LDR is a small device used to detect variation in light intensity.
It is a kind of variable resistor and the resistance depends on the intensity of light falling on it.
In total dark, its resistance is as high as 10 Mega Ohms which reduces to a few Ohms in bright light.
LDR is called as CdS cells because its semiconductor is Cadmium Sulphide.
Its light sensitive area is doped with impurities like Silver, Antimony or Indium.
When the light fall on the semiconductor material, flow of electron- hole pairs occur and electrical conductivity takes place.
In structure, the LDR has a thin Zig-Zag shaped semiconductor line enclosed in a transparent case.
LDR can handle very high current and even AC passes through it without harming the device.
Two leads are arising from the two ends of the semiconductor.
LDR is available in sizes 3mm, 5mm, 10mm 20mm etc