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I'm trying to emulate a fluid level sensor

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skyjumper:
Hi All...

Many marine tank level sensors have an output range of 240 to 33 ohms. Resolution depends on the type of sensor. I'm trying to figure out how to generate this signal from a microcontroller. I considered using a transistor switching array, or several of them, with a whole bunch of resistors but that seems inefficient, yet it would work. I looked at some digital potentiometer ICs but I didn't see one less than 1,000 ohms. Is there a batter approach?  Thanks...
 

floobydust:
It's a hassle because the currents are relatively large and it could go as high as the vehicle's voltage (12-24V).
I've used an 8-bit discrete DAC and switched in resistors using small mosfets for each channel.  250, 125, 62.5 ohms and so on down.
You could consider TPIC6AC596 only has 1 ohm RDS on.

Your sensor range is the same as automotive fuel tank sensors, such as Chevrolet.
Please ensure your (gauge) design can never output hazardous current or voltage to the fuel tank sensor to prevent explosion or fire.
edit: updated TI spec

skyjumper:
Thanks, I'll think about that. This is really just a simulator, it will never be exposed to actual fluid. Thanks for the warning, I'm just trying to simulate a water level sensor. I didn't realize the same type of sensor is used in an auto fuel tank.

floobydust:
I had to emulate a level sensor for vehicles using alternative fuels (CNG or propane) where they wanted to keep using the stock gauge in the instrument cluster.
So the MCU has the resistor DAC. 240-33 ohms is pretty standard for the float sensor. 240 can be full or empty depending on the manufacturer.  The analog gauges are fairly high current devices so a resistor DAC needs 1/2W resistors. You can use lower number of bits, even 32 levels can be good enough.

PWM I also used but it was a hassle because the gauge can make noise if the frequency is in the audible range. If it's not an analog gauge receiving the signal, say a digital gauge then PWM did not work.

ledtester:
Update - changed 630R to 240R.

1. How much resolution do you need? A 1K digital pot with 100 taps will still give you around 21 steps from 33R to 240R. Put a 33R resistor in series to get an additional 3 usable steps.

2. You can always put a 33R resistor in series and a 240R resistor in parallel the digital pot. The result will not be linear, but you can account for the non-linearity in software.

3. FETs and JFETs can be used as a voltage controlled resistor - VCR, so that would reduce the problem to generating a voltage from a microcontroller.

There's a lot of writeups on VCRs on the web, for instance, this one:

https://www.vishay.com/docs/70598/70598.pdf

The main problem to solve is finding a device which has the resistance range you are interested in. Some suggestions are given at the end of the above app note.

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