LDRs (AQA GCSE Combined Science: Trilogy: Physics): Revision Note

Light-Dependent Resistors

• A light-dependent resistor (LDR) is a non-ohmic conductor

• It is represented by the following circuit symbol:

LDR circuit symbol

• The resistance of an LDR changes depending on the light intensity on it

  • As the light intensity increases the resistance of an LDR decreases and vice versa

The resistance of an LDR is dependent on the amount of light intensity on it

Applications of LDRs

• An LDR is a light sensor

  • This means it automatically regulates the amount of light intensity on it or activates a device when the light intensity reaches above or below a certain point

• Therefore, LDRs are found in:

  • Lights that switch on when it gets dark (eg. garden lights, street lights)

  • Alarm clocks

  • Burglar alarm circuits

  • Light intensity meters

  • Security lights

• The main advantage of an LDR is that these circuits are automatic therefore not needing any human time and intervention to function correctly everyday

Automatic street lights use LDRs to switch on and off during different times of day and night