Hi all, I am new to the forum, and from what I have read, new to wiring compared to most of you. My largest hobby is wrenching on cars, which has lead to my inquiry.

Goal: I want to wire accessory interior lights into one of my cars to be controlled by the existing light/dimmer combination switch.

Problem: I understand the on-off function of the switch. I plan to use the outgoing signal from the switch to activate the relays which will be controlling each individual accessory. What I do not understand, is how to use the dimming function to also dim each individual accessory.

It is my understanding that relays are always either on or off. I am not knowledgeable of what type of Hardware I will need to interpret the outgoing signal from the dimmer switch and feed it to each accessory I am adding.

Essentially I want the existing light switch to be used solely as a signal and not a source of power. I will be using fuse blocks and relays as mentioned to power the accessories.

I like to disturb the OEM engineering as little as possible. And I hate splicing wires! I always de-pin and re-crimp connections when possible.

Thanks in advance for any help, if you could point me in the direction of the hardware I need, or take me to school if I'm way off, I would really appreciate it.

 

Hi hatch, what type of lighting are you attempting to trigger? Are they all LED, are they all incandescent, a combination of the two, etc? Seeing as you're not looking to use the dimmer switch circuit as your supply, I'm not sure how you would accomplish a synchronized dimming between the two circuits, aside from perhaps a micro-controller reading the voltage from the factory dimmer switch, and using a PWM (pulse width modulation) output to a MOSFET. There would need to be a program written to adjust the PWM output on the micro-controller as the voltage from the factory dimmer adjusts. This video might help on the use of the PWM output and MOSFET, but you would still need to run the dimmer to an input on the controller, and adjust the output based on the input:

That's probably at least one way to do it, potentially, but I'll let the experts chime in and give their ideas because I'm sure there's a simpler way, perhaps without a micro-controller.

By the way, the reason for the MOSFET is because if you use something like an Arduino for the micro-controller, it only has a 5v output for PWM, which wouldn't handle much draw (amperage), nor a 12v device of course. The MOSFET allows you to tie a 12v power source (a fused lead from your car's battery, perhaps using a relay triggered by your ACC or ignition ("ON") circuit from your ignition switch wiring) to the PWM circuit, so you can adjust the output duty cycle up and down with 12v, effectively adjusting the brightness of the lights up and down.

 

AGrayson, thanks so much for the information, and resources!

I am wiring up gages for engine feedback (boost, trans temp, and EGTs) and the light bulbs are incandescent, bit I could switch to LED if needed.

It sounds like trying to avoid using the existing switch for power is more trouble than its worth.. =/ I was hoping there would be a simple solution on the market (a dimmable relay or something)

There is a product called a Speedohealer (usually used on motorcycles) to correct or modify digital speed input and relay to the information to the speedometer. As you mentioned however, it would need to be reprogrammed for the signal I am dealing with.

 

Glad to try to help! I have a triple gauge A-pillar pod (boost, AFR, trans temp) you can see some of in this pretty old video of my car here:

) which are back-lit with what I believe are LEDs. It has been so long since I examined them many years ago, but I ended up just using a potentiometer to statically dial in the back-lighting where I wanted it at night, because they were blinding me when it got dark out. They were much brighter than the factory gauges could get at their brightest setting. This would be a good solution for both of us, if we can figure this out.

With that I was explaining, you won't be using the voltage from the factory dimmer to power the additional lighting... you'd just be feeding that voltage to the micro-controller, so the micro-controller can simply read the adjustments, in order to use the separate 12v supply from your battery to power the gauge lighting through the MOSFET. The micro-controller would just READ the 12v from the dimmer, but it would have do have something to regulate the power on the micro-controller input down to 5v max.

There actually might be a much simpler approach, now that I think about it, using just a MOSFET and an extra potentiometer. Your factory dimmer may already be using PWM on the output, rather than just current reduction like a standard potentiometer. If the output from the dimmer happens to be PWM, perhaps just feed a wire off that output to the input of a potentiometer, and the output of the potentiometer to the signal on the MOSFET. Run a fused 12v wire from your car battery (or fuse box) to input of the MOSFET, and the output leg of the MOSFET would get tied to the backlighting circuit of your gauges. The factory dimmer would not be powering the backlighting of the gauges still-- it would just be used as a signal to tell the 12v source you're going to run out of the MOSFET how to behave.

Why even add the potentiometer? So in case your aftermarket gauges lights are much brighter than your factory lights at their brightest setting, like mine were, you can tone that lighting down some to closer-match the lighting of the factory gauges, then allow the MOSFET to adjust the brightest up and down from there, in tandem with your factory dimmer circuit. You'd probably never get the brightness of the aftermarket gauges to 100% match the brightness of the lighting of the factory gauges at their brightest and dimmest settings of the dimmer switch, because you're dealing with different wattage lighting and whatnot, but you can use that potentiometer to dial it in as close as you can. Once all wired up, you can find your preferred lighting setting of the factory gauges with the factory dimmer, then dial in the aftermarket gauges with the potentiometer, and as long as you don't adjust the dimmer too far from there, the aftermarket gauge lighting shouldn't be horribly far off from the factory gauges as you adjust the dimmer switch up and down some.

Again, all just theory. I couldn't be totally wrong on this, but that would at least give you something to look into if no one else has anything more appropriate to suggest. Keep in mind, I'm a total newbie to this stuff haha.

 

Such great info!

I need to research potentiometer and MOSFET to understand what they are and re-read your posts, but yes!; that is exactly how I want to run the circuit!

 

Yeah the potentiometer is pretty simple-- it's just a variable resistor, adjusted by a dial. You can find them in various designs. A MOSFET is described in that first video I linked. Both pretty basic components. I already have a slew of resistors, and I just ordered some MOSFETs, as I'm sure I can find use for them for myself for something or other. When they get it on Monday, I'll test my theory for you either a breadboard or a small perfboard and see if it works with dimmer on my car... which I could do by the end of the week next week. If it works, you wouldn't necessarily need to solder things to a small circuit board unless you wanted to. You could always just solder the wires the the legs of the MOSFET and potentiometer ("POT"), and just use heat shrink tubing to protect the terminals, and hide everything behind the dash somewhere once you get the brightness on the aftermarket gauges dialed in.