PHYSICS INVESTIGATORY PROJECT
NAME: Jai Aditya R
CLASS: 12 B
“To study the variations in current flowing in a
circuit containing a LDR”
ACKNOWLEDGEMENT
I express my gratitude towards my physics teacher for his
extended guidance and support for completion of this project
work.
I would give my special thanks to the Principal of Global City
International School for his great support by motivating and
encouraging in every endeavor of ours.
I want to thank my friends for their contribution and co-
operation.
A special thanks to CBSE for the scheme of project introduction
to make students discover their inbuilt abilities.
Last but not least I would like to thank my Parents for their love
and support.
INDEX
❖ ABSTRACT
❖ OBJECTIVE
❖ THEORY
❖ MATERIAL REQUIRED
❖ CIRCUIT DIAGRAM
❖ PROCEDURE
❖ OBSERVATION
❖ APPLICATION
❖ CONCLUSION
❖ REFERENCE
Abstract
A Light Dependent Resistor (LDR) is a special type of resistor whose resistance
changes with the intensity of light falling on it. It is made of semiconductor
materials such as Cadmium Sulphide (CdS) or Cadmium
Selenide (CdSe). When more light falls on the LDR, its resistance
decreases, and when there is less light, its resistance increases. This
simple property makes it very useful in different devices and
experiments. For example, LDRs have been used in automatic
streetlights, smoke detectors, fire alarms, and camera light meters. To protect the
sensitive material inside, the LDR is usually covered with a thin glass layer that
keeps out dust and moisture but allows light to pass through.
Objective
To study the variations, in current flowing in a circuit containing an 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 an incandescent lamp, (of fixed power), used to ‘illuminate’
the LDR.
Theory
1.) LDR and its Characteristics
A Light Dependent Resistor (LDR) is a semiconductor device whose resistance
decreases when the intensity of incident light increases. When photons of sufficient
energy strike the material, electrons are excited from the valence band to the
conduction band, creating mobile charge carriers. This lowers the resistance of the
device.
The threshold wavelength is given by: λ₀ = hc / Eₓ
where,
λ₀ = threshold wavelength,
h = Planck’s constant,
c = speed of light, and
Eₓ = energy gap.
Cadmium Sulphide (CdS) with band gap 2.42 eV and Cadmium Selenide (CdSe)
with 1.74 eV are commonly used materials, as they have very high resistance in
darkness.
2.) Characteristics of Photoconductive Cells
In the absence of light, the resistance of an LDR is called dark resistance, usually
of the order of 10¹²–10¹³ Ω. When light falls on it, the resistance decreases rapidly,
often reaching a few kilo-ohms or even hundreds of ohms depending on light
intensity.
CdS LDRs respond best around 520 nm (green light), while CdSe LDRs have
maximum sensitivity near 615 nm (orange-yellow). Both can detect light up to the
near-infrared region.
3.) Sensitivity
The sensitivity of an LDR indicates how strongly its resistance changes with
variation in incident light. With increasing illumination, resistance falls and the
current through the circuit increases. This property makes LDRs very useful for
measuring light intensity or controlling light-operated circuits.
4.) Spectral Response
Different photoconductive materials respond to different wavelengths. CdS cells
are more sensitive to green light, while CdSe cells respond better to red and orange
light. Since incandescent lamps emit a broad spectrum covering visible and near-
infrared light, they are well suited for experiments involving LDRs.
Materials Required
• Light Dependent Resistor (LDR)
• Connecting Wires
• Source of different power rating (bulbs)
• Bulb Holder
• Metre scale
• Multi Meter
• Battery
Circuit Diagram
Procedure
1. Fix the light source (bulb) securely in a holder so that it remains stable
throughout the experiment. Ensure that the light can fall directly on the LDR
without obstruction.
2. Begin the experiment with a bulb of the lowest power rating. Connect this
bulb to the circuit according to the diagram.
3. Connect the LDR in series with a 6V battery, a switch, and the multimeter.
Make sure all the connections are tight and correct.
4. Switch on the bulb and set the multimeter to the resistance (Ω) mode. Adjust
the range suitably and record the resistance of the LDR when exposed to
light.
5. Now change the setting of the multimeter to the current (µA) range. This
will allow the measurement of current flowing through the circuit. Record
the value obtained.
6. Repeat the above measurements using bulbs of higher power ratings, one by
one, keeping the distance between the bulb and the LDR constant.
7. Next, keep the bulb power fixed but vary the distance between the bulb and
the LDR. For each distance, record both the resistance and the current
values.
8. Take at least three readings for each case (different power ratings and
distances) to ensure accuracy. Note down all the readings in a well-
organized observation table.
9. After completing the experiment, switch off the circuit and carefully
disconnect all components.
Observation Tables And Graphs
Observations
Table 1: Variation with Lamp Power at Fixed Distance (20 cm)
Lamp Power Fixed Distance Resistance (kΩ) Current (µA)
(W) (cm)
15.0 20.0 11.0 540.0
40.0 20.0 4.5 1330.0
Graph for table 1:
Current vs Lamp Power (Fixed Distance = 20 cm)
Table 2: Variation with Distance at Fixed Lamp Power (40 W)
Fixed Lamp Distance (cm) Resistance (kΩ) Current (µA)
Power (W)
40.0 50.0 20.0 300.0
40.0 40.0 13.0 460.0
40.0 30.0 8.5 700.0
40.0 20.0 4.5 1330.0
Graph for table 2:
Current vs Distance (Fixed Lamp Power = 40 W)
Applications of LDR
LDRs are widely used in both analog and digital circuits where detection of light intensity is
required. Some common applications are:
1. Analog Applications
• Camera Exposure Control – To adjust shutter speed and aperture automatically.
• Auto Slide Focus (dual cell) – For focusing projectors based on light intensity.
• Photocopy Machines – To measure the density of toner on paper.
• Colorimetric Test Equipment – Used in laboratories for chemical analysis.
• Densitometer – For measuring optical density of materials.
• Electronic Scales (dual cell) – For weight measurements using light interruption.
• Automatic Gain Control – In circuits with modulated light sources.
• Automated Rear View Mirror – To reduce glare from headlights at night.
2. Digital Applications
• Automatic Headlight Dimmer – For vehicles, adjusts beam intensity depending on
surrounding light.
• Night Light Control – Automatically switches on lights in the dark.
• Oil Burner Flame Out Detector – Monitors light from the flame for safety shut-off.
• Street Light Control – Turns streetlights ON at night and OFF during daytime.
• Position Sensors – Detects the presence or position of objects based on light interruption.
Results
1. At a fixed distance, increasing the power of the incandescent lamp decreases the LDR
resistance and increases the current.
2. For a lamp of fixed power, decreasing the distance reduces resistance and increases
current.
3. The variation confirms that resistance of an LDR is inversely proportional to light
intensity, while current is directly proportional.
Conclusion
The experiment verifies that:
• With higher power incandescent lamps at the same distance, the current in the circuit
increases because the LDR resistance decreases.
• With a fixed lamp power, bringing the lamp closer to the LDR increases the current due
to increased light intensity.
• Hence, both lamp power and distance of the lamp significantly affect the behavior of an
LDR.
Bibliography
1. NCERT Physics – Class XII, Part II (Chapter on Semiconductor Electronics)
2. Practical Physics for Class XII – Lab Manual (CBSE guidelines)
3. K.L. Kapoor – A Textbook of Physics, Volume II
4. Theraja, B.L. – Principles of Electronics
5. “Light Dependent Resistor (LDR) – Working and Applications,” Electronics
Tutorials
6. “Incandescent Lamp – Characteristics and Applications,” ScienceDirect
Articles
