ATTN elec. Gurus, lighting/dimming question

[JonnyWachter]

I mean the aftermarket gauges get brighter as the OEM gauges get darker. the same as they do when the transistor isn't hooked up.

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[Cougar4]

Well then it sounds like you may have wired the aftermarket lights into the illumination circuit incorrectly. It seems to me that you may have installed the aftermarket lights in series with the dash lighting.

Normally, the illumination control dims the lights by adding resistance into the return side of the power to the lights. That makes less voltage available to the lights and they dim. It seems you may have placed the aftermarket lights across the dimmer control instead of in series with it. In the older models there is a violet wire that supplies power to the lights. The newer models may be the same or perhaps different. You need to find the power supply lead and tie that to one side of the new lights. Then tie the other end to the lights to the illumination controller lead that ties to the return side of the dash lights.

[JonnyWachter]

The illum(+) from the transistor is hooked up to the violet, the illum(-) from the resistor was hooked up to the black/white wire. I looked over everything a number of times before I came here. I did have the incorrect resistors on there, I need to grab some 10k tonight. I feel that wont change anything.

[SVXdc]

Originally Posted by JonnyWachter

I built the transistor, hooked it up and the illumination is still reversed. I can completely bypass the unit and plug my illumination wire into the black/white wire and get the same result... what is going on here? Car is a 2010 STI.

You want your circuit's output to be 12V when the input is 0V and output 0V when the input is 12V. That will invert the PWM signal (so that higher % 'on' time makes your gauge's LED appear brighter).

Sounds like your transistor circuit is acting as a buffer rather than inverting the signal (giving you 12V input = 12V output, and 0V in = 0V out).

Are you sure you got an NPN transistor (not a PNP)?

As I said above, the link to the example circuit that I posted was the first thing I found via Google. I have not tested the circuit myself.

You might need to find someone locally with EE skills who can help you sort things out.

ADDED: I assumed you're driving LED(s) in a gauge. If not, what are you trying to dim? This trick probably won't work for most aftermarket head units. They simply want to see the illumination(+) signal (12V when parking or headlights are on, no variable dimming).

[JonnyWachter]

I do have the NPN transistor, ill go over everything again. Ill have to talk to my EE at work and see what he says. I am running LED's to a gauge.

[Cougar4]

The way you have the dimmer control installed now is on the power side of the circuit. So it is limiting the voltage on the positive side of the lights. Install it on the return side of the lights. The controller wire that is currently tied to the violet wire needs to go to the wire that should now be going to ground for the lights. That wire will still be positive with reference to ground when it is changed over correctly. The controller will be in series with the light on the return side of the light.

[JonnyWachter]

these gauges have a common ground which means I cant separate the return side. I read earlier that if I could separate the return side then I would be able to just hook my LEDs straight into the system. In this case that's the point of the transistor, it flips the voltage so I could run it to the power side... I thought.

[Cougar4]

Okay, the picture is getting clearer now. I don't know what the blk/wht wire ties to but your added light control should tie to a true ground point (the emitter lead of the transistor). You can then tie the resistor/base lead of the transistor to return side of the dash lights which is controlled by the dimmer control. You may have to experiment with the base lead resistor value to get the intensity you want for the added lights.

[JonnyWachter]

from my research the BLK/WHT wire is the Illum(-) dimmer signal and the violet is the illum(+) so i would need to connect something to the blk/wht wire to get the correct dimming.

[Cougar4]

Okay, if the blk/wht wire is the control lead then tie that lead to the resistor that goes to the base lead of the transistor. Tie the emitter lead of the transistor to chassis ground of the vehicle. Leave the collector side of the transistor and load resistor tied to the violet power lead of the lights.

[JonnyWachter]

I'll try that then! thank you for the help! so just to make things easier... this is what im doing...


sorry for the lack of electrical knowledge... i'm really trying here.

That leaves me with still needing a lead that goes to the gauges 12v+ lighting lead.

[SVXdc]

I haven't deciphered what you're doing with that circuit (I don't recall and don't have time to look up the pin-out of an LM741 op amp). But you're unnecessarily complicating things.

The purpose of vrg3's original op amp circuit was to invert the older-style Subaru analog dimming signal. So as the car gradually changed the voltage from 12V down to 0V, the circuit would output 0V increasing to 12V -- something you could connect directly to the (+) lead of an incandescent bulb, to have it dim and brighten in tandem with the car's instruments.

Since your 2010 has the newer PWM dimming, just use the simple transistor circuit that I linked to. That should invert the car's illumination(-) PWM signal (from the black/white wire). As I wrote in that post, you can use the illum(+) as your +12V power for that circuit.

So you'll connect 3 wires into the transistor circuit: illum(+), illum(-), and chassis ground. The circuit's output goes to the gauge's LED(+) lead. The LED(-) pin gets chassis ground internally from your gauge.

You can bench test this circuit: Instead of using the car's PWM signal, touch the input wire to either +12V or ground. The output should be the opposite. In other words, the gauge's LED should light when you touch the input wire to ground, and go out when you touch the wire to +12V.

[SVXdc]

BTW, for people who need the original op amp circuit for older Subaru models with the analog dimming signal, this is the updated link to vrg3's circuit:

www.surrealmirage.com/vrg3/dimmer

[Cougar4]

I didn't realize you had an OP amp in the circuit and I'm not sure you need it. I suggest you refer to post #120 of this thread and try using the transistor circuit marked as Transistor Invertor Circuit (NOT Gate) to see if that works for your needs.