An NTC (negative temperature coefficient) thermistor is a resistor whose resistance decreases with increasing temperature. They are low cost and fairly accurate, and can be read by a simple analogue input.
Adafruit include a thermistor on their Circuit Playground Express  and Circuit Playground Bluefruit .
The thermistor is usually connected in a voltage divider arrangement with a fixed resistor:
With this configuration the resistance of the thermistor is defined as follows, where reading is the 10-bit ADC reading, from 0 to 1023, and series-res is the series resistance, 10kΩ:
The following program reads the thermistor and returns the temperature in °C. It uses the following parameters:
|pin||23||The ADC pin|
|series-res||10000||The series resistor|
|nom-res||10000||The resistance of the thermistor at the nominal temperature|
|nom-temp||25||The nominal temperature in °C|
|b-coeff||3380||The B coefficient of the thermistor, from the datasheet|
The values above are suitable for the thermistor used in Adafruit's Circuit Playground.
(defun temperature (pin series-res nom-res nom-temp b-coeff) (analogreference :ar-vdd4) (- (/ (+ (/ (+ nom-temp 273.15)) (/ (log (/ (* series-res (1- (/ 1023 (analogread pin)))) nom-res)) b-coeff))) 273.15))
For example, with the Circuit Playground Bluefruit you can read the temperature with:
> (temperature 23 10000 10000 25 3380) 20.3813
- ^ Circuit Playground Express on Adafruit.
- ^ Circuit Playground Bluefruit on Adafruit.