INVESTIGATORY PROJECT
PHYSICS
TOPIC:
Variation of current in LDR with the intensity of light incident on it

CERTIFICATE
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An LDR or photoresistor is made of any semiconductor material with high resistance. It has a high resistance
because there are very few electrons that are free and able to move, the vast majority of electrons are locked
into the crystal lattice and unable to move. Therefore in this state there is a high LDR resistance .
As the light fall on semi-conductor; the light photons are absorbed by the semi-conductor lattice and some of
their energy is transferred to the electrons. This gives some of them sufficient energy to break free from the
crystal lattice so that they can conduct electricity. This results in a lowering of the resistance of semi-
conductor and hence the overall LDR resistance. The process is progressive and as more light shines on the
LDR semi-conductor, so more electrons are released to conduct electricity and the resistance fall further.
It is a light sensitive device which is generally used to detect the presence or absence of light, and to measure
the intensity of light.
In the dark, their resistance is very high (about 1MW), but when the LDR is exposed to light, the resistance
drops down dramatically even to few Ohms, depending on the light intensity incident on it.
INTRODUCTION OF LIGHT DEPENDENT RESISTANCE(LDR)

PROCEDURE
The LDR is put in series with a voltage source (preferably variable voltage source) and a sensitive micro-ammeter
(or milli-ammeter) as shown in circuit diagram.
1. The LDR is now covered so that no light is incident on it and the reading of the milli-ammeter is noted down.
2. The reading of the milli-ammeter is noted by uncovering the LDR so that normal room light is incident on it
3. A lamp is taken which can glow in three different intensities, i.e low intensity, moderate intensity, and hight
intensity.
The reading of milli-ammeter is noted for each intensity of the lamp

mA
( k )
LDR
CIRCUIT DIAGRAM

S. No. Light Source Current(mA)
1 No light .04
2 Room Light 212
3 Lamp light (low) 230
4 Lamp light(moderate) 250
5 Lamp light(high) 300
OBSERVATION TABLE

CONCLUSIONS
We observe that there is a significant change in the current when the LDR is placed in room light as compared to
the LDR placed in dark.
We also observe that the current increases on increasing the intensity of light (by the help of lamp). The
percentage change in the current is very small due the placement of lamp. The possible reason is that the
intensity of light from lamp is very small compared to the room light.