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I'm looking into building a custom gauge cluster for a project truck. The fuel level sending unit that I have will read 40 ohms when the fuel tank is empty, and 250 ohms when the fuel tank is full (it's just a potentiometer). Most aftermarket fuel gauges are designed to work with a sending unit that is 0 ohms when empty and 90 ohms when full. I am looking into designing a resistance converting/scaling circuit so that I can interface a standard aftermarket gauge to my factory fuel sending unit, but I am not sure where to start.

While researching, I came across the Negative Impedance Converter. Would it make sense to stack two "Negative Impedance Converter's" in series for my application?

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Generally speaking when information is transferred in an analog format, it is by varying a voltage (as opposed to some other electrical quantity like current or resistance). This is because the instrumentation for conditioning and digitizing voltages is simple and easy to use. Somewhere in your truck there are electronics that are measuring the resistance of your variable resistor sending unit by converting it to a voltage; it may be simpler for you modify that circuit to work with your nonstandard unit than to try and make your own middleman variable resistor.

That being said, if you absolutely must convert your 40-250 ohm variable resistor to a 0-90 ohm, the way to do it is by constructing a voltage controlled resistor (VCR). This can be a complicated circuit to build; a good place to start is with this answer: Floating voltage-controlled resistor with LM13700: How does it work?

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  • \$\begingroup\$ Thanks for pointing me towards the VCR. I'll need to do some research on it, but it appears to be what I need. Just for the sake of context, the original fuel gauge that was in this truck was an air motor - the amount of current passing through the air motor would have dictated the motor's position. \$\endgroup\$
    – kubiej21
    Jan 27 '20 at 17:51

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