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how to use car door light switch? Answered

ok so i have added RGB led strips to my car with common +12v and i want to add foot light when the door is open.
the door switch uses ground (-) my led strips have 3 (-) grounds one for each color and when certain color is selected the each have different resistance* (i think) so cant connect 3 wires to door switch.
is there a way (beside using a relay) or a switch where i can use ground to turn on/off +12v wire?

thank you


Are your LED strips the same kind as shown in this,



By the way, regarding the question of how to switch something from the high side, the +12 V rail of your power supply, which happens to be a car battery, like without using a relay...

The only way I know how to do that is by using a PNP transistor with its emitter connected to +12V, and its collector connected to the positive side of your load, and the negative side of your load.

Or, analogously, kind of the same but different, by using a P-channel MOSFET, with source connected to +12V, drain connected to the positive side of your load, etc.

These pages,



kind of sort of, offer some hints on using transistor as a switch.

Also BTW, the search that led to that first link at "behindthesciences.com" was an image search for the phrase "circuit diagram for rgb led strip". Here:


yes those are the same led strip, thnx alot will check out those circuits maybe i`ll learn something

Oops. I think I forgot some words there.

"...and the negative side of your load connected to ground."

I do not see why you cannot connect together all three (or any one or two, leaving the others open) of the three, what you call "(-) grounds" of your RGB LED strips, and then connect that to the door switch, which I am assuming connects to the negative terminal of the battery. I mean, that puts the R, G, and B strings in parallel, but I do not see reason why they should not be in parallel. I am naively guessing these LED strings have some built-in, and appropriate sized, resistors, in series with each string.

Basically I am guessing that the people who designed this RGB LED strip, were, vaguely speaking, um, trying to make things easy for you.

I realize this is a lot of guesses.

Also guessing that door switch closes, becomes conductive, when the door opens.

Maybe I should draw you a picture of the way I am imagining this. That would probably make it more clear. If you want me to draw a picture (circuit diagram) of this, please reply, saying you want me to draw a picture of it.

well since there is one positive and 3 negatives i guess the negatives set the color so if a connect all three negatives together i will have white color no matter what i set from the remote?!
i havent measured the door switch but i also guess it

"becomes conductive, when the door opens."

Uh, there's a remote? Maybe these RGB LED strips are more complicated than I thought.

Here I was thinking that the three ground connections, that was the method by which the user chooses the LED strip's color.

That is to say, the number of permutations from three, on or off, choices is exactly 2^3 = 2*2*2 = 8,

with one of those combinations being all off; i.e. darkness. So that's really 7 different "colors", plus "off".

But it has a remote too? That is confusing to me.

Is there some URL, somewhere you can point to on the web, for to tell us more info about these RGB LED strips?

A manual would be nice, but just a like a product page, or a wiring diagram, or something.

I mean the only mental picture I have so far:

It is intended to be powered by a car's electrical system; i.e. 12 volts DC, nominal.

It has four terminals: one for +12V, three for ground.

It has a remote.

It will maybe make pretty colors, when wired correctly?

read the reply above, in a hury at work didn`t reply to you


10 months ago

Of course a strip will have apparent differing resistances, that is the difference of non-linear RGB LEDs. The LED resistors are set up to draw a specific current such that when all are on there is a white appearance...

You can connect any combination of strip grounds that you eyes want colors to see..

In your place, I would add a color change every door open..

good idea never thought of it but that would be good if i set to white and never change the color (maybe i`ll use this for other projects) since i have speedometer and car heating all the same color i want foot underglow same color but i will/want to be on all the time at reduced glow only when door open brightness @100%