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Page 1 of 22
PHYSICS PROJECT FILE
Light Dependent
Resistor (LDR)
SUBMITTED TO:- SUBMITTED BY:-
Page 2 of 22
ACKNOWLEDGEMENT
I would like to express my special thanks to my Physics
teacher as well as our Principal Mam who gave me the
golden opportunity to do this wonderful project in
PHYSICS, which also helped me in doing a lot of
Research. I came to know about many new things.
This project couldn’t have been satisfactorily completed
without the support and guidance of MY parents.
I would like to thank my parents who helped me in
gathering diverse information, collecting data and
guiding me from time to time in making this project,
despite their busy schedule. They gave me different ideas
in making this project unique.
Page 3 of 22
CERTIFICATE
This is to certify that __________________________ of class
12th has completed the physics project titled
“Light Dependent Resistor (LDR)” in partial
fulfillment of curriculum of CENTRAL BOARD OF
SECONDARY EDUCATION (CBSE). This project
was carried out in the school laboratory of YAK
public school, Khopoli during the academic year
2018-2019.
Sign.:–
Internal examiner External examiner
Date:–
Page 4 of 22
Index
1 Certificate of Excellence
2 Acknowledgement
3 Aim of project
4 Introduction
5 Theory
6 Apparatus required
7 Procedure followed
8 Observation
9 Conclusions
10 Precautions
11 Bibliography
Page 5 of 22
Aim of Project
The aim of the project is to demonstrate the underlying
principles of electrical conduction under the influence of
light. Various applications are possible by employing this
simple principle such as (A) Automatic switching of
street lamps, (b) Intruder’s alarm, (c) Morning wake up
alarm, (d) Fire alarm, (e) Light intensity meters, (f)
Automatic elevator doors etc. to name a few.
In this project, I have demonstrated the intruder’s alarm
in a simple manner which is easy to understand. The
heart of the project is an LDR or Light Dependent
Resistor. This project could have been done using a
photo-voltaic cell in place of LDR but the circuit would
have become complicated in the case of photocell.
Page 6 of 22
Introduction
A photoresistor or light-dependent resistor(LDR) is a
light-controlled variable resistor. The resistance of a
photoresistor decreases with increasing incident light
intensity; in other words, it exhibits Photoconductivity.
A photoresistor can be applied in light-sensitive
detector circuits, and light and dark activated switching
circuits. These resistors use pure semiconductors like
silicon or germanium. When the light falls on the LDR,
then the electrons get excited by the incident photons
and move from the valence band to the conduction band
and therefore the number of charge carriers increases. In
other words, the conductivity goes up.
Distinction needs to be made here between photocells
and LDRs. In a photocell, when it is excited by light
(photons), electricity is generated. Unlike photocells,
LDRs, do not generate electricity but only change their
conductivity.
Page 7 of 22
Theory
Alight dependent resistor works on the principle of photo
conductivity. Photo conductivity is an electro-optical
phenomenon in which the
material’s conductivity is
increased when light is
absorbed by the material.
Modern light dependent
resistors are made of
materials such as lead sulphide, lead selenide, indium
antimonide and most commonly cadmium sulphide
(CdS) and cadmium selenide.
When light falls i.e. when the photons fall on the
material, the electrons in the valence band of the
semiconductor material are excited to the conduction
band. These photons in the incident light should have
energy greater than the band gap of the semiconductor
material to make the electrons jump from the valence
Page 8 of 22
band to the conduction band. Hence when light having
enough energy strikes on
the device, more and more
electrons are excited to the
conduction band which
results in large number of
charge carriers. The result
of this process is more and more current starts flowing
through the device when the circuit is closed and hence
it is said that the resistance of the device has been
decreased. This is the most common working principle of
LDR.
This can be clearly seen from the graph. The resistance of
the LDR falls rapidly with the increasing intensity of the
incident light.
The converse is also true when light intensity is reduced
or cut off.
Page 9 of 22
APPARATUS REQUIRED
The project consists of the following apparatus:
1.Light source (white, ultra-violet and laser)
2.LDR
3.Switching circuit
4.Relay
5.Buzzer
6.Batteries
7.Housing for the above
Page 10 of 22
Procedure followed
An empty cardboard box was used to mimic a house. An
opening was cut on the box to present a door. Behind
the door, a light source is placed and in the line of vision
of the light source, an LDR is also placed.
A circuit is connected to the LDR which switches on a
relay when the light beam is intercepted. A schematic
diagram of the set-up is shown below:
Light
LIGHT SOURCETRANSISTOR
CIRCUIT
RELAY
BUZZER
INTERRUPTION
HOUSE
DOOR
LIGHT BEAM
Page 11 of 22
As seen in the above diagram, a light source is positioned
behind the door on the right hand side and an LDR is
placed in the same line of vision at the opposite end of
the door. When the light beam falls on the LDR, it lowers
the resistance of the same and this activates the
switching transistor circuit. The transistor circuit
(described later) is connected to a relay which is in turn
connected to an external buzzer.
The relay is a special type of switch which is driven by its
magnetic coil. The relay has two positions. In de-
energized condition, position-1 will be active and in
energized condition, position-2 will be active.
When the circuit is powered, the relay goes to position-1
to which the buzzer is connected. Thus, the buzzer will
start ringing. But when the light source is switched on,
the resistance of LDR falls and this drives the transistor
switching circuit and the relay gets energized. This puts
the relay switch in position-2, which cuts off the buzzer.
In this condition, whenever the light beam is intercepted
Page 12 of 22
by an opaque object, the LDR stops receiving the incident
light and its resistance becomes high. This de-energizes
the relay and puts it to position-1 and the buzzer starts
ringing.
Thus, to summarize, we can say that the as long as the
light source is on and the LDR is illuminated, the buzzer
will not ring. But the moment, the light source is
interrupted, the buzzer will start ringing.
We can Imagine the interruption of the light beam to be
caused by an intruder and hence this arrangement can
automatically detect any intruder by turning on the
buzzer.
The buzzer is just one of the devices we have connected
to the relay. As such we can connect many devices to the
relay such as sirens, flashing lights or even an automatic
dialer to the nearest police station. This intruder alarm
can be easily set up in houses, banks, schools etc.
Page 13 of 22
The same circuit can be rewired to reverse the effect of
light. Viz. we can make the buzzer ring whenever, light is
present and interestingly this set up can be used as a
morning alarm.
Circuit Diagram
Now, let us understand how the switching takes place
when the LDR is illuminated by light. For the automatic
switching, we have used the following circuit consisting of
Transistors. One of the most common uses for transistors
in an electronic circuit is as simple switches. In short, a
transistor conducts
current across the
collector-emitter
path only when
sufficient voltage is
applied to its base.
When no sufficient base voltage is present, the switch is
off. When sufficient base voltage is present, the switch is
on.
Page 14 of 22
The circuit uses two transistors, connected in series. The
base voltage of the first transistor is adjusted carefully
through the variable resistor (potentiometer) so that the
slightest increase in the base voltage can fire the
transistor T1. Thus, when there is no light on the LDR,
the transistor remains un-fired. But when light falls on
the LDR, the LDR starts conducting and the net base
voltage of transistor T1 crosses the threshold voltage,
causing T1 to fire. As a consequence, the collector
current of T1 drives transistor T2 and T2 begins to fire.
The collector current of T2 is now large enough to
energize the relay. The relay gets energized and changes
the contact position from 1 to 2. When light is cut off,
the base voltage of T1 falls back to less than the
threshold voltage of its base –emitter junction and
therefore, the relay gets de-energized.
As stated earlier, the relay is connected to a buzzer
through a battery. So, whenever, light is interrupted, the
Page 15 of 22
relay goes to position-1 where, the buzzer is connected
and the buzzer starts ringing.
The ringing buzzer wards off the intruder and also alerts
the neighbors about the intrusion. In addition to the
buzzer, one can also connect a high intensity flash light
to warn the people in the neighborhood about the
intrusion.
Needless to say, one has also to install a stealth switch
that is known only to the owner so that the owner can
disable it for his own entry.
Page 16 of 22
Observation
In the set up described, the following observations were
made by (a) varying the nature of light source, (b)
varying the translucency of the interceptor object.
Table of observations
S. No Type of Light
Source
Material of
Interceptor
Result
1 White light Opaque Buzzer Rings
Transparent Buzzer does
not ring.
2 Near ultra-
violet
Opaque Buzzer Rings
Transparent Buzzer does
not ring.
3 Low intensity
red laser
Opaque Buzzer Rings
Transparent Buzzer does
not ring.
Page 17 of 22
Extension of scope in future
The device was also tested under invisible light using a
TV remote control as an infrared light source. The same
observations were made as above. This extends the scope
of the project to the invisible light sources such as
ultraviolet and infrared. The advantage is that; we can
dupe the intruder by using invisible light as the light rays
will not be visible even in the night.
Another interesting application that can be thought of is
an automatic ‘people counter’ that can be used to
measure the number of people traversing a certain
passage or entrance in an auditorium or theatre. It can
also be used as a ‘traffic counter’ in check posts and toll
gates. For this, we have to simply
replace the buzzer with a digital
counter. A typical digital counter
is shown in the accompanying
picture.
Page 18 of 22
Conclusion
Based on the observation above, it can be concluded that
the LDR based electronic switch can be successfully
deployed for the following applications:
1.Intruder detection / burglar’s alarm for home uses.
2.Anti-theft alarm for Banks and offices.
3.People Counter at Auditoriums
4.Vehicle counter at traffic check posts.
Precautions
The project uses certain devices that can be harmful to
humans and animals if proper caution and care are not
observed.
1.Laser diode: Although we have used a low intensity
laser, this can be harmful if aimed accidently at the
Page 19 of 22
eyes or if the reflected beam is seen through naked
eye. It is suggested to wear protective sun-glasses
while assembling / positioning the light source.
2.Ultraviolet LED: UV LED with wavelengths between
315 and 400 nm, which emits the least energetic
and harmful type of UV light. However, prolonged
exposure should be avoided which can damage the
skin and eyes.
3.Buzzer: This device emits high frequency audio
sounds that can damage the ears when put close to
the ears or heard for prolonged period.
4.The LDR: Should a need arise to destroy the project;
care should be taken to dispose off the LDR properly
as it contains harmful chemicals that can damage
the environment.
Page 20 of 22
Bibliography
sds
1. www.google.com
2. https://en.wikipedia.org/
3. www.electrical4u.com
Help from
The Internet
Help from
Parents

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12th Physics Investigatory project (LDR)

  • 1. Page 1 of 22 PHYSICS PROJECT FILE Light Dependent Resistor (LDR) SUBMITTED TO:- SUBMITTED BY:-
  • 2. Page 2 of 22 ACKNOWLEDGEMENT I would like to express my special thanks to my Physics teacher as well as our Principal Mam who gave me the golden opportunity to do this wonderful project in PHYSICS, which also helped me in doing a lot of Research. I came to know about many new things. This project couldn’t have been satisfactorily completed without the support and guidance of MY parents. I would like to thank my parents who helped me in gathering diverse information, collecting data and guiding me from time to time in making this project, despite their busy schedule. They gave me different ideas in making this project unique.
  • 3. Page 3 of 22 CERTIFICATE This is to certify that __________________________ of class 12th has completed the physics project titled “Light Dependent Resistor (LDR)” in partial fulfillment of curriculum of CENTRAL BOARD OF SECONDARY EDUCATION (CBSE). This project was carried out in the school laboratory of YAK public school, Khopoli during the academic year 2018-2019. Sign.:– Internal examiner External examiner Date:–
  • 4. Page 4 of 22 Index 1 Certificate of Excellence 2 Acknowledgement 3 Aim of project 4 Introduction 5 Theory 6 Apparatus required 7 Procedure followed 8 Observation 9 Conclusions 10 Precautions 11 Bibliography
  • 5. Page 5 of 22 Aim of Project The aim of the project is to demonstrate the underlying principles of electrical conduction under the influence of light. Various applications are possible by employing this simple principle such as (A) Automatic switching of street lamps, (b) Intruder’s alarm, (c) Morning wake up alarm, (d) Fire alarm, (e) Light intensity meters, (f) Automatic elevator doors etc. to name a few. In this project, I have demonstrated the intruder’s alarm in a simple manner which is easy to understand. The heart of the project is an LDR or Light Dependent Resistor. This project could have been done using a photo-voltaic cell in place of LDR but the circuit would have become complicated in the case of photocell.
  • 6. Page 6 of 22 Introduction A photoresistor or light-dependent resistor(LDR) is a light-controlled variable resistor. The resistance of a photoresistor decreases with increasing incident light intensity; in other words, it exhibits Photoconductivity. A photoresistor can be applied in light-sensitive detector circuits, and light and dark activated switching circuits. These resistors use pure semiconductors like silicon or germanium. When the light falls on the LDR, then the electrons get excited by the incident photons and move from the valence band to the conduction band and therefore the number of charge carriers increases. In other words, the conductivity goes up. Distinction needs to be made here between photocells and LDRs. In a photocell, when it is excited by light (photons), electricity is generated. Unlike photocells, LDRs, do not generate electricity but only change their conductivity.
  • 7. Page 7 of 22 Theory Alight dependent resistor works on the principle of photo conductivity. Photo conductivity is an electro-optical phenomenon in which the material’s conductivity is increased when light is absorbed by the material. Modern light dependent resistors are made of materials such as lead sulphide, lead selenide, indium antimonide and most commonly cadmium sulphide (CdS) and cadmium selenide. When light falls i.e. when the photons fall on the material, the electrons in the valence band of the semiconductor material are excited to the conduction band. These photons in the incident light should have energy greater than the band gap of the semiconductor material to make the electrons jump from the valence
  • 8. Page 8 of 22 band to the conduction band. Hence when light having enough energy strikes on the device, more and more electrons are excited to the conduction band which results in large number of charge carriers. The result of this process is more and more current starts flowing through the device when the circuit is closed and hence it is said that the resistance of the device has been decreased. This is the most common working principle of LDR. This can be clearly seen from the graph. The resistance of the LDR falls rapidly with the increasing intensity of the incident light. The converse is also true when light intensity is reduced or cut off.
  • 9. Page 9 of 22 APPARATUS REQUIRED The project consists of the following apparatus: 1.Light source (white, ultra-violet and laser) 2.LDR 3.Switching circuit 4.Relay 5.Buzzer 6.Batteries 7.Housing for the above
  • 10. Page 10 of 22 Procedure followed An empty cardboard box was used to mimic a house. An opening was cut on the box to present a door. Behind the door, a light source is placed and in the line of vision of the light source, an LDR is also placed. A circuit is connected to the LDR which switches on a relay when the light beam is intercepted. A schematic diagram of the set-up is shown below: Light LIGHT SOURCETRANSISTOR CIRCUIT RELAY BUZZER INTERRUPTION HOUSE DOOR LIGHT BEAM
  • 11. Page 11 of 22 As seen in the above diagram, a light source is positioned behind the door on the right hand side and an LDR is placed in the same line of vision at the opposite end of the door. When the light beam falls on the LDR, it lowers the resistance of the same and this activates the switching transistor circuit. The transistor circuit (described later) is connected to a relay which is in turn connected to an external buzzer. The relay is a special type of switch which is driven by its magnetic coil. The relay has two positions. In de- energized condition, position-1 will be active and in energized condition, position-2 will be active. When the circuit is powered, the relay goes to position-1 to which the buzzer is connected. Thus, the buzzer will start ringing. But when the light source is switched on, the resistance of LDR falls and this drives the transistor switching circuit and the relay gets energized. This puts the relay switch in position-2, which cuts off the buzzer. In this condition, whenever the light beam is intercepted
  • 12. Page 12 of 22 by an opaque object, the LDR stops receiving the incident light and its resistance becomes high. This de-energizes the relay and puts it to position-1 and the buzzer starts ringing. Thus, to summarize, we can say that the as long as the light source is on and the LDR is illuminated, the buzzer will not ring. But the moment, the light source is interrupted, the buzzer will start ringing. We can Imagine the interruption of the light beam to be caused by an intruder and hence this arrangement can automatically detect any intruder by turning on the buzzer. The buzzer is just one of the devices we have connected to the relay. As such we can connect many devices to the relay such as sirens, flashing lights or even an automatic dialer to the nearest police station. This intruder alarm can be easily set up in houses, banks, schools etc.
  • 13. Page 13 of 22 The same circuit can be rewired to reverse the effect of light. Viz. we can make the buzzer ring whenever, light is present and interestingly this set up can be used as a morning alarm. Circuit Diagram Now, let us understand how the switching takes place when the LDR is illuminated by light. For the automatic switching, we have used the following circuit consisting of Transistors. One of the most common uses for transistors in an electronic circuit is as simple switches. In short, a transistor conducts current across the collector-emitter path only when sufficient voltage is applied to its base. When no sufficient base voltage is present, the switch is off. When sufficient base voltage is present, the switch is on.
  • 14. Page 14 of 22 The circuit uses two transistors, connected in series. The base voltage of the first transistor is adjusted carefully through the variable resistor (potentiometer) so that the slightest increase in the base voltage can fire the transistor T1. Thus, when there is no light on the LDR, the transistor remains un-fired. But when light falls on the LDR, the LDR starts conducting and the net base voltage of transistor T1 crosses the threshold voltage, causing T1 to fire. As a consequence, the collector current of T1 drives transistor T2 and T2 begins to fire. The collector current of T2 is now large enough to energize the relay. The relay gets energized and changes the contact position from 1 to 2. When light is cut off, the base voltage of T1 falls back to less than the threshold voltage of its base –emitter junction and therefore, the relay gets de-energized. As stated earlier, the relay is connected to a buzzer through a battery. So, whenever, light is interrupted, the
  • 15. Page 15 of 22 relay goes to position-1 where, the buzzer is connected and the buzzer starts ringing. The ringing buzzer wards off the intruder and also alerts the neighbors about the intrusion. In addition to the buzzer, one can also connect a high intensity flash light to warn the people in the neighborhood about the intrusion. Needless to say, one has also to install a stealth switch that is known only to the owner so that the owner can disable it for his own entry.
  • 16. Page 16 of 22 Observation In the set up described, the following observations were made by (a) varying the nature of light source, (b) varying the translucency of the interceptor object. Table of observations S. No Type of Light Source Material of Interceptor Result 1 White light Opaque Buzzer Rings Transparent Buzzer does not ring. 2 Near ultra- violet Opaque Buzzer Rings Transparent Buzzer does not ring. 3 Low intensity red laser Opaque Buzzer Rings Transparent Buzzer does not ring.
  • 17. Page 17 of 22 Extension of scope in future The device was also tested under invisible light using a TV remote control as an infrared light source. The same observations were made as above. This extends the scope of the project to the invisible light sources such as ultraviolet and infrared. The advantage is that; we can dupe the intruder by using invisible light as the light rays will not be visible even in the night. Another interesting application that can be thought of is an automatic ‘people counter’ that can be used to measure the number of people traversing a certain passage or entrance in an auditorium or theatre. It can also be used as a ‘traffic counter’ in check posts and toll gates. For this, we have to simply replace the buzzer with a digital counter. A typical digital counter is shown in the accompanying picture.
  • 18. Page 18 of 22 Conclusion Based on the observation above, it can be concluded that the LDR based electronic switch can be successfully deployed for the following applications: 1.Intruder detection / burglar’s alarm for home uses. 2.Anti-theft alarm for Banks and offices. 3.People Counter at Auditoriums 4.Vehicle counter at traffic check posts. Precautions The project uses certain devices that can be harmful to humans and animals if proper caution and care are not observed. 1.Laser diode: Although we have used a low intensity laser, this can be harmful if aimed accidently at the
  • 19. Page 19 of 22 eyes or if the reflected beam is seen through naked eye. It is suggested to wear protective sun-glasses while assembling / positioning the light source. 2.Ultraviolet LED: UV LED with wavelengths between 315 and 400 nm, which emits the least energetic and harmful type of UV light. However, prolonged exposure should be avoided which can damage the skin and eyes. 3.Buzzer: This device emits high frequency audio sounds that can damage the ears when put close to the ears or heard for prolonged period. 4.The LDR: Should a need arise to destroy the project; care should be taken to dispose off the LDR properly as it contains harmful chemicals that can damage the environment.
  • 20. Page 20 of 22 Bibliography sds 1. www.google.com 2. https://en.wikipedia.org/ 3. www.electrical4u.com Help from The Internet Help from Parents