Light-dependent resistor

Light dependent resistors are electronic components where the resistance of the device decreases with increasing light intensity. They can also be called LDRs, photoresistors or photoconductors.

LDR's are 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.

In intrinsic devices, the only available electrons are in the valence band, and hence the photon must have enough energy to excite the electron across the entire bandgap. Extrinsic devices have impurities added, which have a ground state energy closer to the conduction band - since the electrons don't have so far to jump, lower energy photons (i.e. longer wavelengths and lower frequencies) will suffice to trigger the device.

Applications

LDRs come in many different types. Inexpensive cadmium sulfide[?] (CdS) LDRs can be found in many consumer items such as camera light meters, clock radios, security alarms and street lights. At the other end of the scale, Ge:Cu photoconductors are among the best far-infrared detectors available, and are used for infrared astronomy and infrared spectroscopy.