Light Dependent Resistor

1. PHYSICS POWER
POINT PRESENTATION

2. Aditi Tiwari
Anjali Tiwari
Anshika Sinha

3. TOPIC – LIGHT
DEPENDENT
RESISTOR

4. ◦ These devices depend on the light, when light falls on the
LDR then the resistance decreases, and increases in
the dark. When a LDR is kept in the dark place, its
resistance is high and, when the LDR is kept in the light its
resistance will decrease.
Light dependent resistors, LDRs, or
photoresistors are electronic components that
are used to detect light & change the
operation of a circuit dependent upon the
light levels.
What is a Light
Dependent Resistor.

5. Structure/ Diagram of LDR

6. What is Photo conductivity
Photoconductivity is the tendency of a substance to conduct electricity to
an extent that depends on the intensity of light-radiant energy (usually
infrared transmission or visible light) striking the surface of a sample.
Most semiconductor materials have this property.
The working principle of an LDR is photoconductivity, which is
nothing but an optical phenomenon. When the light is absorbed by
the material then the conductivity of the material enhances. When
the light falls on the LDR, then the electrons in the valence band of
the material are eager to the conduction band.

7. Test1.
Keep the multimeter at Ohms mode. The LDR must be subjected to a
bright light source (day light is enough).Connect the LDR leads to the
multimeter terminals as shown in the figure. Now the multimeter will
show a low resistance reading around 500 Ohms.
Test2.
Keep the multimeter at Ohms mode. The LDR must be subjected to
darkness by covering it with an opaque paper. Connect the LDR leads
to the multimeter terminals as shown in the figure. Now the multimeter
will show a high resistance reading around 200K.
How to test a LDR
If both of the above tests are positive then we can
conclude that the LDR is healthy.

8. ◦Working Principle of LDR
◦ This resistor works on the principle of photo conductivity. It is nothing but, when the light falls on its surface, then
the material conductivity reduces and also the electrons in the valence band of the device are excited to the
conduction band. These photons in the incident light must have energy greater than the band gap of the
semiconductor material.This makes the electrons to jump from the valence band to conduction.
These devices depend on the light, when light falls on the LDR then the resistance decreases, and increases
in the dark.When a LDR is kept in the dark place, its resistance is high and, when the LDR is kept in the light
its resistance will decrease.
Also, the energy in incident light should be greater than the bandgap gap energy so
that the electrons from the valence band got excited and go to the conduction band.

9. 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.
Construction of LDR:

10. Applications
•The photoresistor is generally used in detecting the presence and intensity of light
•Used in automatic lights that switch on and off according to light
•Simple Smoke Detector Alarm, Clock with automatic light
•Optical circuit design
•Photo proximity switch
•Laser-based security systems
•Solar Street Lamps
•Camera light meters
•Clock radios
•Can be used in Dynamic Compressors, some compressors use LDR and LED connected to the signal
source to create changes in signal gain.