ARMY PUBLIC SCHOOL,JODHPUR

2024-25

PROJECT ON
VARIATION OF CURRENT
IN LDR CIRCUIT

SUBM ITTED SUBM ITTED

TO: BY:
M R. AJAY GHELOT KARTIKEY
1) Certificate
2) Acknowledgements
3) Aim
4) Apparatus
5) Introduction
6) Applications
7) Theory
8) Procedure
9) Conclusion
10) Sources of error
11) Bibliography
12) End of project
This is certify that this is the best bonafied
work of KARTIKEY of Class 12-A. He has
prepared his PHYSICS investigatory project
compromising of all dimension of the given
topic VARIATION OF CURRENT IN LDR
CIRCUIT of the syllabus. The student is
initiative cooperation and participation during
the practical classes are recommended.
Content, accuracy, creativity and analysis of
different perception.

Mr. AJAY GEHLOT

External Examiner

PRINCIPAL
I would like to express my special thanks
of gratitude to our school Principal SSN.
MISHRA Sir as well as my PHYSICS
Teacher Mr. AKSHAY MISHRA Sir who
gave me the golden opportunity to do this
wonderful project on the topic
“VARIATION OOF CURRENT IN LDR
CIRCUIT” which also helped me in doing
a lot of research and I came to know
about so many new things. I am really
thankful to them.
Secondly I would like to thank
my parents and friends who helped me a
lot in finalizing this project within the
limited time frame.
To study the variation in current
flowing in a circuit containing LDR
because of a variation in:
a) The power of incandescent lamp,
used to illuminate the LDR
(keeping all the lamps at a fixed
distance).
b) The distance of incandescent lamp
(of fixed power) used to illuminate
the LDR.
1) LDR (Light Dependent Resistor)
2) Connecting wires
3) Bulb holder
4) Bulbs of different power rating
5) Meter scale
6) Multimeter
7) Battery
A photoresistor or light dependent resistor is a
component that is sensitive to light. When light
falls upon it then the resistance changes
changes. Values of resistance of the LDR may
change over many orders of magnitude the value
of resistance falling as the level of light
increases.
With such a wide variations in resistance, LDRs
are easy to use and there are many LDR circuits
available. The sensitivity of light dependent
resistors or photoresistor also varies with the
wavelength of the incident light.
LDRs are made from semiconductor materials to
enable them to have their light sensitive
properties. Many materials can be used, but one
popular material for these
photoresistor is cadmium sulphide.
Analog Applications:
 Camera Exposure Control
 Auto Slide Focus- dual cell
 Photocopy machines
 Electronic scales- dual cell
 Automated Rear View Mirror

Digital Applications:
 Automatic headlight Dimmer
 Night Light Control
 Street Light Control
 Position Sensor

LDR has a disadvantage that when its

temperature changes, its resistance
changes drastically for a particular light
intensity.
Types of photoresistor :
 Intrinsic photoresistor : Intrinsic
photoresistor use un-doped semiconductor
materials including silicon or germanium.
Photons fall on the LDR excite electrons
moving them from the valence band to the
conduction band. As a result these electrons
are free to conduct electricity. The more light
that falls on the device, the more electrons are
liberated and the greater the level of
conductivity, and this results in a lower level of
resistance.
 Extrinsic photoresistor : Extrinsic
photoresistor are manufactured from
semiconductor of material doped with
impurities. These impurities or dopants create
a new energy band above the existing valence
band. As a result, electrons need less energy
to transfer to the conduction band because of
smaller energy gap.
LDR Symbol:
The LDR symbol user in circuits is based around
the resistor circuit symbol, but shows the light, in
the form of arrows shining on it.

Circuit symbols used for light

dependent resistor /
photoresistor
 THEORY
1.) LDR and its characteristics

When light is incident on it, a photon is absorbed

and thereby it excites an electron from valence
band into conduction band. Due to such new
electrons coming up in conduction band area,
the electrical resistance of the device decreases.
Thus the LDR or photo-conductive transducer
has the resistance which is the inverse function
of radiation intensity.
휆 ℎ. 풸
표 e.
Eω
=
λo=threshold wavelength, in meters
e = charge on one electron, in Coulombs Ew
= work function of the metal used, in Ev

Here we must note that any radiation with

wavelength greater than the value obtained in
above equation CANNOT PRODUCE any
change in the resistance of this device.
The gap energy of Cadmium Sulphide is 2.42eV
and for Cadmium Selenide it is 1.74eV. Due to
such large energy gaps, both the materials have
extremely high resistivity at room temperature.
Such large energy gaps, both the materials have
extremely high resistivity at room temperature.
Characteristics of photoconductive cells:
Now when the device kept in darkness, its
resistance is called a dark resistance. This
resistance is typically of the order of 1013ohms.
When light falls on it, its resistance decreases up
to level kilo ohms or even hundreds of ohms,
depending on the intensity of light, falling on it.
The spectral response characteristics of two
commercial cells were compared in our
laboratory. And we found that there is almost no
response to the radiation of a wavelength which
was shorter than 300nm. It was very interesting
to note that the Cadmium Sulphide cell has a
peak response nearer or within the green color of
the spectrum with a range of 520nm. Thus it can
be used nearer to the infrared region up to
750nm. It was found that the maximum response
of Cadmium Sulphoselenide is in the yellow-
orange range at 615nm and also it can be used
in the infrared region up to about 970nm.
Sensitivity :
The sensitivity of a photo detector is the
relationship between the light falling on the
device and the resulting output signal. In the
case of a photocell, one is dealing with the
relationship between the incident light and the
corresponding resistance of the cell.
Spectral Response :
Like the human eye, the relative sensitivity of a
photoconductive cell is dependent on the
wavelength (color) of the incident light. Each
photoconductor material type has its own unique
spectral response curve or plot of the relative
response of the photocell versus wavelength of
light.
 PROCEDURE

1) Draw a diagram showing

the scheme of the
connections as shown in
fig.
2) Make the connections as
given in the diagram.
Arrange the incandescent
lamp and LDR so that they are in same
straight line, and make sure that both are
stable.
3) To find the resistance, set the multimeter to
ohm section and select suitable range and
measure the resistance of LDR with a bulb
on.
4) In multimeter select the switch to
microampere to get current.
5) Switch on the bulb and take different
readings with different power sources
played at same distance.
6) Now repeat these steps again with different
power sources at different distances and
repeat the observations.
 CONCLUSION
o The LDR resistance decreases with increase
in intensity of light and hence there is an
increase in the flow of current.
o There is an increase in current as the
distance from the source decreases.
o The intensity decreases as the distance from
the source increases.

SOURCE OF ERROR

o The LDR may not be perpendicular to the

source.
o Connections may be faulty.
o Experiment should be conducted in a dark
room.
o Measurements should be taken accurately.
Bibliography

 NCERT Class-12 Physics

 www.technologystudent.com/elec1/ldr1

 Physics Lab Manual Class-12

 https://en.wikipedia.org

 https://www.google.co.in

 Physics for you blogs