A light-dependent resistor(LDR) is used to detect the presence or level of light. They work on the basic principle of photo-conductivity. It is also called as photoresistor, photoconductor or photocell. The circuit symbol of an LDR is shown in the figure.
LDR consist of a resistor made up of semiconductor material. Cadmium sulphide and cadmium selenide are the most commonly used materials for constructing these resistors. They have high resistivity and their resistance value varies according to light intensity.
A snake-like track is formed using cadmium sulphide on the surface as shown in the figure. This structure provides maximum surface and reduces resistance.