11/18/2014

To study the variations, in current flowing, in a circuit containing a LDR, because of a variation in the distance of a

laser light, (of fixed power) used to illuminate the LDR.

Chirag has successfully completed the projected work for physics

On the topic To study the variations in current flowing in a circuit

containing a LDR, because of a variation in the distance of a

laser light, (of fixed power) used to illuminate the LDR for the

CBSE 2014-2015 under my guidance & to my complete

satisfaction.

Signature-

Mr. Vinod Chauhan

Physics H.O.D

D.A.V Public school

Sudden development of interest on how light can change the

value of resistance in an circuit.

CIRCUIT DIAGRAM

APPARATUS

THEORY & FORMULA USED

PROCEDURE

OBSERVATIONS

RESULT

PRECAUTIONS

TIME+COST

REFERENCE

FUTURE SCOPE

1) LIGHT DEPENDENT RESISTOR

2) A BATTERY OF 6 VOLTS

3) A SOURCE OF LIGHT

4) MULTIMETER

5) CONNECTING WIRES

6) KEY

A photoresistor or light-dependent resistor (LDR) or photocell is a

light-controlled variable resistor. The resistance of a photoresistor

decreases with increasing incident light intensity.

A photoresistor is made of a high resistance semiconductor. In

the dark, a photoresistor can have a resistance as high as a few

mega ohms (M), while in the light, a photoresistor can have a

resistance as low as a few hundred ohms. If incident light on a

photoresistor exceeds a certain frequency, photons absorbed by

the semiconductor give bound electrons enough energy to jump

into the conduction band. The resulting free electrons conduct

electricity, thereby lowering resistance. The resistance range and

sensitivity of a photoresistor can substantially differ among

dissimilar devices. Moreover, unique photoresistors may react

substantially differently to photons within certain wavelength

bands.

Photoresistors come in many types. Inexpensive cadmium

sulphide cells can be found in many consumer items such as

camera light meters, street lights, clock radios, alarm

devices, night lights, outdoor clocks, solar street lamps and solar

road studs, etc.

FORMULA USED:- R=

1. Arrange the apparatus according to the electric circuit

diagram.

2. Now set the knob of multimeter on the point marked current.

3. Place a thick piece of black paper on the LDR, so that no

light fall on LDR. Record the current in multimeter.

4. Remove the black paper from the LDR. Record the current

in multimeter in presence of normal light in the laboratory

and determine the resistance of LDR by using, R=

.

5. Now measure the resistance of LDR by using the laser light.

6. Now measure the current in the circuit by varying the

distance between the laser light and light dependent

resistor at 10cm & 8cm.

9. Similarly, record the current in multimeter for the distance

6 cm, 4 cm.

1) The emf of battery used = 6 volt.

2) Range of multimeter = 200 mA.

S.NO Distance b/w

source & LDR

(cm)

Current I(mA)

Resistance

R=

1) 10 2.6 2.307

2)

8

5.8

1.034

3)

6

8.6

0.697

4)

4

11.7

0.152

It is concluded that the electric current flowing in a circuit

containing LDR increases with decrease in the distance

between source of fixed power and LDR. Hence, with the

distance between light source and LDR increases the resistance

of the LDR decreases and vice-versa.

TIME

Research- 5 days

Experimental setup- 1 day

Experiment- 3 hours

COST

Light dependent resistor- Rs.80

Laser light- Rs.60

Mount board & black chart paper- Rs.30

File + printout- Rs.230

TOTAL- Rs.400

1. www.google.co.in

2. Practical Physics by Comprehensive.

3. www.sciencebuddies.com

http://www.sciencebuddies.com/

1) The source of light must be adjusted so that light falls normally

on LDR.

2) Source of light must radiate light continuously.

3) Connect LDR carefully to the voltage source.

4) The distance between source and LDR measured

accurately.

1) Automatic Street Light Control System is not only easiest but

also the powerful technique. Relay uses as a automatic

switch in this system. It releases the manual work upto

100%.As soon as the sunlight goes under the visible region

of our eyes this system automatically switches ON lights.

Light dependant resistor (LDR ) is a type of sensor which

actually does this work and senses the light as our eyes

does. As soon as the sunlight comes, visible to our eyes it

automatically switches OFF lights .Such type of system is

also useful for reducing energy consumption.

2) The light sensor is a passive devices that convert this light

energy whether visible or in the infra-red parts of the

spectrum into an electrical signal output. Light sensors are

more commonly known as Photoelectric Devices or Photo

Sensors because the convert light energy (photons) into

electricity (electrons).