Photocell Sensors |Construction | Circuit | 4 Important Types

Photocell Sensors |Construction | Circuit | 4 Important Types

PHOTOCELL SENSORS

What are the photocell sensors?

Photocell sensor is a type of light-sensitive resistor that varies its resistivity depending on the intensity of light it receives. These sensors are generally connected with an electrical or electronic circuit. When the intensity of light is lower, the resistance is higher. This happens as the increase in light energy or intensity allows the flow of more electrons, thereby, reducing the resistance. High resistance semiconductor material cells like Cadmium sulfide cells are used in photocell sensors as they are sensitive to IR light. Materials such as indium antimonide (InSb), lead selenide (PbSe), and lead sulfide (PbS) also replace Cadmium sulfide cells at times.

Contents

How is a photocell sensor constructed?

A Photocell sensor is constructed with an evacuated glass tube which comprises two electrodes like an emitter and a collector. The emitter terminal generally has a semi-hollow cylindrical shape and is arranged at a negative potential always. The collector terminal takes the form or shape of a metal that can be fixed at the axis of the partially cylindrical emitter. The collector terminal is generally kept at a positive terminal. The evacuated glass tube is then fixed over a nonmetallic base and pins are present at the base in order to arrange for an exterior connection.

What is the working principle of a photocell sensor?

The internal photoelectric effect forms the basis of the working principle of photocell sensors. It states that when a metal surface is bombarded with light energy or photons, the free electrons on the metal surface can get excited and jump out of the metal lattice resulting in the flow of electrons or electric current. The electron emission from metal surfaces can occur only when the bombarded photons reach a certain threshold frequency that corresponds with the least energy needed by the electrons for breaking the metal bonds. This photoelectric phenomenon is used to varying the electrical resistance in a photocell.

The emitter plate is connected to the negative terminal and the collector plate is connected to the positive terminal. When the frequency of the light received by the emitter plate exceeds a threshold frequency, the electron flow takes place. the collector plate is connected to the positive terminal so the electron flow would be towards the collector. If the energy of the radiant light is increased then the current flow inside the circuit will also increase.

Photocell sensor circuit

The photocell sensor circuit is also called a dark sensing circuit or a transistor switched circuit. The components required to construct a photocell sensor circuit are breadboard, battery-9V, jumper wires, transistor 2N222A, resistors-22 kilo-ohm, 47 ohms, photocell and LED. The photosensor circuit is capable of working in two conditions: when there is light and when there is dark.

photocell sensor
A Photocell sensor circuit.

When there is light, the resistance of the photocell is less. Hence, the current flows mainly through the photocell and the R2 resistor. In this case, the transistor starts working like an insulator that cuts off the flow of current through the LED and the R1 resistor.

When there is dark, the resistance of the photocell is high. Hence, the current would flow through the transistor towards the emitter. When the power is received by the base terminal, the transistor behaves like a conductor. This allows current flow through the resistance R1 and the LED.

What are the types of photocells?

Photocell sensors can be divided into 4 major types:

Photovoltaic cells:

Photovoltaic cells work on the principle of the photovoltaic effect to convert light energy directly into electrical energy. These cells can generate an electromotive force that depends on the amount of radiant energy received. The commonly used single-junction silicon cells produce a maximum open-circuit voltage of around 0.5 to 0.6 Volts. These cells use selenium as a photovoltaic material. In addition to the generation of electrical energy, photovoltaic cells also work as photodetectors. Photo-conductive cells and photo-emissive cells also serve a similar purpose.

Photocell Sensors |Construction | Circuit | 4 Important Types
A crystalline silicon solar photovoltaic cell. Image source: Unknown author, Solar cell, marked as public domain, more details on Wikimedia Commons

Photo-resistors:

Photo-resistors or light-dependent resistors or LDR are instruments that can alter their own internal resistance depending on the amount of incident light energy. When the intensity of light is lower, the resistance is higher. This happens as the increase in light energy or intensity allows the flow of more electrons, thereby, reducing the resistance. High resistance semiconductor material cells like Cadmium sulfide cells are used in photocell sensors as they are sensitive to IR light. Materials such as indium antimonide (InSb), lead selenide (PbSe), and lead sulfide (PbS) also replace Cadmium sulfide cells at times. Photo-resistors take a much longer time (of about a few seconds) for responding to the exposed light.

Photocell Sensors |Construction | Circuit | 4 Important Types
A Photo-resistor. or LDR. Image source: © Nevit Dilmen, LDR 1480405 6 7 HDR Enhancer 1CC BY-SA 3.0

Golay cells:

A Golay cell is generally used for sensing IR radiations. These cells consists of a blackened metal plate cylinder that is filled with noble gases like Xenon on one end. When, the infrared radiations fall on the metal plate, the xenon gas gets heated up and twists the elastic diaphragm present on the other end of the cylinder. The movement of the diaphragm helps in finding the output of the energy source.

Photocell Sensors |Construction | Circuit | 4 Important Types
A schematic diagram of Golay cell. Image source: Ehab Ebeid, originally Tls60 Golay_Cell_Schematic.pngGolay Cell SchematicCC BY 3.0

Photomultiplier:

A photomultiplier is a device that is used for multiplying or enhancing blurry or unclear light about 100 million times. Photomultipliers are extremely sensitive and are capable of detecting very faint light. Photomultipliers are of three types, magnetic photomultiplier, electronic photomultiplier, and silicon photomultiplier.

Photocell Sensors |Construction | Circuit | 4 Important Types
A schematic diagram of a photomultiplier tube. Image source: Qwerty123uiopPhotoMultiplierTubeAndScintillatorCC BY-SA 3.0

What are the applications of photocell sensors?

Photocell sensors are used for a variety of purposes like:

  • Automatic street lights.
  • Burglar alarms.
  • Race timers.
  • Robot manufacturing.
  • Camera exposure meters.
  • Automatic switches.
  • Lux meters.
  • Automobile lights.
  • Automatic irrigation sprinklers.
  • Automatic doors.

To know more about light sensors visit https://lambdageeks.com/light-sensors/

About Sanchari Chakraborty

Photocell Sensors |Construction | Circuit | 4 Important TypesI am an eager learner, currently invested in the field of Applied Optics and Photonics. I am also an active member of SPIE (International society for optics and photonics) and OSI(Optical Society of India). My articles are aimed towards bringing quality science research topics to light in a simple yet informative way. Science has been evolving since time immemorial. So, I try my bit to tap into the evolution and present it to the readers.

Let's connect through https://www.linkedin.com/in/sanchari-chakraborty-7b33b416a/

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