GLAZE BROOKE PUBLIC SCHOOL CLASS XII
PHYSICS INVESTIGATORY PROJECT
C.DHINAKAR
P H Y S I C S T E A C H E R : M R . S AT H I S H
DDF
Euro International School, Sector 45
Physics Investigatory Project
Certificate This is to certify that C.Dhinakar, a student of class XII has successfully completed the research on the below mentioned project under the guidance of Mr.Sathish (Physics Teacher).
Teacher’s Signature
I would like to express my special thanks to my teacher as well as our principal who gave me the golden opportunity to do this wonderful project in PHYSICS,
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which also helped me in doing a lot of Research. I came to know about many new things. This project project couldn’t have been satisfactorily 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.
1
Certificate of Excellence
2
Acknowledgement
3
Aim of project
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4
Introduction
5
Theory
6
Apparatus required
7
Procedure followed
8
Observation
9
Conclusions
10
Precautions
11
Bibliography
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 Page 4 of 22
simple principle such as (A) Automatic switching of street lamps, (b) Intruder’s 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.
A photoresistor or light-dependent resistor(LDR) is a light-controlled variable resistor. variable resistor. The resistance of a photoresistor decreases with increasing incident light intensity; in other words, it exhibits Photoconductivity. Page 5 of 22
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.
Alight dependent d ependent resistor resistor works works on the principle of photo conductivity.Photo conductivity is an electro-optical phenomenon in which the material’ material’s conductivity is
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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
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incident light should have energy greater than the band gap of the semiconductor material material to make the electrons jump from the valence 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. 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 resistance of the device has been decreased. This is themost 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.
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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
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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:
BUZZER INTERRUPTION
RELAY Light LIGHT BEAM
TRANSISTOR
LIGHT SOURCE
CIRCUIT
HOUSE
DOOR
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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 t he 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 s witch which is driven by its magnetic coil. The relay has two positions. In deenergized 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
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interceptedby 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.
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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 current
conducts 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 ison. Page 13 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
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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.
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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 Material of Result Source Interceptor
1
White light
2
Near ultra- Opaque violet Transparent
3
Low intensity Opaque red laser Transparent
Opaque Transparent
Buzzer Rings Buzzer does not ring. Buzzer Rings Buzzer does not ring. Buzzer Rings Buzzer does not ring.
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Extension of scope in future The device was also tested under invisible light using a TV remote remote control as an infrared light source. source. The same 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
accompanyingpicture.
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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 Anti-theft alarm for Banks and offices. 3. People Counter at Auditoriums 4. Vehicle counter at traffic check posts.
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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 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.
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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.
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sds
1. www.google.com 2. https://en.wikipedia.org/ 2. https://en.wikipedia.org/ 3. www.electrical4u.com
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