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| Type | Passive |
|---|---|
| Working principle | Photoconductivity |
| Electronic symbol | |
 The symbol for a photoresistor | |
Aphotoresistor (also known as alight-dependent resistor,LDR, orphoto-conductive cell) is a passive component that decreases in resistance as a result of increasing luminosity (light) on its sensitive surface, in other words, it exhibitsphotoconductivity. A photoresistor can be used in light-sensitive detector circuits and light-activated and dark-activated switching circuits acting as asemiconductor resistance. In the dark, a photoresistor can have a resistance as high as severalmegaohms (MΩ), while in the light, it can have a resistance as low as a few hundred ohms. If incident light on a photoresistor exceeds a certainfrequency,photons absorbed by the semiconductor give boundelectrons enough energy to jump into theconduction band. The resulting free electrons (and theirhole partners) conduct electricity, thereby loweringresistance. The resistance range and sensitivity of a photoresistor can substantially differ among dissimilar devices. Moreover, unique photoresistors may react substantially differently to photons within certain wavelength bands.
A photoelectric device can be either intrinsic or extrinsic. An intrinsic semiconductor has its owncharge carriers and is not an efficient semiconductor (such as silicon is). In intrinsic devices, most of the available electrons are in thevalence band, and hence the photon must have enough energy to excite the electron across the entirebandgap. Extrinsic devices have impurities, also calleddopants, added whose ground state energy is closer to the conduction band; since the electrons do not have as far to jump, lower energy photons (that is, longer wavelengths and lower frequencies) are sufficient to trigger the device. If a sample of silicon has some of its atoms replaced by phosphorus atoms (impurities), there will be extra electrons available for conduction. This is an example of anextrinsic semiconductor.[1]
A photoresistor is less light-sensitive than aphotodiode or aphototransistor. The latter two components are truesemiconductor devices, while a photoresistor is a passive component that does not have aPN-junction. 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 oflatency 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.[2]
Photoresistors come in many types. Inexpensivecadmium 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.
They are also used in somedynamic compressors together with a smallincandescent orneon lamp, orlight-emitting diode to control gain reduction. A common usage of this application can be found in manyguitar amplifiers that incorporate an onboardtremolo effect, as the oscillating light patterns control the level of signal running through the amplifier circuit.
The use of CdS andCdSe[3] photoresistors is severely restricted in Europe due to theRoHS ban oncadmium.
Lead sulfide (PbS) andindium 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 forinfrared astronomy andinfrared spectroscopy.
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