How is an LDR Works

How an LDR (Light Dependent Resistor) Works. LDR (Light Dependent Resistor) An LDR is a component that has a (variable) resistance that changes with the light intensity that falls upon it. This allows them to be used in light sensing circuits.
A Light Dependent Resistor (LDR) is also called a photo resistor or a cadmium sulfide (CdS) cell.It is also called a photo conductor. It is basically a photocell that works on the principle of photo conductivity. The passive component is basically a resistor whose resistance value decreases when the intensity of light decreases.
The electronic symbol of LDR:
The actual behavior of LDR is clearly explained in yhe picture below. You can see the the resistance value of LDR in Day Light is 500Ω and the same value at night is 200kΩ which is very large value of resistance. Greater value of resistance limits the amount of current flowing across the circuit. Now you have got a clear picture of application of LDR.
Working Principle of LDR.
The snake like track shown below is the Cadmium Sulphide (CdS) film which also passes through the sides. On the top and bottom are metal films which are connected to the terminal leads. It is designed in such a way as to provide maximum possible contact area with the two metal films. The structure is housed in a clear plastic or resin case, to provide free access to external light. As explained above, the main component for the construction of LDR is cadmium sulphide (CdS), which is used as the photoconductor and contains no or very few electrons when not illuminated.
In the absence of light it is designed to have a high resistance in the range of mega ohms. As soon as light falls on the sensor, the electrons are liberated and the conductivity of the material increases. When the light intensity exceeds a certain frequency, the photons absorbed by the semiconductor give band electrons the energy required to jump into the conduction band. This causes the free electrons or holes to conduct electricity and thus dropping the resistance dramatically, 500Ω in above figure.

Applications:
1. LDR Based light Intensity Control for Street Lights
2. Lighting Switch from Sunset to Sunrise
3. LDR Based light Intensity Control for Street Lights
4. Automatic sun tracker solar system.
Video Tutorial: