AQA GCSE Physics

Revision Notes

2.1.8 LDRs

Light-Dependent Resistors

  • A light-dependent resistor (LDR) is a non-ohmic conductor
  • It is represented by the following circuit symbol:

LDR Circuit Symbol, downloadable IGCSE & GCSE Physics revision notes

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

LDR & Light Intensity, downloadable IGCSE & GCSE Physics revision notes

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

LDR Street Lights, downloadable IGCSE & GCSE Physics revision notes

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

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