Transistor as an Automatic Switch - Heat Controlled Switch


  1. In a heat operated switch, the LDR is replaced by a thermistor.
  2. A thermistor is a resistor which its resistance changes as the temperature changes.
  3. There are 2 types of thermistor:
    1. The positive temperature coefficient (PTC) thermistor
    2. The negative temperature coefficient (NTC) thermistor
  4. For the PTC thermistor, the resistance of the thermistor increases as the temperature increases whereas for the NTC thermistor, the resistance of the thermistor decreases as the temperature increases.
  5. In SPM, we assume all the thermistor used is the NTC thermistor, unless it is stated otherwise.

Heat Operated Switch

  1. The circuit of a heat operated switch is similar to the light operated switch, except that the LDR is replaced by an NTC thermistor.
  2. If heat is applied to the thermistor, its resistance drops. As a result, the base voltage will increase and the transistor is switched on and the bulb lights.