Multiple 12 V trigger options to send one 12V output


Old 05-08-14, 12:20 AM
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Join Date: May 2014
Posts: 2
Multiple 12 V trigger options to send one 12V output


I am trying to wire up my foglights to my car, and I want the fog lights to turn on when the low beams turn on or when I turn it on using the push button switch. Can anybody advise me on what I can use to achieve this? I was thinking of using both 12 volt sources as a single input to a simple relay switch, but my concern is what if I have the low beams on and I accidentally press on the push button switch, would it burn the relay with 24 volt input? I appreciate any help you can give in order to achieve this.

Thank you.
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Old 05-08-14, 02:18 AM
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,399
Do you have dual 12 volt batteries in your car? If not then there is no way that you will have 24 volts available.

Back in my youth, well, early twenties, I put a pair of fog lights on my car. As I recall I wired them through a switch using the taillight circuit for power so that I could have the fog lights on with the tail and side marker lights but no headlights. Even on low beam the headlights will have a tendency to scatter their light to a point that makes the advantage of the low-mounted fog lights almost useless. For best results you really want the fog lights mounted as low as possible, under the bumper if possible.

However, if you still want to have the fog lights and headlights together then just take the power from the headlight circuit through the fog light switch to the lights themselves. Remember, if you wire the fog lights the way I described above there is no way that you could have the fog lights on without also having the tail lights and side marker lights also lit...which is a very important safety consideration. You can always turn the light switch from parking lights to headlights if you want both fog lights and headlights illuminated.
Old 05-08-14, 06:25 AM
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Join Date: Mar 2003
Location: NC, USA
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Furd's idea is safest. I can't tell you how many idiots I see driving around here with in the fog or after dark with just fog lights on not remembering that just because they can see doesn't mean others can see them. If you have a need for any sort of light then your tail lights should also be on. Still, you can wire as you mentioned using a relay with the relay being energized either by the light circuit or a separate switch. Just make sure to mind your polarities so if you do happen to bump the button when the lights are on there will be no damage.
Old 05-08-14, 06:53 AM
Join Date: Apr 2007
Location: Near Buffalo, NY
Posts: 4,239
Parallel wiring to the coil of a relay won't double the voltage, but it also won't solve the problem of both sets of lights turning on when either of the switches is on.

Diodes pass current in one direction only. Insert a diode after the fog light push button switch in the line coming from the low beam lights and you isolate the circuits.

Make sure the diode is rated to handle the current (amps) of the circuit.
Old 05-08-14, 08:25 AM
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Join Date: May 2014
Posts: 2

Thank you for all your advice and recommendations guys. I think I'll follow Furd's advice and just wire it with the Park Lights. Thanks again.

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