Circuit challenge! Inverting power

Alright, here's the deal...

I have a 1995 toyota pickup. Have a mediocre system in it, starting to build a big SQ system. Here's the deal. My factory dimmer wire works like no other I've seen. Instead of showing 0 when dash lights are off and fading to +12 when they are full up, it is exactly opposite that. When dash lights are off, it shows +12, and when they are at full, it shows 0. even the metra wiring harness says this lead is negative dimmer. It's ORANGE / black. How can I hook this up to a standard aftermarket receiver? I know a decent amount of elecronics, was jsut hoping you might have a solution to make it easy...

/NyxBass

Hi NyxBass, if you're adding or using an aftermarket head unit, wouldn't  you want the illumination lead (ORANGE / White)  instead of the dimmer lead (ORANGE / Black) ? ....and if no illumination lead is present, you can attach to the parking lamp lead....if it is the dimmer lead you want, what radio are you using that has a dimmer circuit  and/or instead of an illumination circuit?

the12volt  

Just a shot in the dark, not sure if this would work but you may be able to use a relay to convert the signal if the relay can transfer the dimming signal from the dimmer wire to the deck. Never heard of this problem before ?? You must have a short somewhere !!

No, I think my point passed you by. The factory wiring going to the radio harness is negative dimmer. I can just tap the parking lights, acc, whatever...but I was hoping to retain dimming ability. I think the dimmer lead is pretty buried, I was hoping not to totally have to tear apart factory wiring wraps to find the positive dimmer.  I know I want an illum. lead and not a negative dimmer lead, but that isn't  what's at the harness. Any ideas on reversing voltage?

P.S> I was also just trying to see if anyone knows how to do this...I had a few  ideas rattling around, and every curcuit idea I see sparks new ones...

Hi NyxBass, maybe I did miss your point.... you asked "How can I hook this up to a standard aftermarket receiver?"  and  I asked "what radio are you using that has a dimmer circuit  and/or instead of an illumination? ",  because even if you manage to get a positive output (which using a relay alone will not accomplish), I'm curious as to how you're going to maintain the dimming ability if the dimming ability doesn't exist with the head unit you're using. If it does exist, please tell me what radio you are using.

Now if you want to use this to dim other lights in the vehicle and not for the radio itself why not just wire them like the factory lights? One side to constant 12V+ and the other side to the dimmer output. If you still need a positive output for dimming and no positive terminal is available at the factory switch, adding a second dimmer switch would be the easiest and cheapest solution. 

the12volt

It is currently a Jensen cd something or other - mid level, not very good. Soon to be a Kenwood kdc-959 as soon as they are released. I think both have a dimming ability...the Jensen surely does. I just saw this as a challenge to myself as an istaller. I know a realy won't do it, I was just wondering if anyone had any ideas?

NyxBass,  Please don't slam me for I am a newbie to the forum, but here is what I would try.  I would recommend trying this on a breadboard of some sort before doing it in the vehicle.

Connect the vehicle's illumination wire through a 1K ohm resistor to the base of an NPN Transistor, I would recommend a TIP 41 for current capability.  On the other side of the resistor, attach the collector of the transistor back to the base.  Now for the tricky part.   Once again at the collector of the transistor attach the aftermarket headunits dimmer wire input.  I would strongly recommend diode isolating this from the factory illumination wire.  Now at emitter of the transistor I would attach a fuse (1A to 3A depends on the illumination current draw).  I would then attach the vehicle's dimmer wire on the other side of the fuse.  Unless I have had a brain freeze (which happens often)  I think this has a good chance of working.  I once again strongly recommend trying this on a breadboard with a pot acting as your vehicles dimmer wire.

Steve  

Well  Yeah, I know I could just go and dig out the positive dimmer wire, but I wanted the challenge. ;) I assume the lights from the dash run on a positive dimmer, 'cause they're lights, I just wanted to see if I could do it.

/NyxBass

Maybe I'm missing the boat here but did you ever think of this.....maybe your dimer system has a constant 12V and the dimmer is a resistor to ground??? If that were the case, the more resistance that you add would limit the current or reduce the voltage, thus bringing you closer to the 12V source voltage on the other end.  How about this....12V supply...to a bulb...to a resistor...to ground. If you were measuring from the load side of the bulb, you would see exactly what you are seeing. I know that in my street rod the dome light has a constant 12V and the pin switch makes and breaks the ground. Just my thoughts..hope it helps.