Editing Photoresistor

{{Short description|Light dependent resistor}}
{{distinguish|Photoresist}}
{{Infobox electronic component
|name              = Photoresistor
|image             = File:LDR 1480405 6 7 HDR Enhancer 1.jpg
|image_size        = 201
|caption           = 
|type              = [[Passivity (engineering)|Passive]]
|working_principle = [[Photoconductivity]]
|invented          = 
|first_produced    = 
|symbol            = [[File:Photoresistor symbol.svg]]
|symbol_caption    = The symbol for a photoresistor
}}

A '''photoresistor''' (also known as a '''light-dependent resistor''', '''LDR''', or '''photo-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 exhibits [[photoconductivity]]. A photoresistor can be used in light-sensitive detector circuits and light-activated and dark-activated switching circuits acting as a [[semiconductor]] resistance. In the dark, a photoresistor can have a resistance as high as several [[Ohm|megaohms]] (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 certain [[frequency]], [[photon]]s absorbed by the semiconductor give bound [[electron]]s enough energy to jump into the [[conduction band]]. The resulting free electrons (and their [[electron hole|hole]] partners) conduct electricity, thereby lowering [[Electrical resistance|resistance]]. 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 own [[charge carrier]]s and is not an efficient semiconductor (such as silicon is). In intrinsic devices, most of the 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, also called [[dopants]], 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 an [[extrinsic semiconductor]].<ref>{{cite book|last=Diffenderfes|first=Robert|title=Electronic Devices: System and Applications|year=2005|publisher=Delimar|location=New Delhi|isbn=978-1401835149|pages=480}}</ref>

==Design considerations==
[[File:Photoresistors - three sizes - mm scale.jpg|thumb|upright=1.2|Three photoresistors with scale in mm]]
[[File:CdS Photocell.jpg|thumb|Large CdS photocell from a street light]]
A photoresistor is less light-sensitive than a [[photodiode]] or a [[phototransistor]].  The latter two components are true [[semiconductor device]]s, while a photoresistor is a passive component that does not have a [[p–n junction|PN-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 of [[Latency (engineering)|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.<ref>{{cite web|url=http://www.resistorguide.com/photoresistor/|title=Photo resistor - Light Dependent Resistor (LDR) » Resistor Guide|website=resistorguide.com|access-date=19 April 2018}}</ref>

==Applications==
[[Image:Streetlight control.jpg|thumb|The internal components of a photoelectric control for a typical USA [[streetlight]]. 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), [[nightlight]]s, 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 some [[Dynamic range compression|dynamic compressors]] together with a small [[incandescent lamp|incandescent]] or [[neon lamp]], or [[light-emitting diode]] to control gain reduction. A common usage of this application can be found in many [[guitar amplifiers]] that incorporate an onboard [[Tremolo (electronic effect)|tremolo]] effect, as the oscillating light patterns control the level of signal running through the amplifier circuit.

The use of CdS and [[Cadmium selenide|CdSe]]<ref>{{cite web |title=Silonex: ''TO-18 photocells on ceramic substrate'' |url=http://www.silonex.com/datasheets/specs/images/pdf/102899.pdf |format=PDF |access-date=17 October 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130401053725/http://www.silonex.com/datasheets/specs/images/pdf/102899.pdf |archive-date=1 April 2013 }}</ref> photoresistors is severely restricted in Europe due to the [[Restriction of Hazardous Substances Directive|RoHS]] ban on [[cadmium]].

[[Lead(II) sulfide|Lead sulfide]] (PbS) and [[indium antimonide]] (InSb) LDRs (light-dependent resistors) are used for the mid-infrared spectral region. [[germanium|Ge]]:[[copper|Cu]] photoconductors are among the best far-[[infrared]] detectors available, and are used for [[infrared astronomy]] and [[infrared spectroscopy]].

==See also==
*[[Optoelectronics]]
*[[Photodetector]]

==References==
{{reflist}}

==External links==
{{commons category|Photoresistors}}
* [https://web.archive.org/web/20100224041125/http://www.sccs.swarthmore.edu/users/06/adem/engin/e72/lab7/ Using a photoresistor to track light]
* [http://www.acroname.com/howto/photoresistor/photoresistor.html Connecting a photoresistor to a circuit]
*[https://www.electronicsforu.com/technology-trends/learn-electronics/ldr-light-dependent-resistors-basics Limitations of Photoresistor]
* [https://www.electronics-notes.com/articles/electronic_components/resistors/light-dependent-resistor-ldr.php Photoresistor overview] - detailing operation, structure and circuit information

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[[Category:Resistive components]]
[[Category:Optical devices]]
[[Category:Sensors]]