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There are two kinds of of thermistors, NTC (negative temperature coefficient) and PTC (positive temperature coefficient). In general, you will see NTC sensors used for temperature measurement. PTC's are often used as resettable fuses - an increase in temperature increases the resistance which means that as more current passes through them, they heat up and 'choke back' the current, quite handy for protecting circuits!

Thermistors have some benefits over other kinds of temperature sensors such as analog output chips (LM35/TMP36) or digital temperature sensor chips (DS18B20) orthermocouples.

• First off, they are much much cheaper than all the above! A bare 5% thermistor is only 10 cents in bulk.
• They are also much easier to waterproof since its just a resistor.
• They work at any voltage (digital sensors require 3 or 5V logic).
• Compared to a thermocouple, they don't require an amplifier to read the minute voltages - you can use any microcontroller to read a thermistor.
• They can also be incredibly accurate for the price. For example, the 10K 1% thermistor in the shop is good for measuring with ±0.25°C accuracy! (Assuming you have an accurate enough analog converter)
• They are difficult to break or damage - they are much simpler and more reliable

On the other hand, they require a little more work to interpret readings, and they dont work at very high temperatures like thermocouples. Without a digital-to-analog converter on board, you might be better off with a digital temperature sensor.

Their simplicity makes them incredibly popular for basic temperature feedback control. For example, lets say you wanted to have a fan that turns on when the temperature gets high. You could use a microcontroller, a digital sensor, and have that control the relay. Or you could use the thermistor to feed the base of a transistor, as the temperature rises, the resistance goes down, feeding more current into the transistor until it turns on. (This is a rough idea, you would need a few more components to make it work)

Even if you do use a microcontroller or complex system, for the price you can't beat 'em!

• Resistance at 25°C: 10K ±1%
• B25/50: 3950 ±1%
• Thermal time constant ? 15 seconds
• Thermistor temperature range -55°C to 125°C
• Wire temperature range -55°C to 105°C
• 28 AWG PVC Wire
• Diameter: 3.5mm/0.13in
• Length: 18in/45cm

Note that even though the thermistor can go up to 125°C the cable itself maxes out at 105°C so this thermistor is not good for measuring very very hot liquids

Wiring

We can use a simple voltage divider circuit to measure the voltage across the thermistor.

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ADC value = Rt / (Rt + 10K) * Vcc * 1023 / Varef

What is nice is that if you notice, if Vcc (logic voltage) is the same as the ARef, analog reference voltage, the values cancel out!

ADC value = Rt / (Rt + 10K) * 1023

Finally, what we really want to do is get that Rt (the unknown resistance). So we do a little math to move the Rt to one side:

Rt = 10K / (1023/ADC - 1)

// thermistor-1.ino Simple test program for a thermistor for Adafruit Learning System
// https://learn.adafruit.com/thermistor/using-a-thermistor by Limor Fried, Adafruit Industries
// MIT License - please keep attribution and consider buying parts from Adafruit
 
// the value of the 'other' resistor
#define SERIESRESISTOR 10000    
 
// What pin to connect the sensor to
#define THERMISTORPIN A0 
 
void setup(void) {
  Serial.begin(9600);
}
 
void loop(void) {
  float reading;
 
  reading = analogRead(THERMISTORPIN);
 
  Serial.print("Analog reading "); 
  Serial.println(reading);
 
  // convert the value to resistance
  reading = (1023 / reading)  - 1;     // (1023/ADC - 1) 
  reading = SERIESRESISTOR / reading;  // 10K / (1023/ADC - 1)
  Serial.print("Thermistor resistance "); 
  Serial.println(reading);
 
  delay(1000);
}

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