KOTHARI INTERNATIONAL SCHOOL
NOIDA
PHYSICS PRACTICAL
FILE
TO STUDY THE VARIATION IN CURRENT FLOWING
IN A CIRCUIT CONTIANING AN LDR
SUBMITTED BY: KRISH JAIN
CLASS AND SECTION: 12 B
BOARD ROLL NO:
CERTIFICATE
This is to certify that Krish Jain student
of class XII “B” Kothari International
School has completed the project
titled “ To study the variation in
current containing LDR” during the
academic year 2023-2024 towards
partial fulfilment of credits for the
physics practical evaluation of CBSE
2023-2024, under the supervision on
Ms. Samta Sharma
Sign. of internal
Examiner
Sign. of external
Examiner
ACKNOWLEDGEMENT
“There are times when silence peaks much more
loudly than words of praise to only as good as
belittle a person whose words do not express,
but only put a veneer over true feelings , which
are gratitude at this point of time “. I would like
to express my sincere gratitude to our Physics
teacher, Ms Samta Sharma for her vital support,
guidance and encouragement- without which
this project would have come forth. I wouldalso
like to express my gratitude to the staff of the
Physics Department at Kothari International
School for their support during the making of this
project. I would also like to thank my parents and
friends who helped and supported me for this
project. This project was an amiable as well as
inspiring assignment to me for achieving success
INDEX
T
TOPIC:
PAGE NO. :
INTRODUCTION
01
APPLICATIONS
02
AIM
03
APPARATUS
04
THEORY
05
PROCEDURE
08
OBSERVATION
11
PRECAUTIONS
13
SOURCE OF ERROR
13
CONCLUSION
14
BIBLIOGRAPHY
15
INTRODUCTION
Light Dependent resistor also known as the
photoresistor is a two terminal passive
electronic component that is made up of a
semiconductor element or compound. As
the name implies the resistance of the
Photoresistor depends upon the intensity
of light projected onto it. With the increase
in the intensity of light the resistance of the
light dependent resistor decreases.
The LDRs are used in a bridge sensor
circuit on order to convert the light into the
voltage and thus different actions can be
performed for example by feeding the
voltage signal into the microcontroller the
circuit can be used to determine the
darkness in the room and turn on or off the
light. Two of its earliest applications were
as part of smoke and fire detection systems
and in camera light meters. There are two
common types of materials found
photoconductive cells. They are cadmium
sulphide ( Cds) and Cadmium Selenide
(CdSe)
APPLICATIONS
There are some applications of LDR which are
given below:
-> The LDR is used in the infrared astronomy.
-> The LDR is used in light failure alarm circuits
and used in light meter.
-> The LDR used in smoke detectors.
-> It is used for automatic contrast and
brightness control in television receivers.
-> It is used in photosensitive relay
-> It is used in optical coding.
-> It is used in street light control circuits.
-> It is used in camera light meters.
-> It is used in the security alarm.
-> It is used as a proximity switch.
-> It is used in light activated control circuits.
APPARATUS
1)Light Dependent Resistor (LDR)
2)Connecting Wires
3)Source of different power rating (bulbs)
4)Bulb Holder
5)Metre scale
6)Multi Meter
7)Battery
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 photoconductive transducer has the resistance which is the inverse function of radiation intensity.
λ0 = 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 band gap energy of
Cadmium Sulphide is 2.42eV and for Cadmium Selenide it is1.74eV. Due to such large energy gaps,
both the materials have extremely high resistivity at room temperature.
Characteristics of photoconductive cells
Now when the device is kept in darkness, its resistance is called as dark resistance. This resistance
is typically of the order of 1013 ohms. When light falls on it, its resistance decreases up to several
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 within 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 infra-red 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
•Choose a specific position for the source and mount it using a holder, make sure it is
stable.
• Select the bulb with the lowest power rating and connect it to the holder as shown
in the figure.
• Connect the LDR, battery(6V) and the multimeter in series.
• Set the multimeter to ohm section and select suitable range and measure the
resistance with a bulb on.
• Similarly switch to current section and move to micro ampere in the multimeter.
This gives the value of the current.
• Repeat these steps with different power sources at different distances and note
down observations
OBSERVATIONS
• The experiment has been conducted by using various sources
with different power ratings. Voltage of the battery = 6 V
SECOND TABLE ON NEXT PAGE-->
GRAPHS
RESISTANCE AS A FUNCTION
OF ILLUMINATION
RESULT
• The LDR resistance decreases with increase in intensity of
light and hence there is an increase in the flow of current.
• There is an increase in the current as the distance from
the source decreases.
• The intensity decreases as the distance from the source
increases
ERROR
• The error lies within the experimental limit.
BIBLIOGRAPHY
• NCERT physics class XII
• Art of Electronics by Paul worowitz
• www.wikipedia.com/
• www.electronics2000.co.uk/links/educationhobby/
• www.ecelab.com/
AIM
To study the variation in current, flowing in a
circuit containing an LDR, because of a variation
A. In the power of the incandescent lamp, used to
illuminate the LDR (keeping all the lamps at fixed
distance)
B. In the distance of an incandescent lamp ( of
fixed power), used to ‘illuminate’ the LDR