Rajendra public

school
Project Report
On
To study the effect of intensity
of light on current flowing
through the LDR circuit.

Submitted to

Mrs priya
Submitted by

Harmanpreet

Acknowledgement
The project on study the effect of
intensity of light on current flowing
through the LDR circuit would never
have become to happen without the
encouragement of many individuals. I
am deeply indebted to many teachers
who gave me their support to
understand this project.
It gave me immense pleasure to
embellish my heartiest sense of
gratitude and profound privilege to
express my sincere thank to mrs

priya for his training imparted to me.

Finally, it is my pleasure to
acknowledge moral support and cooperation to my teachers parents and
relatives , without them I could have
not completed this task.
Thank you
Harmanpreet

preface
As per requirement of senior
secondary syllabus under CBSE
board , new delhi a project report has
to be submitted on many topic
included in the syllabus. Such an
effort when organised with a define
aim is known project.
The objective of project has taken
scientific thinking . I completed this on

the topic to study the effect of

intensity of light on current flowing
through the ldr circuit. Welcome
comments , criticism and suggestion
from the reader of the project.

Introduction
A light dependent resistor (LDR) is a
semiconductor product either cadmium sulphide
or cadmium selenide arranged in a zigzag
pattern as shown in Figure-1. The resistance of
the material varies with incident light. When it
is connected to an external battery current flow
depending on the light intensity. The current is
controlled by incident light. There is no pn

junction in LDR as in the case of diode, solar

cell or LED. It is a surface phenomenon.

Figure-1, 20mm LDR

The light photons falling on LDR surface excite
electrons in the valence band. The electrons
move to the conduction band by absorbing
energy. This movement decreases the resistance
of the material. In the presence of external
battery current flows through the LDR. The
current increases with the light intensity. As the
light intensity is increased more and more
electrons move to the conduction band
increasing the current or decreasing LDR
resistance. The effect of light intensity is seen as
resistance variation in LDR. The LDR resistance
is given by

RLDR = Rdark L-b

1
Where RLDR is LDR resistance
Rdark is the LDR dark
resistance (Resistance
without light) L is the light
intensity at the surface of the
LDR in Lux b is a constant
depends on the material
Taking logarithm of Equation-1, and rewriting it
as
Log RLDR = Log Rdark b Log L
2
Substituting, y =Log RLDR, C = Log Rdark, m = -b
and x =Log L Equation-2 is rewritten as
y=mx+C
3
Which represents a straight line with slope m,
and y intercept C. Therefore by drawing a graph

of Log L versus Log RLDR , the two constants

Rdark and b can be estimated.
As per equation-1, the resistance variation with
light intensity is non-linear. Hence LDR is not
used to measure light intensity but used as light
control device such as street light, counting
machine and to record sun light variations
during day or during the solar eclipse [1].
When different color light is allowed to fall on
LDR different current is observed for different
colors resulting in spectral response similar to
human eye response.
To study the variation of LDR resistance with
light intensity an electric bulb is selected as
white light source and LED of known
wavelength is used as different wavelength
light. An electric bulb produces 11 lumen of
light per watt. (This is the standard value used in
industry) [2]. Hence if we know the wattage or
power consumption by the light source the
amount of light produced can be estimated.

Further the light intensity at a distance d from

the light source is given by
Light produced in Lumen
L = -------------------------------4
d2
Where L is the light intensity at the surface in
LUX
d is the normal distance between the source
and the surface in meters
Measuring the distance between the LDR and
light source the light intensity can be
determined from Equation-4.

To study the variation LDR resistance with light

LED

LDR

intensity in this experimentBa 12V/3A-auto bulb

is selected and bulb is energized
with dc power
ul
supply. The LDR is placedbat a distance of 20cm
from the light source such that the filament of
the bulb and LDR center lies on the same
straight

Figure-2, Light arrangement for LDR

characteristics
line. The LEDs are placed on the other side of
the LDR. The LDR can be made to face auto
bulb or LED light just by reversing its position.
The complete light arrangement used in the
experiment is shown in Figure-2. The LED light

source is energized with 5V dc regulated power

supply.
Apparatus Used
LDR characteristics experimental setup
consisting of light arrangement with white light
and blue, green and red LED lights, digital dc
milliammeter, digital dc ammeter, digital dc
voltmeter, rheostat 22 ohms/3A, dc regulated
power supply 5V/0.5A and 0-12V/0.5A in step
variation.
Experimental Procedure
The experiment consists of 2 parts,
Part-A, LDR response to white light
Part-B, LDR response to blue, green and red
lights
Part-A, LDR response to white light
1. The circuit is rigged as shown in Figure-3.
The LDR is mounted with its active surface

facing white light source at a distance of 20cm

from the light source.
2. Adjusting the rheostat the bulb current is
set. The voltage and current to the bulb is noted.
VB = 2V and IB = 1A and power P = 2 watt, 2
watt power generates 22 lumen light in
electric bulb. Therefore light intensity is given
by
L= 22 lumen/ 0.22 = 22/0.04 = 550
lumen/meter2 = 550 lux.
0-3A

IB

12V/3A
VB

Rehostat
22 Ohms/3A
0-20V

0-20mA
VLDR

LDR

1.2-12
V
12V/3A
Auto
Bulb

Figure-3, Circuit connection for LDR

characteristics
3. The corresponding LDR current in the
milliammeter is noted in Table-1. Noting LDR

supply voltage, LDR resistance is calculated and

presented inTable-1.
RLDR = VLDR / ILDR = 1.2V/0.5mA = 24K.
4. Trial is repeated by varying bulb voltage
using rheostat. The corresponding LDR current
is noted and LDR resistance is calculated. The
readings obtained are presented in Table-1.
5. A graph showing the variation of light
intensity and LDR resistance is drawn taking
light intensity on X-axis and LDR resistance on
Y-axis. The graph is shown in Figure-4.
6. From the graph the slope of the straight line
is calculated and the Y intercept is noted.
Slope = m = -b = 1.19 and
Y intercept = C = log Rdark = 6.64
Rdark = Antilog 6.64 = 4.38 M.
Using these constants the Equation 1 is rewritten
as

RLDR = 4.38x 106 L1.19

8
Table-1
Bulb
Light
LDR
VB( IB( Power Lu I(LU ILDR( RLDR(
V) A) (W)
men X) mA) )
2.0 1.0 2.00 22.00 550 0.5 2400
0
3.0 1.2 3.75 41.25 1037 1.1 1090
5
4.0 1.5 6.00 66.00 1650 1.7
700
0
5.0 1.7 8.75 96.25 2406 3.1
387
5
6.0 1.8 11.10 122.1 3052 4.5
266
5
7.0 2.0 14.00 154.0 3850 5.9
203
0
8.0 2.2 17.60 193.6 4840 7.9
152
0

9.0 2.3
0
10. 2.5
2 0
11.0 2.6
0
12. 2.8
5 0

20.70 227.7 5692

9.4

127

25.50 280.5 7012 11.6

103

28.60 314.6 7865 12.8

93

35.00 385.0 9625 15.4

78

Variation of LDR resistance with light

intensity

8
6
Log R(Ohms)

4
2
0

Log I (Lux)

Figure-4, Variation of light

intensity with LDR resistance Part-B,
LDR response to blue, green and red
lights

7. The LDR is now placed its active face

facing LED in the light arrangement. The LED
power supply is put ON and blue LED is lit. The
blue light now falls on the LDR. The cover of
the light arrangement is closed.
8. LDR supply voltage is set to 1.2 volts. And
the corresponding LDR current for blue light is
noted on Table-2.
9. Trial is repeated by varying LDR voltage up
to 12 volts and the corresponding LDR current
is noted and recorded in Table-2.
10. Experiment is repeated with Red and Green
light falling on the LDR and the corresponding
LDR current is noted in Table-2.
VLDR
(V)

Table-2
LDR
Current ILDR
(mA)
Bl Gr Re

ue
1.2 1.
5
2.0 2.
3
4.0 4.
8
6.0 7.
1
8.0 9.
6
10.0 12
.6
12.0 14
.9
1/Sl 79
ope
6

een d
0.4 0.9
0.6 1.5
1.4 3.1
2.3 4.3
3.1 6.2
3.9 8.0
4.6 9.5
250 125
0 0

Table-2, LDR current variation with LDR

voltage for different color light

11. A graph showing the variation of LDR

current with LDR voltage is drawn as shown in
Figure-5.
12. From the straight-line graph slope is
calculated. The 1/slope gives LDR resistance for
that color light.
13. The wavelengths of the LED lights are
predetermined using Newtons rings microscope
[3]. The values are presented in Table-3 along
with the LDR resistance value obtained from the
graph shown in Figure-5. Using these data
another graph showing LDR the response to
wavelength is drawn taking wavelength on Xaxis and LDR resistance on Y-axis as shown in
Figure-6.
Table-3
Colo Wavelengt RLDR
r
h (nm)
()
Blue
470
796
Gree
560
250
n
0

Red

620

125
0

Variation LDR resistance with Wavelength

Red

Green

Blue

20
15(mA)
LDR Current
10
5
0
0

10
LDR voltage (V)

15

Figure-5, Variation of LDR current with LDR

voltage for blue, green and red light

Figure-6, Variation of LDR resistance with

wavelength
Results
The results obtained are tabulated in Table-4
Table-4
Paramet Experime
ers
ntal
Dark
4.38 M
resistanc
e Rdark
Saturated 78

resistanc
e Rmin
Constant 1.19
b
Discussions
LDR is a useful semiconductor device for
applications in light control instruments. The
dark resistance is of the order of few Meg ohms
and saturated resistance is an about 80 ohms
with LDR current up to 50mA. The constant b
appearing in the LDR equation is of the
dimension of ohm/lux. The exact nature of this
constant is not known. We assumed that it
depends on the material and size of the LDR.
Surprisingly we have seen a spectral response
for LDR. For green light LDR resistance is the
maximum. The response is similar to human
eye response. However, by physical observation
of the three LED lights it is found that blue had
very high intensity followed red and then green.
The response may be due to the intensity, may

not be because of wavelength. This aspect we

have not verified. We will take up this work
shortly and make sure that LDR responds to
color or not.
Reference
1. Light measurements during the total solar
eclipse of August 11th, 1999 at Quatre-Champs
near Vouziers (France).
2. Indian Standard Specifications for Tungsten
filament general service electric lamp No
15148- 1978.
3. Wavelength and Energy gap determination
using Newtons Rings microscope in case of
light emitting diode, LE Vol-2, N0-2, Setp-2002,
Page-10