NAME: ASHMIT CHAUHAN

CLASS: XII
SEC: B
ROLL NO.: 11
SUBJECT: PHYSICS
INVESTIGATORY PROJECT
SCHOOL: SUNBEAM SCHOOL,
VARUNA
TOPIC: LIGHT DEPENDENT
RESISTOR

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 INDEX
S. HEADINGS PAG
NO E
. NO.
1. CERTIFICATE
2. ACKNOWLEDGEMENT
3. AIM
4. APPARATUS
5. PHOTORESISTOR
DISCOVERY
6. PRINCIPLE
7. USES
8. BRIEF DESCRIPTION OF
COMPONENTS
9. APPLICATIONS
10. PROCEDURE
11. OBSERVATION
12. CONCLUSION&RESULT
13. BIBLIOGRAPHY
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 CERTIFICATE
THIS IS TO CERTIFY THAT ASHMIT CHAUHAN OF
CLASS XII B HAS SUCCESSFULLY COMPLETED
THE RESEARCH PROJECT ON THE TOPIC “LIGHT
DEPENDENT RESISTOR” UNDER THE GUIDANCE
OF MR. SANJEEV SRIVASTAVA (SUBJECT
TEACHER) IN THE YEAR 2019-2020. THE
REFERENCES TAKEN IN MAKING THIS PROJECT
HAVE BEEN DECLARED AT THE END OF THIS
REPORT.

SIGNATURE SIGNATURE
MR. SANJEEV SRIVASTAVA (EXAMINER)
(SUBJECT TEACHER)

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 ACKNOWLEDGEMENT
I WISH TO EXPRESS MY DEEP
GRATITUDE AND SINCERE THANKS TO
THE PRINCIPAL MRS. ANUPMA MISHRA
FOR HER ENCOURAGEMENT AND FOR
ALL THE FACILITIES THAT SHE
PROVIDES FOR THIS PROJECT WORK. I
SINCERELY APPRECIATE THIS
MAGNANIMITY BY TAKING ME INTO
HER FOLD FOR WHICH I SHALL REMAIN
INDEPTED TO HER. I EXTEND MY
HEARTLY THANKS TO MR. SANJEEV
SRIVASTAVA(PHYSICS TEACHER), WHO
GUIDED ME TO THE SUCCESSFUL
COMPLETION OF THIS PROJECT. I TAKE
THIS OPPORTUNITY TO EXPRESS MY
DEEP SENSE OF GRATITUDE FOR HIS
INVALUABLE GUIDANCE, CONSTANT

4 | Page
 ENCOURAGEMENT, WHICH HAS
SUSTAINED MY EFFORTS AT ALL
STAGES OF THIS PROJECT WORK. I
CAN’T FORGET TO OFFER MY SINCERE
THANKS TO MY CLASSMATES WHO
HELPED ME TO CARRY OUT THIS
PROJECT SUCCESSFULLY AND FOR
THEIR VALUABLE ADVICE AND
SUPPORT, WHICH I RECEIVED FROM
THEM TIME TO TIME. I WOULD ALSO
LIKE TO THANK MR. ARVIND(LAB ASST.)
FOR HIS SUPPORT AND GUIDANCE.

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 AIM
To study thevariations, in current
flowingin a circuitcontaininga LDR,
because of a variation:-
(a) In the power of the incandescent
lamp, used to‘illuminate’ the LDR.
(Keeping all the lamps at a
fixeddistance).
(b) In the distance of a
incandescentlamp, (of fixedpower),
used to ‘illuminate’ the LDR.

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 APPARATUS

Light Dependent
Resistor(LDR),Connecting
Wires, Source of different
power rating(bulbs), Bulb
Holder , Metrescale, Multi
Meter Battery.

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PHOTORESISTOR
DISCOVERY
Photoresistors, or light dependent
resistors have been in use for very
many years. Photoresistors 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
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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 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

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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.

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 Extriniscphotoresistors 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.

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 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.

If LDR places in darkness, the

resistance of LDR increases and when
light falls on it, the resistance of LDR
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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.

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 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.

 It can be used in jail lights.

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 BRIEF
DESCRIPTION
OF COMPONENTS
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.

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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 vice-versa.
We use a transformer of 6-0-6V for
the circuit.
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7. CAPACITOR:- Capacitor is used
to block DC. In the
PHOTORESISTOR
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.
Type Passive
9.PHOTORESISTOR-
A photoresistor (or lig Working Photocondu
ht-dependent principle
resistor, LDR,
Electronic symbol
or photocell) is a light-
controlled
variable resistor.
The resistance of a The symbol fo
photoresistor photoresistor
decreases with
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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.
A photoresistor is made of a high
resistance semiconductor. In the dark,
a photoresistor can have a resistance
as high as several 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
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lowering resistance. The resistance
range and sensitivity of a
photoresistor can substantially differ
among dissimilar devices. Moreover,
unique photoresistors may 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 bandgap. 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
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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.

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 APPLICATIONS
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

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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.
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The use of CdS and CdSe photo
resistors is severely restricted in
Europe due to the RoHS ban
on cadmium.
Lead sulphide (PbS) and indium
antimonide (InSb) LDRs (light-
dependent resistors) are used for the
mid-infrared spectral
region. Ge:Cuphotoconductors are
among the best far-infrared detectors
available, and are used for infrared
astronomy and infrared spectroscopy.

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 PROCEDURE
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.

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6. Repeat these steps with
different power sources at different
distances and note down
observations.

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 OBSERVATIONS
The experiment has been conducted
by using various sources with
different power ratings. Voltage of the
battery = 6 V.
OBSERVATIONS TABLE :-

(a) Variation in current of LDR with

lamps of different power, keeping
distance fixed.

Dist Source(
ance Bulb)
S.
betw Of Resista Curren
No
een power nce(Ω ) t(mA)
.
LDR inputs(
and W)
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 the
sour
ce
d(c
m)
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. 20 15 2800 2.14
2. 20 60 650 9.00
3. 20 100 435 13.79

1. 30 15 4300 1.4
2. 30 60 1000 6.00
3. 30 100 660 9.00

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(b) Variation in current of LDR with
distance :-

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 FOR 15 W
POWER SOURCE
Distan
ce
betwe
S.N en LDR Resistance Current(µ
o. and (Ω ) A)
the
source
d(cm)
1. 4 1010 5.94
2. 6 1350 4.44
3. 8 1490 4.03
4. 10 1610 3.73
5. 12 1740 3.45
6. 14 1880 3.19
7. 16 2300 2.61
8. 18 2540 2.36
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9. 20 2800 2.14
10. 22 3000 2.00
11. 24 3240 1.85

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 FOR 60 W
POWER SOURCE
Distan
ce
betwe
en
S.N Resistanc Current(
LDR
o. e(Ω ) mA)
and
the
source
d(cm)
1. 4 105 57
2. 6 180 33
3. 8 230 26
4. 10 280 21
5. 12 360 16
6. 14 440 13
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7. 16 500 12
8. 18 575 10
9. 20 675 8.8
10. 22 740 8.1
11. 24 820 7.3

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 FOR 100 W
POWER SOURCE
Distan
ce
betwe
en
S.N Resistanc Current(
LDR
o. e(Ω ) mA)
and
the
source
d(cm)
1. 4 80 75
2. 6 130 46
3. 8 170 35
4. 10 205 29
5. 12 270 22
6. 14 300 20
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7. 16 360 16
8. 18 410 14
9. 20 460 13
10. 22 525 11
11. 24 585 10

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 CONCLUSION
& RESULT

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.

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 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.

BIBLIOGRAPHY
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1. NCERT CLASS 12 PHYSICS
TEXTBOOK
2. www.google.co.in
3. www.wikipedia.com
4. www.yahoo.co.in

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