Sudan University of Science and Technology College of Graduate Studies Department of Electrical Engineering

 
Inductance, eddy current, hall effect, 
photoelectric, capacitance, etc. 
Proximity sensors
 
Robust
noncontact
switching
action
The digital outputs are often directly fed to the digital controller
 
Photo 
resistors, 
photodiodes, 
photo 
transistors, photo conductors, etc.
Charge-coupled diode
Light sensors
 
Measure light intensity with high sensitivity
Inexpensive, reliable, and noncontact sensor
Charge-coupled diode
Light Dependent Resistor: 
A Light Dependent Resistor (LDR) or a photo resistor is a device 
whose resistivity is a function of the incident electromagnetic radiation. Hence, 
they are light sensitive devices. They are also called as photo conductors, 
photoconductive cells or simply photocells, hey are made up of semiconductor 

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materials having high resistance. There are many different symbols used to 
indicate a LDR, one of the most commonly used symbol is shown in the figure 
4.8. The arrow indicates light falling on it. 
Figure 4.8 Light Dependent Resistor (LDR) 
Characteristics of LDR: 
LDR’s are light dependent devices whose resistance is decreased when 
light falls on them and that is increased in the dark. When a light dependent 
resistor is kept in dark, its resistance is very high. This resistance is called as dark 
resistance. It can be as high as 1012 Ω and if the device is allowed to absorb light 
its resistance will be decreased drastically. If a constant voltage is applied to it 
and intensity of, light is increased, the current starts increasing. Figure 4.9 shows 
resistance vs. illumination curve for a particular LDR. 
Figure 4.9 resistance vs. illumination curve for a particular LDR

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Based on the materials used they are classified as: 
1.
Intrinsic photo resistors (Un doped semiconductor): These are made of pure 
semiconductor materials such as silicon or germanium. Electrons get excited 
from valance band to conduction band when photons of enough energy fall on 
it and number charge carriers is increased. 
2.
Extrinsic photo resistors: These are semiconductor materials doped with 
impurities, which are called as dopants. Theses dopants create new energy 
bands above the valence band, which are filled with electrons. Hence, this 
reduces the band gap and less energy is required in exciting them. Extrinsic 
photo resistors are generally used for long wavelengths. 
Construction of a Photocell: 
The structure of a light dependent resistor consists of a light sensitive material, 
which is deposited on an insulating substrate such as ceramic. The material is 
deposited in zigzag pattern in order to obtain the desired resistance and power 
rating. This zigzag area separates the metal-deposited areas into two regions. 
Then the ohmic contacts are made on the either sides of the area. The resistances 
of these contacts should be as less as possible to make sure that the resistance 
mainly changes due to the effect of light only. Materials normally used are 
cadmium sulphide, cadmium selenide, indium antimonide and cadmium 
sulphonide. The use of lead and cadmium is avoided, as they are harmful to the 
environment See figure 4.10. 
Figure 4.10 Materials of light dependent resistor (LDR)
 

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