# Dimmer/Rheostat help...converting negative into positive

Discussion in 'The Projects Forum' started by SilverJester, Sep 24, 2010.

1. ### SilverJester Thread Starter New Member

Sep 24, 2010
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I'm hoping someone on this forum can help me out as I'm not very good with wiring or hardware components (more of a software kinda guy).

I'm wiring in some new gauges into my car and need a little help dimming the lights on the gauges. The gauge has 1 wire (which requires a +12v source) to adjust the brightness of the lights. However the car's factory dimmer/rheostat has 3 wires (+12V into the rheostat, ground into the rheostat, and ground out of the rheostat). The rheostat dims the lights by increasing the resistance to current ground coming out of it.

I'd like to be able to dim the new gauges using the factory rheostat (which also dims other lights in the car). But as you read above, the factory rheostat effects only the ground, not a positive wire that the new gauges could use for dimming. So my question is, is there a way with some sort of electrical component, to take the ground that is coming out of the rheostat as an input, an output a + source that is in proportion to the resistance on the ground wire?

2. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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1,125
It should still work whether the resistance is at ground or at the 12 volt. The electrons must go from the negative battery terminal to the positive terminal. It doesn't matter much to them if they have a resistance at one end of the circuit or the other, they must STILL go through that resistance to complete the circuit, no matter where it is placed in the circuit.

whether you lower the 12 volt line relative to ground, or you raise ground potential relative to 12 volts. The difference will be the same. The potential difference between those points will decrease.

3. ### SilverJester Thread Starter New Member

Sep 24, 2010
3
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Perhaps I was unclear. The new gauges have (among other wires) a +12v for power, a ground, and another + wire with max input of 12v for the lighting. Attaching the gauge's ground to the rheostat would effect it's ability to preform it's functions because it would cause resistance on the main +12v power wire. I believe internally, the gauges use + wire for powering the gauges core functions, and another + input for lighting but both share a ground. Basically the only way to adjust the light would be to vary the voltage supplied to the + lighting wire. But the car's factory setup only varies resistance to a ground wire.

4. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,158
1,125
That is different then. I would suggest a voltage controlled pot for that then. I think they are sometimes called a digital potentiometer. Some experimentation would be required to get the right values for a voltage divider network from the rheostat to the battery and have that output feed the digital pot which would then regulate a seperate 12 volt line to feed the dimmer function.

5. ### marshallf3 Well-Known Member

Jul 26, 2010
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If at all possible you could disassemble the gauge and change the wiring, that being of course if they're all lit with incandescent lights.

6. ### SilverJester Thread Starter New Member

Sep 24, 2010
3
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I don't want to attempt to open the gauges and rewire things in there. They were kinda expensive and I don't want to mess them up. Also they're lit with LEDs.

I was thinking about just having it switch to a predetermined dim state by using a relay (hooked to the headlights for a signal wire) that would switch between a 12v source for full daytime brightness and a potentiometer that I'll adjust to my desired brightness level for nighttime driving. Like I said though I don't know much about electronic compnents and I went to radio shack and they said they didn't have anything rated for DC. Can anyone help me with a link to one that would work for me?

7. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
You need something like the attached schematic.

This takes your existing PWM signal, and inverts it's polarity to source current instead of sink it. The change in brightness of your added gauge will match the change in brightness of your existing instruments.

V1/M1 represents your instrument clusters' existing low-side PWM for the lamps.

In the interest of keeping the load on the existing PWM system low, I used Q1 as a voltage follower/buffer to supply Q2's base current.

R1 limits the current through Q1's base. R3 ensures that Q1 turns off quickly when the existing PWM circuit is not sinking current.
R2 (a 2 Watt resistor) limits the base current through Q2. R4 ensures that Q2 turns off quickly when Q1 is not supplying base current.

You could use a TIP42G for Q2; Radio Shack sells them. http://www.radioshack.com/product/index.jsp?productId=2062613

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8. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
203
Depending of course on the current required a common CdS photocell might work?