A photo resistor or LDR is an electronic component whose resistance decreases or we can say conductivity increases with increasing incident light intensity.
It can also be referred to as a light dependent resistor (LDR), photoconductor, or photocell. A photo resistor is made of a high-resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.
Cadmium sulfide (CdS) cells rely on the material's ability to vary its resistance according to the amount of light striking the cell. The more light that strikes the cell, the lower the resistance. Although not accurate, even a simple CdS cell can have a wide range of resistance from less than 100 ? in bright light to in excess of 10 M? in darkness. Many commercially available CdS cells have a peak sensitivity in the region of 500nm - 600nm (green light). The cells are also capable of reacting to a broad range of frequencies, including infrared (IR), visible light, and ultraviolet (UV). They are often found on street lights as automatic on/off switches.
Apparatus Required
- Photo Conductive Experiment Kit (As shown in Diagram).
- 5 watt white LED mounted with Lense housing and adjustable height stand.
- Variable Light Source Power Supply.
- Photoconductive sensor mounted with housing with adjustable height stand.