# Circuit needed

Discussion in 'The Projects Forum' started by honey_ryder62, Feb 2, 2009.

1. ### honey_ryder62 Thread Starter New Member

Feb 2, 2009
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Need to get 0-5 volt range out of a 20-300 ohm internal pot. Thinking of using a constant current source and ?something? but can't find much in circuit design. 5, 10 and 12V available.

This is for creating a digital sensor for a gas tank.

2. ### triggernum5 Senior Member

May 4, 2008
216
1
Thats a basic job that pots do.. I'm confused as to what you mean when you say you need a circuit to do that.. Its kind of like asking for a circuit that drops voltage across a resistor..
A pot has 3 legs.. You can think of one like two resistors in series that form a voltage divider.. If you have 5 volts across the two end terminals, and the slider is at the half way point so the resistance between the slider, and either end is equal then you will have 2.5V measured between each end and the slider.. If that slider is all the way to one side, then you'll measure 0V between that end and the slider, and 5V between the far end and the slider..

3. ### Bernard Expert

Aug 7, 2008
5,020
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This thumbnail will approximate your requirement. Meter resistor with a 1 mA meter =5kΩ. Now it's a circuit.

4. ### Bernard Expert

Aug 7, 2008
5,020
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Forgot attachment, another senior moment.

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5. ### honey_ryder62 Thread Starter New Member

Feb 2, 2009
4
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There is already a pot in the tank with a float to set the resistance from 20 to 300 ohm but the ECU needs 0 to 5 volts to set the digital equivilant of the amount of gas in the tank. I could get away with 1-4 volts but with the current resistance range I'm looking at about 2.5 to 5 volts, I don't remember the exact numbers but it's no where near where it needs to be. I was also thinking of something with a constant current source but again the range is no where near wide enough to give 0 to 5 volts.

6. ### beenthere Retired Moderator

Apr 20, 2004
15,808
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The resistive element of the pot does not vary. If you have one end to ground and feed the other end with 5 volts, then the wiper will pick off a voltage between 0 and 5 depending on what part of the fixed resistance is is in contact with.

If the pot is 300 ohms total resistance, then a 5 volt source will only have to feed 16.7 ma current. A 7805 running off the car's 12 volts wiring can do that. It will have to dissipate about 120 mw, so a small heat sink on the tab will keep it cool.

7. ### Ron H AAC Fanatic!

Apr 14, 2005
7,018
682
I think the sensor is actually a rheostat, not a true potentiometer. A current source would potentially give more sensitivity than a series resistor, but the resulting (non)linearity is a function of how the rheostat is wound, assuming that it is wirewound.
A small current to develop a voltage,followed by an amplifier, might be the most efficient solution.

8. ### Bernard Expert

Aug 7, 2008
5,020
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Before we can go further, we need to know if i's a pot or reostat, is one end grounded??

9. ### Bernard Expert

Aug 7, 2008
5,020
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It's Gotta Be This Or That from an old song.

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10. ### beenthere Retired Moderator

Apr 20, 2004
15,808
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Or just that the travel of the wiper is mechanically limited because the float bottoms before the wiper reaches end of travel.

11. ### Bernard Expert

Aug 7, 2008
5,020
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Most likley, but same result, or it could be an intentional 20 ohms to keep from beeing a short??

12. ### Ron H AAC Fanatic!

Apr 14, 2005
7,018
682
Howstuffworks has a good description of how fuel gauges work. I've referenced the second page, which describes the sending unit and the old type indicator, but you can also read about microprocessor-controlled gauges.

13. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,182
415

HI Bernard:

I would opt for the bridge circuit too...as long as an absolute ground reference isn't needed.

Eric