Physics
Photoresistor
A photoresistor is a type of resistor that changes its resistance based on the amount of light that falls on its surface. It is also known as a light-dependent resistor (LDR) and is commonly used in light-sensing applications such as streetlights, cameras, and solar panels. The resistance of a photoresistor decreases as the intensity of light increases.
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- Inamuddin, Mohd Imran Ahamed, Rajender Boddula, Tariq Altalhi, Inamuddin, Mohd Imran Ahamed, Rajender Boddula, Tariq Altalhi(Authors)
- 2022(Publication Date)
- CRC Press(Publisher)
6 Semiconductor Photoresistors Anoop Singh, Aamir Ahmed and Sandeep Arya DOI: 10.1201/9781003188582-6 CONTENTS 6.1 Introduction 6.2 Types of Photoresistors 6.2.1 Intrinsic Photoresistor 6.2.2 Extrinsic Photoresistor 6.3 Working Principle of Semiconductor Photoresistor 6.4 Characteristics of Semiconductor Photoresistor 6.4.1 Wavelength Dependency 6.4.2 Dependent on Sensitivity 6.4.3 Latency Dependent 6.5 Applications of Semiconductor Photoresistor 6.5.1 Light Sensor 6.5.2 Audio Compressors 6.5.3 Measurement of Incident Light Intensity 6.5.4 Light Control Circuit 6.5.5 Automatic Street Lighting 6.5.6 Wearable Image Sensing Applications 6.5.7 Photogate Timing with a Smartphone 6.6 Conclusion References 6.1 INTRODUCTION The word “Photoresistor” is made of two words “photons” and “resistor”. The Photoresistor is a special kind of resistor that is used in electronic circuits. The Photoresistor may be defined as a resistor, the resistance of which depends upon the intensity of light incident upon it. The resistance of the Photoresistor changes with the intensity of light. It comprises a conductor material that is sensitive towards light, i.e., photo-conductive material. The phenomenon governing Photoresistor is very simple, light is made incident on a Photoresistor, and excitation of electrons takes place from the valence to conduction band. More is the number of electrons transferred towards the conduction band, less will be the resistance offered by the Photoresistor. The material that is most commonly used for fabricating Photoresistors is a highly resistive semiconductor. In terms of conductivity, the materials are characterized as conductors, semiconductors, and insulators. In a conductor, the electrons can move on applying a potential across the conductor, whereas in an insulator, the electrons cannot move freely [ 1 ]. The semiconductors have attained huge interest in recent times as it has the conductivity in between the insulators and conductors
eBook - ePub- R M MARSTON(Author)
- 1999(Publication Date)
- Newnes(Publisher)
6Light-sensitive circuits
The previous four chapters have dealt with the theory and applications of light-generating and light-reflecting devices such as LEDs and LCDs. The present chapter concentrates on the operating principles and applications of light-sensitive devices such as LDRs and photodiodes, etc.LDR basics
Electronic optosensors are devices that alter their electrical characteristics in the presence of visible or invisible light. The best known devices of these types are the LDR (light dependent resistor), the photodiode, the phototransistor, and the PIR (passive infra-red) detector.LDR operation relies on the fact that the conductive resistance of a film of cadmium sulphide (CdS) varies with the intensity of light falling on the face of the film. This resistance is very high under dark conditions and low under bright conditions. Figure 6.1 shows the LDR’s circuit symbol and basic construction, which consists of a pair of metal film contacts separated by a snake-like track of light-sensitive cadmium sulphide film, which is designed to provide the maximum possible contact area with the two metal films. The structure is housed in a clear plastic or resin case, to provide free access to external light.Figure 6.1 LDR symbol (a) and basic structure (b)Practical LDRs are available in a variety of sizes and package styles, the most popular size having a face diameter of roughly 10mm. Figure 6.2 shows the typical characteristic curve of such a device, which has a resistance of about 900R at a light intensity of 100 Lux (typical of a well lit room) or about 30R at an intensity of 8000 Lux (typical of bright sunlight). The resistance rises to several megohms under dark conditions.Figure 6.2 Typical characteristics curve of a LDR with a 10mm face diameterLDRs are sensitive, inexpensive, and readily available devices with power and voltage handling capabilities similar to those of conventional resistors. Their only significant defect is that they are fairly slow acting, taking tens or hundreds of milliseconds to respond to sudden changes in light level. Useful LDR applications include light- and dark-activated switches and alarms, light-beam alarms, and reflective smoke alarms, etc. Figures 6.3 to 6.22
eBook - PDFRemote Sensing Physics
An Introduction to Observing Earth from Space
- Rick Chapman, Richard Gasparovic(Authors)
- 2022(Publication Date)
- American Geophysical Union(Publisher)
433 Appendix D Optical Sensors Optical sensors can be categorized as either thermal or photon detectors. Thermal sensors work by absorbing incom-ing radiation, causing a change in temperature of the detector. This change in temperature then impacts some measurable parameter – for example, resistance. Thermal sensors are often used in the infrared because they are typi-cally sensitive across a wide range of incident wavelengths. Photon or quantum detectors depend on the direct interaction of the incoming light with the detector materials, for example a photon hitting a semiconductor sensor creates an electron-hole pair. Such photon-generated carriers can then be measured from either: • The charge collected during an integration period. 10 – 4 10 – 3 10 – 2 10 – 1 10 0 10 1 10 2 Responsivity (A · cm 2 · W – 1 ) 2000 1800 1600 1400 1200 1000 800 600 400 200 Wavelength (nm) High Gain Sillcon S-20 Photomultiplier High Gain Vacuum Photodiode Vacuum Photodiode Silicon InGaAs GaAsP Multijunction Thermopile Figure D.1 Area responsivity of various sensor types. From Ryer (1997). • A photocurrent. • A change in resistance. • By voltage generation across a junction. No matter how the incoming radiation is converted to an electrical response, all optical sensors can be characterized by certain param-eters. The first of these is responsivity, which is defined as the ratio of the photocurrent output by the sensor to the radiant power incident on the sensor: R 𝜆 = I p P inc = photocurrent (A) power incident on sensor (W) (D.1) The units for responsivity are amps per watt. Figure D.1 shows the sensitivity of various sen-sor types to irradiance, which equals the sensor responsivity per unit area. Remote Sensing Physics: An Introduction to Observing Earth from Space, Advanced Textbook 3 , First Edition. Rick Chapman and Richard Gasparovic. © 2022 American Geophysical Union.
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