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I have been given a humidity sensor to work with and the only info I have is an AliExpress listing that says that it's based on the HR202L sensor.

The datasheet of the HR202L shows that its resistance changes with humidity, and gives a table of humidity value, resistance and temperature.

The sensor combines the resistor and a conditioning circuit; the circuit seems to be all analog. It has a quad op-amp LM324 and passives. It is shown in the figures below:

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Using a multimeter to trace paths on the single sided board, I arrived at this:

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I have drawn the orange diode D1 as the Zener diode and the HR202L as the thermistor. The two other diodes are on the 3-pad package with makings as C3.

From my interpretation, U1C is an oscillator, U1D is a kind of precision rectifier, U?A is a summing amplifier, and U1B generates Vref. However, I fail to arrive at an equation relating the resistance of the HR202L to the output voltage; without the equation, the sensor is not useful as it is not linear. The precision rectifier part is quite confusing to me.

I can go for another humidity sensor but I really want to understand this conditioning circuit and how to do the calculations. I don't want to give up on the challenge :)

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  • \$\begingroup\$ please add component labels that match the silkscreen on the PCB \$\endgroup\$
    – jsotola
    Nov 19, 2022 at 0:11

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It’s measuring the impedance of the sensor. Such sensors don’t like DC, so AC is created, and that is used to produce output DC via the precision rectifier. You can correct the errors and omissions in your schematic and attempt to analyze (or perhaps simulate) the circuit.

Alternatively you can temporarily remove the sensor and replace it with a largeish low leakage capacitor (say 1uF film) in series with known resistances based on the datasheet for the sensor. Do that at several points and you will determine the transfer function RH->voltage.

Without correct component values (including at least one unmarked component) you won’t be able to get a number analytically.

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  • \$\begingroup\$ Thanks, what is the reason for adding the capacitor in series with the resistors. \$\endgroup\$
    – k.h
    Nov 19, 2022 at 5:37
  • \$\begingroup\$ Because I think that will better model the sensor (especially at lower resistances). You can try it both ways and see if there is any difference. \$\endgroup\$ Nov 19, 2022 at 5:45
  • \$\begingroup\$ Thank You, will give feedback if I try \$\endgroup\$
    – k.h
    Nov 19, 2022 at 8:26

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