The Light-Dependent Resistor (LDR) (OCR A Level Physics): Revision Note

The Light-Dependent Resistor (LDR)

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

• Its resistance automatically changes depending on the light energy falling onto it (illumination)

• As the light intensity increases, the resistance of an LDR decreases

Resistance of an LDR depends on the light intensity falling on it

• This is shown by the following graph:

Graph of light intensity and resistance for an LDR

• LDRs can be used as light sensors, so, they are useful in circuits which automatically switch on lights when it gets dark, for example, street lighting and garden lights

  • In the dark, its resistance is very large (millions of ohms)

  • In bright light, its resistance is small (tens of ohms)

LDRs are used for automatic street lights