Electronics > Beginners
I'm trying to emulate a fluid level sensor
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floobydust:
Depending on the era, older gauges were balanced coils and needed lots of current at up to 12V.
As the car's system voltage moves around, current in both coils moves around yet the gauge reads the same. The (level sensor simulator) circuit needs to be constant resistance. How would you set a constant-current circuit to 100 ohms? You need to include the voltage too.
I had trouble finding the gauge's current requirements and designed for pretty much anything using the discrete DAC. OP should confirm what type he dealing with, it could save money and complexity.

Chevy Suburban dash uses a 250 ohm pullup to 5V, so 10-16mA typ. on 240R full, 33R empty sensor.  That would be easy to interface to. VDO sensors are the opposite 240R empty, 33R full.
KMoffett:
In my attached link the opamp drives the MOSFET so its resistance allows a current through R3 that causes a voltage drop that proportional the input voltage. At 12v the resistance of the MOSFET(variable) +R3(fixed) is a simulation of the fuel gauge resistance controlled by a scaled DC input to the opamp.
floobydust:
That circuit is a constant-current source, but we need constant resistance.
Depending on the actual gauge, it could be 5,9,12,14V. So commanding a 40mA current could result in 125,225,300,350 ohms respectively.

Rerouter's circuit is clever in taking voltage feedback to the DAC VREF pin, to give constant resistance.
The Microchip DAC is very low cost, it would be a good solution.
KMoffett:
For a fixed input voltage to the opamp, the constant current through the MOSFET+R3 is the equivalent to a constant resistance. For the current set by the input, the voltage drop across the MOSFET+R3 is essentially the same as the drop across the fuel gauge resistance in series with the gauge coils. Changing the input voltage changes the resistance of the MOSFET+R3.  Am I wrong?
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