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I am looking for a way to create a variable resistance signal to mimic the signal that would typically be provided by a thermistor, but by using a digital signal that switches to ground.

I currently have an older car that for the coolant temperature on the cluster gauge is usually run with a thermistor on the engine. I have upgraded the engine management (ECU) and am trying to duplicate that signal but the ECU only has digital outputs that switch to ground (DPO). I can change the duty cycle OR frequency of this DPO.

Other important details: -the gauge cluster puts out a constant voltage (still need to verify how much but it's either 5v or 12v), the single wire would typically be grounded by the now removed thermistor sensor -the DPO can only change the frequency or the duty cycle but the other needs to be fixed. In the ECU I can create a table of frequency/duty cycle vs temperature (reading from a new/different type of sensor) -the frequency range available for the DPO is from 0-1200Hz -the resistance range for the thermistor being replaced is roughly 10 Ohms-350 Ohms

I've had someone mention a capacitor being used somehow for this but my circuits knowledge stops shortly after Ohms law and vehicle wiring diagrams lol

Any help that could be provided would be great, even better if it's easily digestible!

If further info is needed let me know too

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  • \$\begingroup\$ So what is your question? Do you just want that a display on the existing instrument on the dash shows the temperature? \$\endgroup\$
    – kruemi
    May 11 at 4:32
  • \$\begingroup\$ Yes I am looking to use the dash gauge to display temperature.y question would be how can I create a variable resistor controlled by frequency/duty cycle of a digital switch that switches to ground \$\endgroup\$
    – CSla
    May 11 at 11:12
  • \$\begingroup\$ What kind of car is this? Do you know if the gauge is a bimetallic strip or d'arsonval movement? \$\endgroup\$
    – Bryan
    May 11 at 16:48
  • \$\begingroup\$ It's a 1989 BMW E30. Im not sure the difference in those but if there's a way to determine that I can find out \$\endgroup\$
    – CSla
    May 11 at 19:51

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I think for the first step, if you want to mimic something, you must know what the original one is. As far as I know, the "signal" provided by a thermistor is normally "sensed" through current or voltage by some analog-digital converter. Or, in some other cases, if the thermistor is advanced enough, it senses the current (or voltage) by itself through its built-in computer, and sends it out in digital forms.

If your thermistor is the former case (a "pure" thermistor), the "signal" can't be reproduced through a digital I/O pin on your ECU as your wish, since it's a difference between analog and digital.

If it's the latter case, which means your thermistor is digital, it's technially possible but you need to decrypt the communication protocol of your thermistor and reproduce it with your ECU, which is not an easy job. Perhaps some 3rd party customized MCUs could help.

Edit: I get your idea after your comments. I think you don't need to really create the variable resistor. You only need to create a variable voltage (or current, which is the same due to Ohm's law), because actually, electronics can't "read" resistance directly. In electronics' eyes, the resistance is expressed in terms of voltage or current. I think you can try to find a PWM to voltage D/A converter, for example, this one. It outputs 0-10V voltage depending on the duty-cycle of the input PWM wave. And what's more, most analog gauges (that's, a gauge that has a waving pointer needle) are directly driven by a variable voltage. You can directly use this output voltage to drive your analog temperature gauge with proper calibration.

Btw, theoretically, a capacitor could also do the work, but I doubt the accuracy and feasibility in practice.

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  • \$\begingroup\$ The thermistor is a very basic single wire Bosch sensor from the 80s. It decreases resistance to the grounding point as the temperature increases \$\endgroup\$
    – CSla
    May 11 at 11:08

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