Rheostat input -> Voltage output

tuffzc · 2005-10-21T07:54:38+00:00

I need a wizard to design a circuit to allow a 1970 fuel gauge senderto correctly operate a 1986 fuel gauge.The vehicle is a 12 volt, negative earth systemThe 1970 sender is a rheostat that normally operates an ammeter stylegauge.The rheostat reads 100 ohms to earth when the tank is empty and 13 ohmsto earth when it is fullThe 1986 sender is a capacitance thingy.It has 3 wires going to it: battery and earth in, signal out.It outputs 0 volts when the tank is empty and 4.5 volts when full.So I need a circuit to take the varying restance to earth of therheostat and convert it to the voltage output that the gauge needs.It seems like a pretty simple thing to do, but I have no electronicsdesign skills (although I would have no trouble soldering up theadaptor if I had the design)Can anyone help?

Oct 21, 2005 24 Replies

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Don Foreman 20 years ago

See updated schematic at

formatting link

I added the 240 ohm resistor and the damping circuitry.

I also added a capacitor on the output of the LM317L to maintain stability (after a SPICE simulation revealed an oscillation), and corrected the offset bias resistors so the output behaves as desired.

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britt 20 years ago

I have a similar question but my factor level unit reads 13-75 ohms and the new gauge requires 33-240 ohms. Can a circuit convert those readings?

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Don Foreman 20 years ago

Neat!

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Fred Bloggs 20 years ago

Yes- you might first try a simple resistance multiplier x3. It requires a negative supply, and works more or less depending upon how much voltage the meter wants to see and how much current it delivers: View in a fixed-width font such as Courier.

. . BATT . | . | . METER . CKT . | . | . +-------------------------+ . | | . | | . >| 10K | . 2N2907A |-----/\\/\\-------+ | . /| | | . | | / . | | 200K . | V+ | / . | |\\| | \\ . +------------|+\\ | | . | | >--+ | . | +--|-/ | . | | |/| | . | +---------------+ . /_ | | . \\/| V- / . SENDER / 100K . /\\ / . 13-75 | \\ . | | . --- --- . /// /// .

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