The structure of a light dependent resistor consists of a light sensitive material which is deposited on an insulating substrate such as ceramic. The material is deposited in zigzag pattern in order to obtain the desired resistance and power rating. This zigzag area separates the metal deposited areas into two regions. Then the ohmic contacts are made on the either sides of the area. The resistances of these contacts should be as less as possible to make sure that the resistance mainly changes due to the effect of light only. Materials normally used are cadmium sulphide, cadmium selenide, indium antimonide and cadmium sulphonide. It is basically a photocell that works on the principle of photoconductivity. The passive component is basically a resistor whose resistance value decreases when the intensity of light decreases.
Working of LDR:
Due to the radiant energy supplied to the semiconductor, the covalent bonds are broken and the electron hole pairs are generated resulting into flow of current.
These increased current increase the conductivity of the material and hence decrease the resistivity. Such a device is called as a photo resistor or photoconductor.
The photoconductive cell or a light dependent resistor (LDR) makes use of the principle of photoconductivity.
It is semiconductor device in which resistance is dependent on the intensity of incident light.
The resistance of the LDR will decrease with increase in the intensity of incident light.