light dependent resistor (LDR)
Photoconductors, Light Dependent Resistors (LDR), are semiconductor components of optoelectronics whose resistance changes when light falls on them. The semiconductor materials used include cadmium sulfide( CdS) and selenium( Se), which is placed as a thin layer in a meandering pattern between the comb-shaped conductor paths. When exposed to light, the cadmium sulfide increases the electric current through enhanced hole-electron formation.
Since photoresistors are resistors, they can be operated in either direction and control the current flow via the incidence of light by changing the resistance value. If the light incidence increases, the resistor value decreases. The dynamic range of the resistance value may well be two to three powers of ten, for example between a few ohms and mega-ohms.
The spectral light sensitivity of standard photoresistors corresponds to the range of visible wavelengths, the maximum being at about 600 nm, i.e. in the yellow-orange range. However, there are also special versions that have their highest sensitivity at infrared. In addition, these components have a non-negligible inertia.
In a Light Dependent Resistor, two copper combs are mounted on an insulated base. Between them is a thin layer of cadmium sulfide (CdS) in the form of a coiled ribbon. Cadmium sulfide is a semiconductor material in which the electrical conductivity depends on the amount of incident light. The more light falls on the cadmium sulfide layer, the greater the recombination between holes and electrons, the greater the electric current.
Depending on the material, photoresistors can have a dark resistance between 1 mega-ohm and 100 mega-ohms, which can drop to less than 100 ohms to 1 kilo-ohm when illuminated at 1,000 lux( lx). The rates of change are in the millisecond range, and per second the resistance value can change by several hundred kilo-ohms. Among other applications, photoresistors are used in twilight switches. Other semiconductor materials for photoresistors include cadmium selenide (CdSe), lead sulfide( PbS), lead selenide (PbSe), cadmium telluride( CdTe), zinc oxide ( ZnO), selenium (Se), indium antimonide (InSb), indium arsenide (InAs), germanium( Ge) or silicon( Si).