PHOTORESISTOR or LIGHT DEPENDENT RESISTOR
A Light Dependent Resistor (LDR) is a small, round semiconductor device which has a resistance which varies according to the amount of light falling on its surface. Light dependent resistors are used to re-charge a light during different changes in the light, or they are made to turn a light on during certain changes in lights.
Other names for light dependent resistors are: photoconductors, photo resistor, or a CdS cell.
Working of LDR:
A photoresistor or photocell or LDR is a component that uses a photoconductor between two contacts. When this is exposed to light a change in resistance is noted.
Photoconductivity – the mechanism behind the photoresistor – results from the generation of mobile carriers when photons are absorbed by the semiconductor material used for the photoconductor. While the different types of material used for light dependent resistors are semiconductors, when used as a photo-resistor, they are used only as a resistive element and there are no PN junctions. Accordingly the device is purely passive.
How to identify the LDRs
A light dependent resistor is shaped like a quarter. They are small, and can be nearly any size. There are black lines on one side of the light dependent resistor. The overall color of a light dependent resistor is gold. Usually other electrical components are attached to the light dependent resistor by metal tubes soldered to the sides of the light dependent resistor.
1) One of the most common uses for light dependent resistors is in traffic lights.
2) They are also very useful in watches and cars so that the lights can turn on automatically when it becomes dark.
3) They are also used in photographic meters, flame or smoke detectors, burglar alarms, card readers, controls for street lighting and many others.
1) They allow less power to be used in many different kinds of lights.
2) They help lights last much longer.
3) They can be triggered by several different kinds of triggers, which is very useful for motion lights and security systems.