11/25/2016
Submitted by
Gopi Nath Sah
Class XII A
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Acknowledgement
I would like to express my special thanks of
gratitude to my teacher (Mrs. Alka Chopra)
as well as our principal (R K Tyagi) who
gave me the golden opportunity to do this
wonderful project on the topic (Write the
topic name), 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 also like to thank my
parents and friends who helped me a lot in
finalizing this project within the limited time
frame.
Gopi Nath Sah
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Bonafide certificate
This is to certify that Gopi Nath Sah of class XII A has
successfully completed the investigatory project on
“TO STUDY VARIATION OF CURRENT USING A
LDR" under the guidance of Mrs. Alka Chopra .This
project is absolutely genuine and doesn't not involve in
any kind of plagiarism. This is in partial fulfillment of
Physics practical examination CBSE 2016.
Department of Physics
(Mrs Alka Chopra)
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INDEX
ACKNOWLEDGEMENT
BONAFIDE CERTIFICATE
1. INTRODUCTION
2. AIM & APPARATUS
3. PHOTORESISTOR DISCOVERY
AND MECHANISM
4. PRINCIPLE & THEORY
5. BRIEF DESCRIPTION OF
COMPONENTS
6. APPLICATIONS
7. PROCEDURE
8. OBSERVATIONS
9. RESULT & CONCLUSIONS
10. BIBLOGRAPHY
i.
ii.
2
3
4-5
5-6
6-8
9
10
11-14
15
16
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LIGHT DEPENDENT RESISTANCE
Project Report Physics
INTRODUCTION
In the society, sometimes street lights glow in day time due to any
reason. In mines area people face many difficulties due to absence
of light in the nights. In frontier and hilly areas, people face many
problems due to damaged street lights.
For solve above these problems, we create a device in which the
lights glow in night and in day time, they off automatically and
don't glow. Due to use of it, we can solve above problems and can
also save electricity and men's power.
AIM: To study the variations, in current flowing in a circuit
containing a LDR, because of a variation:(a) In the power of the incandescent lamp, used to ‘illuminate’ the
LDR. (Keeping all the lamps at a fixed distance).
(b) In the distance of a incandescent lamp, (of fixed power), used to
‘illuminate’ the LDR.
APPARATUS:
Light Dependent Resistor(LDR),Connecting Wires, Source of
different power rating(bulbs), Bulb Holder , Metre scale, Multi
Meter Battery.
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Photo resistor discovery
Photo resistors, or light dependent resistors have
been in use for very many years. Photo resistors
have been seen in early forms since the nineteenth
century when photoconductivity in selenium was
discovered by Smith in 1873. Since then many
variants of photoconductive devices have been
made.
Much useful work was conducted by T. W. Case in
1920 when he published a paper entitled "Thalofide
Cell - a new photo-electric cell".
Other substances including PbS, PbSe and PbTe
were studied in the 1930s and 1940s, and then in
1952, Rollin and Simmons developed their
photoconductors using silicon and germanium.
Photoresistor mechanism
A photoresistor or photocell is a component that uses a
photoconductor between two contacts. When this is exposed to
light a change in resistance is noted. Photoconductivity - the
mechanism behind the photoresistor - results from the
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generation of mobile carriers when photons are absorbed by the
semiconductor material used for the photoconductor. While the
different types of material used for light dependent resistors are
semiconductors, when used as a photo-resistor, they are used
only as a resistive element and there are no PN junctions.
Accordingly the device is purely passive.
There are two types of photoconductor and hence photoresistor:

Intrinsic photoresistor: This type of photoresistor
uses a photoconductive material that involves excitation of
charge carriers from the valence bands to the conduction
band.

Extrinsic photoresistor: This type of photoresistor uses a
photoconductive material that involves excitation of charge
carriers between an impurity and the valence band or
conduction band. It requires shallow impurity dopants that
are
not
ionised
in
the
presence
of
light.

Extrinisc photoresistors or photocells are generally
designed for long wavelength radiation - often infra-red, but
to avoid thermal generation they need to be operated at low
temperatures.
PRINCIPLE
This project is based on Light Dependent Resistance
(L.D.R.). Light Dependent Resistance [LDR] is a resistance, in
which opposing power of current depends on the presence of
quantity of light present, i.e. the resistance of LDR increases or
decreases, according to quantity of light which falls on it.
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If LDR places in darkness, the resistance of LDR increases and
when light falls on it, the resistance of LDR decreases and act as
a conductor. Any LDR in the presence of light and darkness
changes its resistance is depends on the different types of LDR.
ADVANTAGES






Collection of parts of the circuit are easily available.
Accuracy of this circuit is more than accuracy of other circuits.
By using laser, it can be used for security purposes.
It can be used to stop the wastage of electricity.
The cost of circuit is low.
This circuit saves the men's power.
USES






It can be used in street lights.
It can be used in mines areas.
It can be used in hilly areas.
By using laser, it can be used for safety purposes.
It can be used in frontier areas.
It can be used in houses.
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 It can be used in jail lights.
BRIEF DISCRIPTION OF COM PONENTS
1. TRANSISTOR:-When a thin slice of p-type is sandwiched between
two blocks of n-type, then n-p-n transistor is formed. It consists of
emitter, base, and collector. In the project, common emitter n-p-n
transistor ( BC-147 & SL-100) is used.
2. DIODE:- When a p-type and n-type semiconductors are joined a
diode is formed. It conducts when forward biased and does not conduct
when reverse biased. In the project, IN-4007diode is used.
3. RELAY:- It helps to contact and discontact. In the project, we use
relay of 6 ohms.
4. CARBON RESISTOR:- A carbon resistor has generally four rings
or bands A,B,C and D of different colours corresponding to the value
of resistance. In project, we use carbon resistance of 220 kilo-ohms, 1.5
kilo-ohms and 820 ohms.
5. LDR:- LDR means light dependent resistance which is used to
complete the circuit.
6. TRANSFORMER:- Transformer is used to convert low alternating
voltage to high alternating voltage by decreasing the current and viceversa. We use a transformer of 6-0-6V for the circuit.
Photoresistor
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7. CAPACITOR:- Capacitor is used to
block DC. In the circuit, we use the
capacitor of 220mfd and 1000mfd.
8.BULB:- An electric bulb is connected
to the circuit when LDR comes in the
darkness.
9.PHOTORESISTORA photoresistor (or lightdependent
resistor, LDR,
or photocell) is a light-controlled
variable resistor. The resistance of a Type
Passive
photoresistor
decreases
with
increasing incident light intensity; in Working Photoconductivity
other
words,
it principle
exhibits photoconductivity.
A
photoresistor can be applied in light- Electronic symbol
sensitive detector circuits, and lightand dark-activated switching circuits.
A photoresistor is made of a high
resistance semiconductor. In the
symbol
for
a
dark, a photoresistor can have a The
resistance as high as several photoresistor
megohms (MΩ), while in the light, a
photoresistor can have a resistance as low as a few hundred
ohms. If incident light on a photoresistor exceeds a
certain frequency, photons absorbed by the semiconductor give
bound electrons enough energy to jump into the conduction
band. The resulting free electrons (and their hole partners)
conduct electricity, thereby lowering resistance. The resistance
range and sensitivity of a photoresistor can substantially differ
among dissimilar devices. Moreover, unique photoresistors may
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react substantially differently to photons within certain
wavelength bands.
A photoelectric device can be either intrinsic or extrinsic. An
intrinsic semiconductor has its own charge carriers and is not an
efficient semiconductor, for example, silicon. In intrinsic devices
the only available electrons are in the valence band, and hence
the photon must have enough energy to excite the electron
across the entire band gap. Extrinsic devices have impurities,
also called dopants, added whose ground state energy is closer
to the conduction band; since the electrons do not have as far to
jump, lower energy photons (that is, longer wavelengths and
lower frequencies) are sufficient to trigger the device. If a sample
of silicon has some of its atoms replaced by phosphorus atoms
(impurities), there will be extra electrons available for conduction.
This is an example of an extrinsic semiconductor.
Applications
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The internal components of a photoelectric control for a typical
American streetlight. The photoresistor is facing rightwards, and
controls whether current flows through the heater which opens
the main power contacts. At night, the heater cools, closing the
power contacts, energizing the street light.
Photoresistors come in many types. Inexpensive cadmium
sulphide cells can be found in many consumer items such as
camera light meters, clock radios, alarm devices (as the detector
for a light beam), nightlights, outdoor clocks, solar street lamps
and solar road studs, etc.
Photoresistors can be placed in streetlights to control when the
light is on. Ambient light falling on the photoresistor causes the
streetlight to turn off. Thus energy is saved by ensuring the light
is only on during hours of darkness.
They are also used in some dynamic compressors together with
a small incandescent or neon lamp, or light-emitting diode to
control gain reduction. A common usage of this application can
be found in many guitar amplifiers that incorporate an
onboard tremolo effect, as the oscillating light patterns control
the level of signal running through the amp circuit.
The use of CdS and CdSe[3] photoresistors is severely restricted
in Europe due to the RoHS ban on cadmium.
Lead sulphide (PbS) and indium antimonide (InSb) LDRs (lightdependent resistors) are used for the mid-infrared spectral
region. Ge:Cu photoconductors are among the best farinfrared detectors available, and are used for infrared
astronomy and infrared spectroscopy.
PROCEDURE
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1. Choose a specific position for the source and mount
it using a holder, make sure it is stable.
2. Select the bulb with the lowest power rating and
connect it to the holder as shown in the figure.
3. Connect the LDR, battery (6V) and the multimeter
in series.
4. Set the multimeter to ohm section and select suitable
range and measure the resistance with a bulb on.
5. Similarly switch to current section and move to
micro ampere in the multimeter. This gives the value
of the current.
6. 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.
OBSERVATIONS TABLE :-
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(a) Variation in current of LDR with lamps of different power, keeping
distance fixed.
Distance
between Source(Bulb)
S.No. LDR and
Of power
Resistance(Ω ) Current(mA)
the source imputs(W)
d(cm)
1.
10
15
1310
4.58
2.
10
30
770
7.00
3.
10
60
285
21.00
4.
10
100
195
30.00
1.
2.
3.
20
20
20
15
60
100
2800
650
435
2.14
9.00
13.79
1.
2.
3.
30
30
30
15
60
100
4300
1000
660
1.4
6.00
9.00
(b) Variation in current of LDR with distance :-
FOR 15 W POWER SOURCE
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S.No.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Distance between
LDR and the Resistance(Ω )
source d(cm)
4
1010
6
1350
8
1490
10
1610
12
1740
14
1880
16
2300
18
2540
20
2800
22
3000
24
3240
Current(µA)
5.94
4.44
4.03
3.73
3.45
3.19
2.61
2.36
2.14
2.00
1.85
FOR 60 W POWER SOURCE
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Distance between
S.No. LDR and the Resistance(Ω )
source d(cm)
1.
4
105
2.
6
180
3.
8
230
4.
10
280
5.
12
360
6.
14
440
7.
16
500
8.
18
575
9.
20
675
10.
22
740
11.
24
820
Current(mA)
57
33
26
21
16
13
12
10
8.8
8.1
7.3
FOR 100 W POWER SOURCE
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Distance between
S.No. LDR and the Resistance(Ω )
source d(cm)
1.
4
80
2.
6
130
3.
8
170
4.
10
205
5.
12
270
6.
14
300
7.
16
360
8.
18
410
9.
20
460
10.
22
525
11.
24
585
CONCLUSION & RESULT
Current(mA)
75
46
35
29
22
20
16
14
13
11
10
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1. The LDR resistance decreases with increase in intensity of light
and hence there is an increase in the flow of current.
2. There is an increase in the current as the distance from the
source decreases.
3. The intensity decreases as the distance from the source increases
4. The error lies within the experimental limit.
SOURCES OF ERROR
1. The LDR may not be perpendicular to the source.
2. Connections may be faulty.
3. The experiment should be conducted in a dark room.
4. Measurements should be taken accurately.
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Biblography
1. NCERT CLASS 12 PHYSICS TEXTBOOK
2. www.google.co.in
3. www.wikipedia.com
4. www.yahoo.co.in
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