when open, +12 when closed). I had to replace the buzzer a few times in 17 years. The high temperature kills them.
Now, one car has 1. Lights stay on until the driver’s door is opened (actually radio too) 2. A few different delays are possible for how long the lights stay on.
The other has 1. Lights turn off when the door is opened and car is off. Both have auto-on by daylight sense. In both of these cases, in order to turn-off the lights when the engine is on, you have to do something heroic.
Car #2 Turn-off car and open the door. Turn off the headlights. if they are on.
Car #1 Stop. Turn off car, engage parking brake. Turn on car which will turn off lights. If you want to start the engine with the lights off, the parking brake must be set. Turning them off is not so easy. Blinking the high beams is easy on car #2 (momentary and full on stalk switch). In car #1 it’s not possible. For auto-on; timed off, there are a lot of issues to deal with. Turning the lights off briefly is very useful if you confront a deer.
You just need a 12 volt, single pole single throw relay. Wire the contacts in series with the headlight power wire and power the relay from the ignition switch accessory terminal. You may have to find it by trial and error among the wires coming out of the steering column, or a auto repair manual may tell you the color of the wire.
Typically, the easiest way to do this is to connect a diode, a suitable resistor and a sonalert from the headlight switched positive to the RUN position of the ignition switch. If the headlights are left on when the ignition switch is turned off, the sonalert will sound. The diode prevents the sonalert from sounding when the car is running and the headlights are off. The resistor scales the 12VDC to a lower voltage for the sonalert if necessary. This is much simpler then an automatic switch, and it consumes no power.
