2. CERTIFICATE
This is to certify that the project report entitled “To study
the variations in current flowing in a circuit containing a
LDR” This project is considered as a part of practical
exam of AISSCE Physics conducted by Central Board of
Secondary Education, is a bonafide record of project work
carried out under our guidance and supervision at Manav
Rachna international school This project is evaluated on:-
INTERNAL EXAMINER NAME:- ______________
SIGNATURE:-
3. ACKNOWLEDGEMENT
We express our gratitude towards our guide Ms. Deepmala
for her extended guidance and support for completion of
this project work.
Signature: -
Date: -
Name of the student: - Damini Kaushik Class :- XII Dalton
Name of the School:- Manav Rachna International School,
Charmwood
4. Abstract
The general purpose photoconductive cell is also known as LDR (light
dependent resistor).
It is a type of semiconductor and it’s conductivity changes with
proportional change in the intensity of light.
There are two common types of materials used to manufacture the
photoconductive cells. 1) Cadmium Sulphide (CdS), 2) Cadmium
Selenide (CdSe).
Extrinsic devices have impurities added, which have a ground state
energy closer to the conduction band – since the electrons don’t have
as far to jump, lower energy photons (i.e. longer wavelengths and
lower frequencies) are sufficient to trigger the device.
Two of its earliest applications were as part of smoke and fire
detection systems and camera light meters. The structure is covered
with glass sheet to protect it from moisture and dust and allows only
light to fall on it.
5. Objective
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
6. Theory
1.) LDR and its characteristics
When light is incident on it, a photon is absorbed and thereby it excites an
electron from valence band into conduction band. Due to such new electrons
coming up in conduction band area, the electrical resistance of the device
decreases. Thus the LDR or photo-conductive transducer has the resistance
which is the inverse function of radiation intensity.
Λ0 = threshold wavelength, in meters
e = charge on one electron, in Coulombs
Ew = work function of the metal used, in ev
Here we must note that any radiation with wavelength greater than the value
obtained in above equation, CANNOT PRODUCE any change in the resistance
of this device. The band gap energy of Cadmium Sulphide is 2.42eV and for
Cadmium Selenide it is1.74 ev. Due to such large energy gaps, both the
materials have extremely high resistivity at room temperature.
7. Theory
2) Characteristics of photoconductive cells
Now when the device is kept in darkness, its resistance is called as
dark resistance. This resistance is typically of the order of 1013 ohms.
When light falls on it, its resistance decreases up to several kilo ohms
or even hundreds of ohms, depending on the intensity of light, falling
on it.
Cadmium Sulphide cell has a peak response nearer or within the green
color of the spectrum within a range of 520nm. Thus it can be used
nearer to the infra-red region up to 750nm. the maximum response of
Cadmium Sulphoselenide is in the yellow-orange range at 615nm and
also it can be used in the infra-red region up to about 970nm.
8. Theory
3) Sensitivity
Sensitivity of a photo detector is the relationship between the light
falling on the device and the resulting output signal. In the case of a
photocell, one is dealing with the relationship between the incident
light and the corresponding resistance of the cell.
4) Spectral Response
Like the human eye, the relative sensitivity of a photoconductive cell
is dependent on the wavelength (color) of the incident light. Each
photoconductor material type has its own unique spectral response
curve or plot of the relative response of the photocell versus
wavelength of light.
9. Materials required
Light Dependent Resistor (LDR)
Connecting Wires
Source of different power rating (bulbs)
Bulb Holder
Meter scale
Multi Meter
Battery
11. Procedure
Choose a specific position for the source and mount it using a holder,
make sure it is stable.
Select the bulb with the lowest power rating and connect it to the
holder as shown in the figure.
Connect the LDR, battery(6V) and the multimeter in series.
Set the multimeter to ohm section and select suitable range and
measure the resistance with a bulb on.
Similarly switch to current section and move to micro ampere in the
multimeter. This gives the value of the current.
Repeat these steps with different power sources at different distances
and note down observations .
12. Observations
The experiment has been conducted by
using various sources with different power
ratings. Voltage of the battery= 6 V
