LDR - Light Dependent Resistor.
LDR is also called as Photo resistor, Photo conductor or Photo cell.
LDR is a variable resistor whose value decreases with increase in the intensity of the light.
It is made up of high resistance semiconductor.
If light falling on the device is of high frequency (i.e., brighter), then photons absorbed by the semiconductor gain sufficient energy to jump electrons into the conduction band.
Due to increase of electrons in the conduction band the conductivity increases which means the current increases which in turn lowers resistance.
If light falling on the device is of low frequency, then conductivity decreased. Hence resistance increases.
Type of semiconductor for LDR:
The semiconductor used may be either intrinsic or extrinsic.
Intrinsic devices have zero impurity. There are only valence and conduction bands. The energy of photon should be high to excite electrons to conduction band.
Extrinsic devices have impurities. They have other energy level closer to conduction band. Hence even the small amount of photon energy excites electrons to conduction band.
LDR is widely used in security alarms, street lamps and clock radios.