# Photoresistor > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Photoresistor > Markdown URL: https://mediated.wiki/source/Photoresistor.md > Source: https://en.wikipedia.org/wiki/Photoresistor > Source revision: 1350428263 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) Light dependent resistor Not to be confused with [Photoresist](/source/Photoresist). Photoresistor Component type Passive Working principle‍ Photoconductivity Electronic symbol The symbol for a photoresistor A **photoresistor** (also known as a **light-dependent resistor**, **LDR**, or **photo-conductive cell**) is a [passive](/source/Passive_electrical_component) component that decreases in [resistance](/source/Electrical_resistivity_and_conductivity) as a result of increasing [illuminance](/source/Illuminance) (light) on its sensitive surface, in other words, it exhibits [photoconductivity](/source/Photoconductivity). A photoresistor can be used in light-sensitive detector circuits and light-activated and dark-activated switching circuits acting as a [semiconductor](/source/Semiconductor) resistance. In the dark, a photoresistor can have a resistance as high as several [megaohms](/source/Ohm) (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](/source/Frequency), [photons](/source/Photon) absorbed by the semiconductor give bound [electrons](/source/Electron) enough energy to jump into the [conduction band](/source/Conduction_band). The resulting free electrons (and their [hole](/source/Electron_hole) partners) conduct electricity, thereby lowering [resistance](/source/Electrical_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](/source/Charge_carrier) and is not an efficient semiconductor (such as silicon is). In intrinsic devices, most of the available electrons are in the [valence band](/source/Valence_band), and hence the photon must have enough energy to excite the electron across the entire [bandgap](/source/Bandgap). Extrinsic devices have impurities, also called [dopants](/source/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](/source/Extrinsic_semiconductor).[1] ## Design considerations Three photoresistors with scale in mm Large CdS photocell from a street light A photoresistor is less light-sensitive than a [photodiode](/source/Photodiode) or a [phototransistor](/source/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](/source/Latency_(engineering)) 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 The internal components of a photoelectric control for a typical USA [streetlight](/source/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](/source/Cadmium_sulfide) (CdS) cells can be found in many consumer items such as camera light meters, clock radios, [alarm devices](/source/Alarm_devices) (as the detector for a light beam), [nightlights](/source/Nightlight), 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](/source/Resistive_opto-isolator), also known as vactrols, are formed of a photoresistor in conjunction with a light source. The light source may be a small [incandescent](/source/Incandescent_lamp) or [neon lamp](/source/Neon_lamp), or [light-emitting diode](/source/Light-emitting_diode). Popularity has waned, but they are still used in audio applications, such as [dynamic compressors](/source/Dynamic_range_compression). A common usage of this application can be found in many [guitar amplifiers](/source/Guitar_amplifiers) that incorporate an onboard [tremolo](/source/Tremolo_(electronic_effect)) effect, as the oscillating light patterns control the level of signal running through the amplifier circuit. The use of CdS and [CdSe](/source/Cadmium_selenide)[3] photoresistors is severely restricted in Europe due to the [RoHS](/source/Restriction_of_Hazardous_Substances_Directive) ban on [cadmium](/source/Cadmium). [Lead sulfide](/source/Lead(II)_sulfide) (PbS) and [indium antimonide](/source/Indium_antimonide) (InSb) LDRs (light-dependent resistors) are used for the mid-infrared spectral region. [Ge](/source/Germanium):[Cu](/source/Copper) photoconductors are among the best far-[infrared](/source/Infrared) detectors available, and are used for [infrared astronomy](/source/Infrared_astronomy) and [infrared spectroscopy](/source/Infrared_spectroscopy). ## See also - [Optoelectronics](/source/Optoelectronics) - [Photodetector](/source/Photodetector) ## References 1. **[^](#cite_ref-1)** Diffenderfes, Robert (2005). *Electronic Devices: System and Applications*. New Delhi: Delimar. p. 480. [ISBN](/source/ISBN_(identifier)) [978-1401835149](https://en.wikipedia.org/wiki/Special:BookSources/978-1401835149). 1. **[^](#cite_ref-2)** ["Photo resistor - Light Dependent Resistor (LDR) » Resistor Guide"](http://www.resistorguide.com/photoresistor/). *resistorguide.com*. Retrieved 19 April 2018. 1. **[^](#cite_ref-3)** ["Silonex: *TO-18 photocells on ceramic substrate*"](https://web.archive.org/web/20130401053725/http://www.silonex.com/datasheets/specs/images/pdf/102899.pdf) (PDF). Archived from [the original](http://www.silonex.com/datasheets/specs/images/pdf/102899.pdf) (PDF) on 1 April 2013. Retrieved 17 October 2013. ## External links Wikimedia Commons has media related to [Photoresistors](https://commons.wikimedia.org/wiki/Category:Photoresistors). - [Using a photoresistor to track light](https://web.archive.org/web/20100224041125/http://www.sccs.swarthmore.edu/users/06/adem/engin/e72/lab7/) - [Connecting a photoresistor to a circuit](http://www.acroname.com/howto/photoresistor/photoresistor.html) - [Limitations of Photoresistor](https://www.electronicsforu.com/technology-trends/learn-electronics/ldr-light-dependent-resistors-basics) - [Photoresistor overview](https://www.electronics-notes.com/articles/electronic_components/resistors/light-dependent-resistor-ldr.php) - detailing operation, structure and circuit information Authority control databases GND v t e Electronic components Semiconductor devices MOS transistors Transistor NMOS PMOS BiCMOS BioFET Chemical field-effect transistor (ChemFET) Complementary MOS (CMOS) Depletion-load NMOS Fin field-effect transistor (FinFET) Floating-gate MOSFET (FGMOS) Insulated-gate bipolar transistor (IGBT) ISFET LDMOS MOS field-effect transistor (MOSFET) Multi-gate field-effect transistor (MuGFET) Power MOSFET Thin-film transistor (TFT) VMOS UMOS QFET Tunnel field-effect transistor (TFET) High-electron-mobility transistor (HEMT) RF CMOS Native transistor Other transistors Bipolar junction transistor (BJT) Darlington transistor Diffused junction transistor Field-effect transistor (FET) Junction Gate FET (JFET) Organic FET (OFET) Light-emitting transistor (LET) Organic LET (OLET) Pentode transistor Point-contact transistor Programmable unijunction transistor (PUT) Static induction transistor (SIT) Tetrode transistor Nanoscale vacuum-channel transistor (NVCT) Single-electron transistor (SET) Heterojunction bipolar transistor (HBT) MESFET Heterostructure-emitter bipolar transistor (HEBT) ITFET Avalanche transistor Junctionless nanowire transistor (JLNT) Schottky transistor Spin transistor Ballistic collection 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Chua's diode BARITT diode Stabistor Phosphorescent organic light-emitting diode (PHOLED) Thin-film diode Superluminescent diode (SLD) Josephson diode Hole accumulation diode (HAD) Silicon photomultiplier (SiPM) Metal rectifier Bifacial solar cells (BSC) Integrated Circuits Hybrid integrated circuit (HIC) Mixed-signal integrated circuit MOS integrated circuit (MOS IC) Three-dimensional integrated circuit (3D IC) Photonic integrated circuit (PIC) Application-specific integrated circuit (ASIC) Field-programmable gate array (FPGA) System on a chip (SoC) Other devices DIAC Heterostructure barrier varactor Light emitting capacitor (LEC) Memistor Memristor Memtransistor Memory cell Metal-oxide varistor (MOV) Organic semiconductor Photodetector Silicon controlled rectifier (SCR) Solaristor Static induction thyristor (SITh) Thyristor Trancitor TRIAC Varicap Vertical-cavity surface-emitting laser (VCSEL) Gate turn-off thyristor (GTO) Integrated gate-commutated thyristor (IGCT) Unijunction transistor (UJT) Quantum-cascade laser (QCL) Photoresistor Quantum dot display Quantum dot solar cell Photoelectrochemical cell Dye-sensitized solar cell Plasmonic solar cell Quantum dot laser Quadrac Interband cascade laser (ICL) Oscillistor MOS composite static induction thyristor (CSMT) MOS-controlled thyristor (MCT) Resistive opto-isolator (RO) Emitter turn off thyristor (ETO) Trisil Quantum well infrared photodetector (QWIP) Quantum cascade detector (QCD) Resonant-cavity-enhanced photo detector (RCE) Golay cell Hybrid pixel detector Semiconductor detector Voltage regulators Linear regulator Low-dropout regulator Switching regulator Buck Boost Buck–boost Split-pi Ćuk SEPIC Charge pump Switched capacitor Vacuum tubes Standard Acorn tube Audion Beam tetrode Barretter Compactron Diode Fleming valve Neutron tube Nonode Nuvistor Pentagrid (Hexode, Heptode, Octode) Pentode Photomultiplier (PMT) Phototube Tetrode Triode X-ray tube Double diode triode Additron tube Computron tube Micropup RF Backward-wave oscillator (BWO) Cavity magnetron Crossed-field amplifier (CFA) Gyrotron Inductive output tube (IOT) Klystron Maser Sutton tube Traveling-wave tube (TWT) Vircator Cathode ray tubes Beam deflection tube Charactron Iconoscope Magic eye tube Monoscope Selectron tube Storage tube Trochotron Video camera tube Williams tube Gas-filled tubes Crossatron Dekatron Ignitron Krytron Mercury-arc valve Nixie tube Thyratron Trigatron Voltage-regulator tube Geiger–Müller tube Wire chamber Neon lamp Adjustable Potentiometer digital Variable capacitor Varicap Passive Connector audio and video electrical power RF Ferrite Antifuse Fuse resettable eFuse Resistor Switch Thermistor Varistor Reactive Capacitor types Ceramic resonator Crystal oscillator Inductor Relay reed relay mercury relay Other devices Hall effect sensor SQUID Spin valve Magnetic tunnel junction (MTJ) Planar Hall sensor ggNMOS --- Adapted from the Wikipedia article [Photoresistor](https://en.wikipedia.org/wiki/Photoresistor) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Photoresistor?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.