Thermistor

A thermistor, as the name indicates, is nothing more than a resistor, whose resistance value is sensitive to temperature. Like a normal resistor, it has no polarity, that is, you can't hook it up backwards. Depending on the type of thermistor, the resistance may increase or decrease with increasing temperature.

To use a thermistor to measure temperature in a project, you must monitor the resistance across the thermistor. Generally, this is done by connecting a resistor of a known value in series with the thermistor, bridging a higher voltage (say, + 5V) to ground. To eliminate confusion, let's just say that the thermistor is the component connected to ground. Then, between the thermistor and resistor can be connected to an ADC or "analog in" to measure the voltage across the thermistor.

Knowing that Vtherm / Rtherm = Vres / Rres, the resistance of the thermistor can be measured to be Rtherm = VthermRres / Vres. If the supply voltage is indeed 5 volts, then Rtherm = VthermRres / (5 − Vtherm).

That just leaves converting resistance to temperature. This conversion will depend on the particular thermistor and the information supplied. If given a B or β and R0 value (the resistance at T0 which is usually taken to be 298 K), the equation for temperature as a function of resistance can be written as: $$T = \frac{\beta T_{0}}{\beta+T_{0}ln\frac{R_{therm}}{R_{0}}}$$ For instance, if you have a 10kΩ, β = 3977K thermistor, the hard-coded equation for temperature (in Kelvin) would become: $$T = \frac{1}{0.003354+0.0002502ln\frac{R_{therm}}{10,000 \Omega}}$$

Thermistor