Photoresistor

A photoresistor is less light-sensitive than a photodiode or a phototransistor. The photoresistivity of any photoresistor may vary widely depending on ambient temperature, making them unsuitable for applications requiring precise measurement of or sensitivity to light photons. Photoresistors also exhibit a certain degree of latency between exposure to light and the subsequent decrease in resistance, usually around 10 milliseconds. The lag time when going from lit to dark environments is even greater, often as long as one second. This property makes them unsuitable for sensing rapidly flashing lights, but is sometimes used to smooth the response of audio signal compression. ==Applications==

. The photoresistor is facing rightwards and controls whether current flows through the heater which opens the main power contacts. At night, the heater cools, closing the power contacts, energizing the street light. Photoresistors come in many types. Inexpensive cadmium sulfide (CdS) cells can be found in many consumer items such as camera light meters, clock radios, alarm devices (as the detector for a light beam), nightlights, outdoor clocks, solar street lamps, and solar road studs, etc. Photoresistors can be placed in streetlights to control when the light is on. Ambient light falling on the photoresistor causes the streetlight to turn off. Thus energy is saved by ensuring the light is only on during hours of darkness. Photoresistors are also used in laser-based security systems to detect the change in the light intensity when a person or object passes through the laser beam. Resistive opto-isolators, also known as vactrols, are formed of a photoresistor in conjunction with a light source. The light source may be a small incandescent or neon lamp, or light-emitting diode. Popularity has waned, but they are still used in audio applications, such as dynamic compressors. A common usage of this application can be found in many guitar amplifiers that incorporate an onboard tremolo effect, as the oscillating light patterns control the level of signal running through the amplifier circuit. The use of CdS and CdSe photoresistors is severely restricted in Europe due to the RoHS ban on cadmium. Lead sulfide (PbS) and indium antimonide (InSb) LDRs (light-dependent resistors) are used for the mid-infrared spectral region. Ge:Cu photoconductors are among the best far-infrared detectors available, and are used for infrared astronomy and infrared spectroscopy. ==See also==