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Old 12-02-2008, 03:43 PM   #1 (permalink)
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a low fuel light circuit?

ive been searching this long and hard and i havent really found anything. i know some of you fuel nuts have your eyes glued to your gas needle to watch it move ever so slowly. but for those of us that dont, wouldnt it be nice to have a lil light or annoying beep to come on at a selectable fuel level to let you know that you have barely enough to get to work on?

this mostly applies to older cars that dont have one. like my fiero. i would love to have one on it. and other old cars like it, you cant really rely on what the gas gauge says.

the fiero gas gauge, and probably the same for most other GM's and some others uses a resistance level to tell empty from full. the fiero's is 90 ohms @ full, and 0 ohms @ empty.

it doesnt seem too hard to do since its acting like a varisistor. but i would like others thoughts and opions on this.

comments that only apply to creating a circuit please

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Old 12-02-2008, 06:52 PM   #2 (permalink)
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This sounds like a job for the MPGuino
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Old 12-03-2008, 03:30 AM   #3 (permalink)
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I don't see why you couldn't setup a circuit that when it reaches a certain low resistance, allows the flow of current to a bulb to alert you. I'm sure that's how most standard ones are done. I honestly don't know enough about building circuits to tell you what would work, but I'm sure there's a very simple (under 10 individual circuits) that you can build together.
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Old 12-03-2008, 08:32 AM   #4 (permalink)
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i dont know enough about electronics yet either. im sure theres gotta be an electronics engineer in here somewhere that could chime in.
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Old 12-03-2008, 10:02 AM   #5 (permalink)
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In my limited experience, I can't think of a resistance switch circuit.... But, voltage switches are fairly simple.... To convert resistance to voltage, you'd need to use something called a Wheatstone bridge - basically four resistors, one of which would be your gauge's.

Not sure if that would cause any problems because you're applying an additional voltage to the resistor.... But now you've got some research to do
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Old 12-03-2008, 10:30 AM   #6 (permalink)
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I'd be willing to bet that your guage is already applying a voltage across that resistor. I can help you out, but i'll need you to put a volt meter on the resistor at full and near-empty... even better, at the emptiness that you want the light to come on.

Most likely you can do this with 1 comparitor chip (8-pin, under $1), 3 resistors and an LED. Let me know what you find

Oh, and don't just measure across the guage leads, measure from each guage lead to ground as well.
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Old 12-03-2008, 01:03 PM   #7 (permalink)
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I would use an integrated circuit called a voltage comparator to solve this problem. It compares one voltage to another to set the output on or off. You can set the fixed voltage with a voltage divider circuit which can be variable resistor connected between battery voltage and ground.

I once solved a computer problem with this circuit. The computer had a very weak output from the "Integrated Woz Machine" chip and the disk drive couldn't detect it. I used the voltage comparator to strengthen the output to the voltage expected by the disk drive. Replacing the bad computer chip would have been very difficult.
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Old 12-03-2008, 01:32 PM   #8 (permalink)
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What I would do is go to the junkyard and look for a GM car that has a low fuel light. Pull the gauge cluster, and pull the circuit board from the cluster. Trace the circuit and note which wire goes where first. Or just take the whole cluster if its cheap enough.

I know my 92 Trans Sport van has one. There was a webpage once about how to do this with an 85ish Riveria, and the board new was available from GM for $35.

Knowing GM, you might very well have a spot in your cluster for a low fuel light, just no board.
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Old 12-03-2008, 06:27 PM   #9 (permalink)
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i had a 88 6000 and swear it had a low fuel light spot. mine was the base model, so it didnt have anything. does yours have a low fuel light?
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Old 12-04-2008, 09:19 AM   #10 (permalink)
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I used to wonder that myself. When I was young and clueless I tried running it out of gas once to see if the light worked. No light.

Both of my 6000s have the full gauge cluster. I pulled another one for parts once. I believe the light spot is there for visual symmetry only, since it would look kinda corny otherwise. There was nothing printed in the low fuel light spot, like there is in every other unused spot. There is no bulb hole, though one could be carved. And right behind the bulb spot is the tachometer control board. So Im sure it didnt come with one, at least in the analog cluster. But Im sure one could be put in without too much modification.

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