Thermistors and LDRs

Thermistors

A wide rectangle centred on a horizontal line, with a line cutting across at 45 degrees from bottom to top. This line levels off to the bottom left.Thermistor

Thermistors are used as temperature sensors, for example, in fire alarms. In the most common type of thermistor, the resistance decreases as the temperature increases:

  • at low temperatures, the resistance of a thermistor is high and little current can flow through them
  • at high temperatures, the resistance of a thermistor is low and more current can flow through them

LDRs

A wide rectangle centred on a horizontal line, enclosed in a circle. Two arrows at the top left are pointing down and right at the circle.Light dependent resistor (LDR)

LDRs (light-dependent resistors) are used to detect light levels, for example, in automatic security lights. Their resistance decreases as the light intensity increases:

  • in the dark and at low light levels, the resistance of an LDR is high and little current can flow through it
  • in bright light, the resistance of an LDR is low and more current can flow through it