Light Dependent Resistor (LDR)

An LDR is an input transducer (sensor) which converts brightness (light) to resistance. It is

Photograph © Rapid Electronics

circuit symbol

made from cadmium sulphide (CdS) and the resistance decreases as the brightness of light falling on the LDR increases.
A multimeter can be used to find the resistance in darkness and bright light, these are the typical results for a standard LDR:

• Darkness: maximum resistance, about 1M.
• Very bright light: minimum resistance, about 100.

For many years the standard LDR has been the ORP12, now the NORPS12, which is about 13mm diameter. Miniature LDRs are also available and their diameter is about 5mm.An LDR may be connected either way round and no special precautions are required when soldering.