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 its resistance changes with the light intensity on it, so it can be used in a circuit that switches a device on or off when the light intensity rises above or falls below a certain level

• Therefore, LDRs are found in:

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

  • Burglar alarm circuits

  • Light intensity meters

  • Security lights

• In a circuit with a fixed resistor in series with an LDR, the potential difference is shared in proportion to the resistance

  • When the light intensity decreases, the resistance of the LDR increases

  • The potential difference across the LDR increases and the current decreases

  • A larger potential difference across the LDR can be used to switch a lamp on when it gets dark

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

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