Recently, while studying the relationship between the resistance of a thermistor and temperature using Phyphox and ESP32 Bluetooth, the author needed to plot the relationship between the thermistor resistance and time. By pinching or releasing the thermistor, they observed changes in the resistance-time graph, thus understanding the effect of temperature on the thermistor.
The thermistor is connected in series with a 10,000-ohm fixed resistor, using the ESP32’s 5V and GND pins to create a power circuit. The voltage across the thermistor is measured using pin IO33 to calculate the resistance of the thermistor, which is then sent via Bluetooth to Phyphox for display output. This allows the resistance-time graph of the thermistor to be viewed on a mobile screen.
When pinching the thermistor, the resistance-time graph drops; when releasing the thermistor, the graph rises. This demonstrates that the thermistor has a negative temperature coefficient, meaning the resistance decreases as the temperature increases.
During this experiment, the author had a sudden idea: if they removed the thermistor and replaced it with another unknown resistor, they could also plot the relationship between that resistor and time, measuring the unknown resistance.
Without hesitation, the author replaced the previous thermistor with a 30,000-ohm fixed resistor and found that the resistance-time graph output by Phyphox was a straight line, indicating a resistance value of approximately 29,600 ohms. This measurement accuracy is already very high compared to the nominal value.
The author decided to change the original title from “Exploring the Effect of Temperature on Thermistor Resistance” to “Ohmmeter”.
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