# Help with an LED dimming project



## BigT (Apr 15, 2008)

Hey everyone,

I am a regular on a car forum and haven't been able to find the information I am looking for on there. I have installed an aftermarket boost gauge that uses LED lighting. There is only one level of brightness which is great for the day, but makes it far too bright at night. I could tint the gauge, but that would make it not bright enough during the day. I'm looking for a way to dim the gauge when I put the lights on at night and the instrument cluster dims. I know that my options are a PWM or audio taper Pot. Although I have no idea what either of these are. Can someone suggest a method that won't damage my gauge and will be easy enough for me to do, since I have no experience with any of this?

Thanks!
-T


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## ktronik (Apr 15, 2008)

Not much info to work with, but keeping it simple would be the best bet for you & your car...

The simplest way to dim a LED is via a regular resistor, 'ballast' resistor.

you would need to get in to the LED circuit & mod the current level with the resistor(s) to get the 'dim' mode. This 'dim' mode would be switched in & out via a 12v relay on the light circuit of your cars front headlights.

Thats the first thing that comes to mind... but saying is easier that doing...

Their may be many reasons why you can't mod you LED gauge to have a 'dim' mode & many other reason why you don't want to mod your car...

My 2 cents...

Ktronik


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## LukeA (Apr 15, 2008)

I would put in an SPDT switch after the power switch for the gauges. I'd wire it so that one postition is just a passthrough and on the other position I'd wire a small resistor. This way, you turn on the gauge lights and then you can switch between high and low. 

Alternatively, you could get a 3-position switch and make it off-dim-bright or whatever you want.


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## VidPro (Apr 16, 2008)

LukeA said:


> I would put in an SPDT switch after the power switch for the gauges. I'd wire it so that one postition is just a passthrough and on the other position I'd wire a small resistor. This way, you turn on the gauge lights and then you can switch between high and low.
> 
> Alternatively, you could get a 3-position switch and make it off-dim-bright or whatever you want.


 
that could be done within a small space for sure.
a SPDT *CenterOFF* can be configured to have 3 "levels" 

lemme see if i can draw that.
________
|~~|~~|

in the center off, both resisters are in the curcuit
when you switch one way only one resister, switch the other way, and its the other resister.

something like that, as long as you can connect it straight up (which i assume it already is) you cant screw it up by testing.
we use the single throw center off (which can be locally purchaced) idea to do 3 levels, and it works good, if i could just draw it so it could be understood , because i do a lot of guessing, and ON HANDS, to see what happens.

i make picture (hope its right)





ok this time for sure 
the 2 gray lines at the bottom, are one of the incomming wires to the led.
when its switched one way, it is a direct connection.
when it switched the other way, there is the low resistance
when its in the center position, the its on the high resistance.
3 ways one switch , simple right :tinfoil: if i could just make it look as simple as it is.

in the center Off position, nothing connects but the resister that is on the bottom there in cyan color, Or if the switch breaks  there is still some connection in existance. That is of course the LOW.

when you switch the one way, a direct connection occurs, bypassing the resistance there on the bottom in cyan color. that is the HIGH.

when you switch the other way, the red resister is ADDED in parellel to the cyan resister, so that is why it should be the lower ammount of resistance. that is the MED.

plus you want center off to be dimmest, and for UP to be a direct connection, as that will be the brightest.

NOTES: the pic is done with the assumption that the led or bulb or whatever is already resistered or has a driver (yes you can cut back drivers too) as it makes a DIRECT connection. if you need added resistance for driving of the led that is NOT show in the above picture.
so this is an ADDITION TO your correctally driven item. with that you do not remove any resistance you already have in driving the led.

the other thing, is using a wire wound pot, which is how they used to do the indicator lights in the car , before excessive electronics stuff. but that would be bigger and mabey overkill.


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## BigT (Apr 16, 2008)

awesome responses guys! I'm going to try to source some parts and I'll update with more quesitons I'm sure!


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## adnj (May 1, 2008)

The boost gauge probably has a 12v lead that down converts to 3 volt nominal circuit with a resistor. If your boost gauge is vacuum: Run the input to the IP illumination circuit directly from your boost gauge illumination circuit. This IP circuit is usually 12v PWM and should work for your gauge also. If your gauge is digital, you will need to isolate the gauge illumination circuit, too.

I will guess that the lighting will not match in intensity but should rise/fall with the cluster dimmer.


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