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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)