AUTOMATIC STREET LIGHTS:-

In our daily life we see glowing of street lights even though there is sufficient light outside.There is a lot of wastage of energy occurring due to this.

Thus to avoid this energy wastage ,the circuit shown below can reduce this and save lot of energy in this circuit we use the light sensor names LDR (LIGHT DEPENDANT RESISTANCE) as shown below to detect the presence of light .

now first we learn about,

How an LDR (Light Dependent Resistor) Works ?

An LDR is a component whose value of resistance ,changes with the light intensity that falls upon it. This allows them to be used in light sensing circuits.

A typical LDR looks like as shown below,

The symbol for LDR looks like as shown in the below picture

when there is abundance of light there is a low resistance in the the LDR because all the electrons gain energy from incident light (photons)go to conduction band and participate in the conduction,

but in the absence of light there is no sufficient energy for the electrons to go to the conduction band so there is no available free electrons for conduction,so it acts as high resistance .The curve showing the variation of resistance in shown in the figure  below  

CIRCUIT DIAGRAM

FOR USING LDR IN AUTOMATIC STREET LIGHT SYSTEM

In the above circuit we use the help of transistor, the LDR is connected between the Base and emitter,consider the case when there is light .

since there is light there is low resistance of LDR so there is less voltage drop across LDR.This voltage drop across LDR is equal to voltage across "Vbe",this "Vbe" cannot drive the transistor into ON condition.So there wont be any collector current,So LED  dosen't glow.

While in another case , when there is no light the LDR  becomes very high resistive and there will be a high voltage drop across the LDR, this implies high drop across the  "Vbe".So the transistor turns ON and there will be collector current by which "LED GLOWS"

This  is how we made use of light sensor and transistor for our convenience.

In practical application the street light can be used in the place of LED

Other Applications of LDRs:-

Camera shutter control

LDRs can be used to control the shutter speed on a camera. The LDR would be used to measure the light intensity which then adjusts the camera shutter speed to the appropriate level.