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Photoresistor: The Best Choice for Your Applications

Several electronic components detect the presence of light levels, such as phototransistor or photodiodes. However, the electronic components have PN junctions and semiconductor materials with low resistance change due to light intensity. Therefore, you'll need a particular type of variable resistor known as a photoresistor to carry out the light-sensing operation. 

They are easy to manufacture and simple to use. Because of the advantages and more, they have varied applications such as controlling gain reduction in dynamic audio compressors. 

 

The Definition and Working Principle of a Photoresistor

 

What is a photoresistor?

 

A photoresistor, a light-dependent resistor (LDR)/photocell/photoconductor. It is an electronic component constructed of semiconductors. The high resistance semiconductor materials can include germanium, silicon, cadmium selenide, cadmium sulfide, etc. 

They have high sensitivity to light and therefore decrease in resistance value (loses a few ohms) as light level increases. As such, photoresistors become low resistance materials when there’s a bright light and high resistance materials in light absence. 

 

photoresistors in white background

(photoresistors in white background) 

 

Their general function is to show whether there’s light or not, thus acting as a light sensor. Also, it measures the intensity of light and only changes when exposed to light energy. 

 

Working principle of a photoresistor

 

A photoresistor works by the following process;

  • First, light falls on a photoresistor. Some valence electrons absorb light energy. Then free valence electrons break the atom bonds. 
  • If the photoconductor receives a highly increased amount of light energy, valence electrons receive the photon energy. 
  • After breaking the parent atom bond, the valence electrons form a conduction band. Atoms in a conduction band move freely within the space since they don’t belong to a specific atomic group. 
  • Next, the valence electron leaves the atom, therefore, creating a vacancy known as a hole. The holes and free electrons pair up in the electric field. 
  • Finally, free electrons carry the electric current as they move freely. The holes also have electrical current and move in the valence band. The process is gradual in that, as the light increases on the photocell conductor, more charge carriers conduct electricity. 

Generally, the amount of electric current flowing through a photocell is dependent on several holes and free electrons available. That is to say, the greater the light energy, the more the number of free electrons and holes (charge carriers). Subsequently, the flow of electric current through an LDR increases. 

 

The Structure of a Photoresistor

 

Structure of a photoresistor 

Structure of a photoresistor 

Source: Wikimedia Commons

 

A photoresistor has a horizontal body that faces the light. The basic structure is as shown above.

  • The sensitive material is an active semiconductor region that overlies the semi-insulating substrate. Often, the active material gets slight doping. 
  • Then, the visible pattern of the photocell on the surface increases the area for light exposure. 
  • The two metal contacts should also be wide to reduce the resistance range of the active area's contact with light. 
  • Lastly, there are many semiconductor materials in LDRs with different properties, including wavelengths of light sensitivity. Examples are InSb (indium antimonide), PBS, GaAs, Is, and Ge. 

 

Photoresistor Symbol

 

An international standard and American-standard symbol of photoresistors differ, as shown in the diagram below. 

 

A photoresistor symbol (International standard)

A photoresistor symbol (International standard)

Source: Wikimedia Commons

 

Photoresistor symbol in American Standard

(Photoresistor symbol in American Standard)

Source: Wikimedia Commons

 

The arrow pointing to the resistor circuit symbol is the light energy. The zig-zag line is for older resistor circuit symbols, while the rectangular circuit symbol is for newer resistor circuits.  

 

Type of Photoresistor

 

There are two types of photoresistors based on the materials manufacturers use to build them. They include; 

 

  • Intrinsic Photoresistor

 

Intrinsic photocells have pure semiconductor materials like germanium or silicon, and it has atoms whose outermost shells can hold at most eight valence electrons. But, each bit in the semiconductor materials has four valence electrons only. 

The four valence electrons join (using covalent bonds) neighboring four atoms, thus forming an outermost shell with eight valence electrons. The bonding leaves no free electron. 

Additionally, an intrinsic photoresistor gains only a small amount of valence electrons when light energy passes through it. Therefore, it'll only generate a few charge carriers, making it unreliable for practical applications since it has less light sensitivity. 

 

  • Extrinsic Photoresistor

 

Extrinsic LDRs have extrinsic semiconductor materials such as a combination of impure phosphorus (a doping agent) and silicon atoms. 

Practically, we have a phosphorus atom with five valence electrons and a silicon atom with four valence electrons. Four valence electrons of each of the two particles form four covalent bonds with each other. However, fifth valence electrons in phosphorus atoms roam accessible in bits since they have no corresponding electron from silicon atoms.

Often, free electrons collide with valence electrons from other atoms hence making them accessible. Subsequently, there will be a massive generation of free flow of electrons in the energy band.  

 

Dark circles are free electrons, and white circles are holes (charge carriers)

Dark circles are free electrons, and white circles are holes (charge carriers)

Source: Wikipedia

 

Finally, an extrinsic photoresistor generates many charge carriers, therefore, increasing the electric current flow. And so, they are recommendable in practical applications. 

  1. Applications of Photoresistor

Photoresistors are applicable in a wide range of areas that include;

  • Automatic street lights where it saves on electricity by turning the lights on only during nightfall, 

 

street lights at night

(street lights at night)

 

  • Solar street lamps,
  • Solar road studs, 
  • Light meters in camera, 
  • Clock radios or outdoor clocks,
  • Infrared astronomy where it acts as an excellent infra-red detector,
  • Light sensors in robotic projects, 
  • Alarm devices like smoke and burglar alarms, and 
  • Night light. 

 

Conclusion

 

Photoresistors are electric devices that benefit lighting technology since they are compact, widely available from electronic component distributors, and cost-effective. They may have low accuracy, but they still serve the electrical devices sufficiently. A photocell lacks PN junction in construction materials, making it a passive device.

 All in all, that sums up the photoresistor technology. Would you please do contact us if you’ve got any inquiries or comments? We will appreciate it.