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I'd like to unobtrusively read the voltage of a simple thermistor & voltage divider circuit. This is a hot tub/spa controller and it is using 5v -> 20k (R1) -> thermistor (R2) -> ground circuit. I put my multimeter on it which reads fine, but when I connect an esp8266 dev board ADC to this circuit (320k ohm) I am greatly affecting the resistance of the thermistor/R2 and messing up the spa controller temp reading.

Would I be able to get away with a megaohm level voltage divider for my ADC? I have a feeling it wouldn't work though so I haven't tried it yet. Also read a bit about buffers / voltage followers but I'm not sure if that's really as simple as adding one of those inline before my ESP's ADC?

I'm looking for a "good enough" approach. It doesn't have to be perfect, just better than what I have now (which is changing the thermistor resistance by kilo-ohms).

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Use an op-amp as a voltage follower.

schematic

simulate this circuit – Schematic created using CircuitLab

Pick R1 and R2 to divide 0-5V to no more than 0-3.3V but maybe 0-1V if there is no divider in your board.

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    \$\begingroup\$ Thanks! So it really is as simple as that? I will try this out. \$\endgroup\$
    – Graham McMicken
    Commented Sep 21, 2020 at 17:43

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