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Trick automotive light switching circuit

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Hello, and Thank You in advance for reading...

The Problem:

How to switch on a new additional set of driving lights on my Subaru using the existing fog-lamp switch. I know what you're thinking... what I am after is not that simple.


My new work takes me over the Sierra Nevadas on highway 80 sometimes at night and when quarter size snow flakes are falling, it is very difficult to see the edges of the roadway. My Subaru has installed and working fog lamps and they help a little. My solution is to mount two of these LED lamps

XIL-S1102 Spot Beam Solstice Solo Square LED Light by Vision X S1102 Spot Beam

pointing out at around a 45 degree angle to better illuminate the edges of the roadway and especially the concrete barrier on the left, basically - to be able to improve the visibility of ANY reference point on the sides of the road that will allow the driver of the vehicle to steer a more accurate course...

The Ideal:

There are times when the existing fog lights are useful and the newly installed 45 degree pointing outward LED lights might present a hazard to other motorists, so wiring both sets of lights into the fog lamp switch is out of the question. Yes, I know, a separate switch - Duh! A separate switch is what I am attempting to avoid ergo the reason for this post.

What I am after:

Very simple really. The stock fog light switch turns the stock fog lights on and off.
The new roadway side illuminating lights are switched on by turning the existing fog lamps on, then off for one to two seconds and then on again which somehow triggers a circuit that switches on the after-market lamps. In this scenario the after-market lamps use the existing fog lamp wiring for power since their amperage is low and I believe that this should not over-load the existing wiring and switch assemblies. This scenario allows for not having to run extra wires through the firewall and installing a potentially awkwardly accessible switch somewhere on the dashboard.

My initial solution:

A 555 triggered by the fog lamp switch that charges a capacitor 3/4 of the way. When the fog lamps are turned off and on again the 555 then fully charges the cap which dumps its charge into a solid state relay that then turns on the new side lamps. I am not sure that this would even work, since the relay might drain the cap before it ever is able to reach a full charge...

Anyway, obviously I need help - any ideas or suggestions are most welcome.

Thanks again in advance...
Which lead of the fog lamp is switched? The positive or negative?

it doesn't matter to me if the high or low side of the lamp is switched.

Here is a dual toggle circuit which will probably fit your needs.

Functions: Switch ON/OFF - nothing happens.
1st button push: Lamp1 (L1) - on
2nd button push: Lamp1 (L1) - off, Lamp2 (L2) - on
3rd button push: Lamp1 (L1) and Lamp2 (L2) - on
4th button push: both lamps off again.

Use a low drop voltage regulator (12V) and some means of filtering to protect the CMOS-IC from alternator spikes.

Connect the lamps directly to the relays which are also connected directly to the car power supply.

I guess using indicators (LEDs) is not a bad idea.

Attached are three possible switching conditions (Off is omitted).



Edit: power supply pins of the IC are not shown in schematic. Pin8 connects to ground and pin16 connects to +12 (13.8V).


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Last edited:
Wow, Thank You Bonkcuk for the quick reply and thorough diagrams.

What you propose is very sophisticated and if I read the schematic correctly, still requires an additional switch (the toggle button) to be run through the firewall (something I'm attempting to avoid). Also, I apologize if I didn't make it sufficiently clear, but the fog light do not need to have an off state with the curb lights on.

Anyway thanks to your great reply, which has given me a direction to start to look, I think I have found a very simple solution which should work quite nicely for this application and only requires one part (I think):

The only remaining question is weather the 711 Impulse Sequencing Relay minds having its control circuit continually energized with 12 V or wether I now still need to build a momentary pulse circuit that sends only a momentary pulse to toggle the relay on and off with each on and off cycle of the fog-lamps switch.

Thanks again,

Solution to Trick automotive light switching circuit...

Thank You Boncuk.

Thanks to your reply I now have the solution. Utilizing a 711 Impulse Sequencing Relay/DPDT w. 12 Amp Rating -

**broken link removed**

(I talked to Magnecraft tech support to verify that this will work) the 12V from the the fog lamp switch connected to the switching pins will toggle the relay on and off. The relay remembers its last state, even if all power is removed. There is no problem leaving 12V continuously connected to the switching pins. So to turn the curb lights on all I have to do is cycle the fog lamp switch once. Of course they will come on every other cycle, but that's not a major draw-back. I'm even thinking of getting another relay and using the first to trigger the second, turning on a set of these lights:


Can you say "daylight" in-front of my Subie? Anyway, that's stage two...

Thank you again Boncuk...
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