The Light-Dependent Resistor (LDR)

In a light-dependent resistor the resistance decreases as the light intensity (brightness of light) increases.

It has the symbol:

  • At low light levels or in the dark the resistance of an LDR is high and little current flows through it.

  • In bright light the resistance of an LDR is low and more current flows through it.

The properties of an LDR are put to use in digital camera. The shutter speed of a camera can be controlled by an LDR. If light levels are low changes in the resistance of the LDR can be measured causing the shutter to stay open for longer. An LDR can also be used in a burglar alarm circuit for e.g. inside a safe. When the safe is closed it is dark and so the resistance of the LDR is high. If it is opened and the LDR exposed to light the resistance drops allowing current to flow which can then trigger a relay circuit which rings an alarm.


Current Potential graph for a Light-Dependent Resistor (LDR)