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Photoresistor

A photoresistor (also known as a photocell, or 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 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.

Photoresistor
TypePassive
Working principlePhotoconductivity
Electronic symbol

The symbol for a photoresistor

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 edit

 
Three photoresistors with scale in mm
 
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 devices, while a photoresistor is a passive component that does not have a 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 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 edit

 
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), 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 or LDRs are also used in laser-based security systems to detect the change in the light intensity when a person/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 edit

References edit

  1. ^ Diffenderfes, Robert (2005). Electronic Devices: System and Applications. New Delhi: Delimar. p. 480. ISBN 978-1401835149.
  2. ^ "Photo resistor - Light Dependent Resistor (LDR) » Resistor Guide". resistorguide.com. Retrieved 19 April 2018.
  3. ^ (PDF). Archived from the original (PDF) on 1 April 2013. Retrieved 17 October 2013.

External links edit

  • Connecting a photoresistor to a circuit
  • Photoresistor overview - detailing operation, structure and circuit information

photoresistor, confused, with, photoresist, photoresistor, also, known, photocell, light, dependent, resistor, photo, conductive, cell, passive, component, that, decreases, resistance, result, increasing, luminosity, light, sensitive, surface, other, words, ex. Not to be confused with Photoresist A photoresistor also known as a photocell or 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 megaohms MW 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 PhotoresistorTypePassiveWorking principle PhotoconductivityElectronic symbolThe symbol for a photoresistor 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 Contents 1 Design considerations 2 Applications 3 See also 4 References 5 External linksDesign considerations edit nbsp Three photoresistors with scale in mm nbsp 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 devices while a photoresistor is a passive component that does not have a 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 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 edit nbsp 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 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 or LDRs are also used in laser based security systems to detect the change in the light intensity when a person 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 editOptoelectronics PhotodetectorReferences edit Diffenderfes Robert 2005 Electronic Devices System and Applications New Delhi Delimar p 480 ISBN 978 1401835149 Photo resistor Light Dependent Resistor LDR Resistor Guide resistorguide com Retrieved 19 April 2018 Silonex TO 18 photocells on ceramic substrate PDF Archived from the original PDF on 1 April 2013 Retrieved 17 October 2013 External links edit nbsp Wikimedia Commons has media related to Photoresistors Using a photoresistor to track light Connecting a photoresistor to a circuit Photoresistor overview detailing operation structure and circuit information Retrieved from https en wikipedia org w index php title Photoresistor amp oldid 1218989179, wikipedia, wiki, book, books, library,

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