Light Dependent Resistor (LDR), Construction, Working , Characteristics and Applications

 What is Light Dependent Resistor (LDR)

Light Dependent Resistor is called by many names such as photoresistor, photocell, photoconductor, photoconductive cells. The resistivity of LDR depends on the light incident on it and the sensitivity of LDR depends on the wavelength of the incident light. Thus it is a light sensitive device. Mostly it is used in circuits to detect the presence of the level of the light.

Symbol of LDR:

 

Symbol of LDR

There are many symbols. The most commonly used symbol of LDR is shown in the figure. The arrow in the symbol indicates the light falling on it.

Construction of LDR:

Construction of LDR

The LDR is constructed by placing a thin zigzag shaped strip of photosensitive device upon the insulating material. The light sensitive materials used in LDR are Cadmium Sulphide (Cds), Cadmium Selenide (CdSe) or lead Sulphide (Pbs). The insulating material used in LDR is ceramic. The metal films are connected with the terminal leads.

The whole structure is placed inside a plastic or resin case to have direct exposure to the sunlight. When there is no light the resistance is very high in Mega Ohms. When the light is incident the resistance value decreases and the conductivity increases.

Working of LDR:

 

Working of LDR

The photoconductive material does not consists of any free electrons or it consists of  few free electrons when it is not exposed to light. When the light is incident on the LDR the covalent bond breaks and many free electrons and holes are formed. The free electrons and holes gains energy and they jump from valence band to the conduction band. Thus current is generated. The resistivity of LDR decreases with increase in the incident light. More the light, more the charge carriers.

Illumination characteristics of LDR:

 

Illumination characteristics of LDR

When there is no light resistivity is very high of Mega Ohms. When the light is incident the resistance decreases to kilo Ohms.

Advantages:

• Low cost
• Available in many shapes and sizes
• Low power operation
• High sensitivity

Disadvantages:

• Large response time- The variation in resistance value is slow to the light action.

Application:

• Used in automatic street lighting
• Used in cameras
• Used in alarm clocks
• Used in burglar circuits
• Used to count objects in conveyor belts.