Some resistors are dependent on physical quantities like light, heat, magnetic force, etc. These resistors can be divided into 4 types which includes:
• Photo resistor (LDR)
• Thermistors (NTC and PTC)
• Varistor (VDR)
• Magneto resistor (MDR)
Photo resistor is an electronic component whose resistivity depends on the presence or absence of light.Photo resistor is also known as Light Dependent Resistor (LDR), photo conductors, photo conductive cells or photocells. Hence, they are light sensitive components. They are made from light sensitive material like camdium sulfide. Photo resistor has no fixed value of resistance, it’s resistivity varies with light level available. Thus,it acts like a variable resistor. When the light level decreases, the resistance of the photo resistor increases and when the light level increases, the resistance of the photo resistor decreases. Therefore the resistance of a photo resistor is maximum in the dark.
Working Principle of Photo Resistor
It works on the principle of photo conductivity. Photo conductivity is an optical phenomenon in which the materials conductivity is increased in the presence of light. In the presence of light, photo resistor absorbs the light rays (photons). Then the electrons in the valence band of the semiconductor material are excited to the conduction band. These photons in the incident light should have energy greater than the band gap of the semiconductor material to make the electrons jump from the valence band to the conduction band. This process is known as exication. Therefore when light having adequate energy hits on the device, more and more electrons are excited to the conduction band which results in huge number of charge carriers . The result of this process is more and more current starts flowing through the device when the circuit is closed and hence it is said that the resistance of the device has been decreased.
Types Of Photo Resistor
• Intrinsic Photo Resistor– This type of LDR is made of undoped materials like germanium or silicon. Light rays (photons) that fall on the device excite electrons from the valence band to the conduction band, and the result of this process are more free electrons in the material, which can carry current, and therefore less resistance.
• Extrinsic Photo Resistor– This type of LDR is made of materials doped with impurities, also known as dopants. The dopants create a new energy band above the existing valence band, populated by electrons. These electrons need less energy to make the transition to the conduction band thanks to the smaller energy gap. The result is a device sensitive to different wavelengths of light.
Applications Of Photo Resistor
• They are usually used as light sensors or in making automatic night light.
• They are usually used in a camera light meter where there is a need to detect absences or presences of light.
• They used in burglar alarm circuits and light intensity meters.