Photoresistor: Difference between revisions
imported>ASIS GTM |
imported>Citation bot Removed parameters. | Use this bot. Report bugs. | Suggested by Abductive | Category:Sensors | #UCB_Category 31/154 |
||
| Line 14: | Line 14: | ||
}} | }} | ||
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 | A '''photoresistor''' (also known as a '''light-dependent resistor''', '''LDR''', or '''photo-conductive cell''') is a [[Passive electrical component|passive]] component that decreases in [[Electrical resistivity and conductivity|resistance]] as a result of increasing [[illuminance]] (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> | 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> | ||
| Line 21: | Line 21: | ||
[[File:Photoresistors - three sizes - mm scale.jpg|thumb|upright=1.2|Three photoresistors with scale in mm]] | [[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]] | [[File:CdS Photocell.jpg|thumb|Large CdS photocell from a street light]] | ||
A photoresistor is less light-sensitive than a [[photodiode]] or a [[phototransistor]]. | 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 (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> | 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> | ||
| Line 36: | Line 36: | ||
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. | 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 | 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 |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]]. | [[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]]. | ||
Latest revision as of 06:38, 3 October 2025
Template:Short description Script error: No such module "Distinguish". Template:Infobox electronic component
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 illuminance (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 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, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electrons (and their hole partners) conduct electricity, thereby lowering 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 carriers 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.[1]
Design considerations
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.[2]
Applications
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.
They are also used in some dynamic compressors together with a small 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 effect, as the oscillating light patterns control the level of signal running through the amplifier circuit.
The use of CdS and CdSe[3] 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
References
External links
- Using a photoresistor to track light
- Connecting a photoresistor to a circuit
- Limitations of Photoresistor
- Photoresistor overview - detailing operation, structure and circuit information