5 Volts to 240 Ohms

[MavMitchell]

Hi,

This is my challenge, I have an automotive type fuel gauge that uses a 40-240ohm resistive level sensor. It is biased high with 5 volts.
What I have in my fuel tank is a level sensor that produces 0-5 volts.
I need some type of circuit that will take the 0-5 volts and make it look like the 40-240 ohms so the gauge works.

Ideas.

[Rerouter]

That 40-240 ohm gauge is a VDO spec, but biased to 5V... I'm going to say its a compatible clone

Measure the current through that signal lead at full, empty and half reading, then should be easy to scale a howland current source for your input voltage.

otherwise you can drive these gauges by driving the signal pin to a certain voltage (e.g. an op amp that can sink current) If you want you can find the empty half and full points there,

In both cases the curve will likely be non-linear, however you can still match it to be right at those 3 points.

[Zero999]

Presumably it's fine to have steps?

The gauge won't respond very quickly, so some glitches will be tolerated.

I'd connect five resistors in series and bypass four of them with relays to make a digitally controllable resistor. The resistors are connected in series with relay contacts in parallel with each, except for the 39R. 4-bit will give 16 steps, which is probably enough. 39R, 12R, 33R 56R and 100R, will give 39 to 240R in roughly 12R steps.  Want more steps? 6-bit will give 64, use 39R, 2R2, 5R6, 10R, 27R, 56R and 100R.

[nali]

Or if it's an analogue meter you could probably just stick with the low value 40R, and PWM it with a small MCU and MOSFET. You could even get cute and compensate in code if the sensor is non-linear.

[Zero999]

Or if it's an analogue meter you could probably just stick with the low value 40R, and PWM it with a small MCU and MOSFET. You could even get cute and compensate in code if the sensor is non-linear.

That would work. Put 200R in parallel with the MOSFET and now it will PWM between 40R and 240R.

[MavMitchell]

Could I measure the voltage on the gauge over the resistance range, then just use an OP amp to produce that voltage from the 0-5?

[Rerouter]

voltage between the sense wire and ground, yes