# Need 0-3v output from 0-12v input

Atomic178
Hi, this isnt a homework question, but this is one of the best technical forums Ive seen for stuff like this so I thought I would give it a shot.

Im an aerospace engineering student (3rd year) and like all engineers I like to tinker with stuff, but electrical wiring like this is not my specialty.

I have a gauge in my car that runs off a 3v source. I want to be able to dim it with the interior control knob that controls all the other lights (and 2 gauges right beside this one). I emailed the manufacturer about it and they said just throw in a resistor until its where you want it. I dont really like that idea since I want to be able to adjust it with the other gauges.
Let me back up and say the gauge lighting is run on a constant 3v from the sending unit. The typical car system is 12v, obviously. What I want is basically a linear control over the gauge, ie, I have the knob at 6v I want the gauge at 1.5v; 12v to 3v; 0v to 0v.

I asked one of my professors and he mentioned a voltage regulator, but I have never used those before...

waht
A simple two resistor voltage divider might work if the gauge doesn't draw too much current.

http://web.mit.edu/rec/www/workshop/voltage-divider.gif

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Atomic178
Yea, I considered a voltage divider, but will it always give the same ratio no matter the input voltage since it has a range of 0-12?

For testing it, would R1=3ohm and R2=1ohm work fine for a 1/4 scale?

Im not sure of how much current the display draws, but its a couple LEDs so I imagine its not much. Any way to easily find out?

Atomic178
Anyone else?

story645
Im not sure of how much current the display draws, but its a couple LEDs so I imagine its not much. Any way to easily find out?
Hook it up to a multimeter. You can use the voltage divider idea for this test.

I emailed the manufacturer about it and they said just throw in a resistor until its where you want it. I dont really like that idea since I want to be able to adjust it with the other gauges.
What about using a variable resistor?

skeptic2
This circuit should work. The diode and transistor models are not important. Just about any diode or transistor should work. Likewise it's the ratio of the values of R1 and R2 that is important not their actual values although they should be kept in the same range as the example. It may be easier to replace the diode with a second transistor and not connect the collector.

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Bob S
The electrical voltage in the car depends on whether the alternator is charging (~13,8 volts) or not (~12 volts). If this bothers you, go to the nearest electronics store and buy a 3-pin 7806 or 7805 voltage regulator.

Bob S