PHYSICS PROJECT
ACADEMIC YEAR 2024-2025
TOPIC: VARIATION OF CURRENT THROUGH A LDR
NAME: S . J . Samhitha
CLASS AND SECTION: 12 I
ACKNOWLEDGEMENT
I would like to state that this project is my original work and would like to
thank all those people who have wholeheartedly extended their cooperation
and guidance for making it possible to complete this project on time.
My sincere gratitude to Our School Management for providing us the best
infrastructure and all the required resources. My special thanks to school
Principal Mrs. Sharadha Ramamurthy, Vice Principal Mrs.
M.V.Mahalakshmi for their unconditional support. Many many thanks to my
Physics teacher Mrs. Annie Manoj Kumar for her valuable guidance and
support. I would also like to thank my family members and friends for their
cooperation in completing this project within stipulated time.
INTRODUCTION
A photoresistor (also known as a light-dependent resistor, LDR, or
photo-conductive cell) is a passive component that decreases in resistance
as a result of increasing luminosity (light) on its sensitive surface, in other
words, it exhibits photoconductivity.
A photoresistor can be used in light-sensitive detector circuits, and
light-activated and dark-activated switching circuits acting as a
semiconductor resistance. In the dark. a photoresistor can have a
resistance as high as several megaohms (ΜΩ), while in the light, it
can have a resistance as low as a few hundred ohms.
COMPOSITION OF AN LDR
Since the discovery of photoconductivity in selenium, many other materials
have been found that are light-dependent. By the 1930s and 1940s, PbS,
PbSe, and PbTe were studied, followed by silicon and germanium
photoconductors. Modern LDRs are made from lead sulfide, lead, selenide,
indium antimonide, cadmium sulfide, and cadmium selenide. To create a
CdS LDR. purified cadmium sulfide powder is mixed with binding materials,
pressed, and sintered. Electrodes are evaporated onto one side, and the
disc is mounted in glass or plastic to prevent contamination.
WORKING PRINCIPLE
The photoconductivity theory underlies the operation of this resistor. It is
nothing more than the fact that when light strikes its surface, the material's
conductivity decreases and the electrons in the device's valence band are
stimulated to the conduction band.
These incident light photons must have energy larger than the
semiconductor material's band gap. Consequently, the electrons quickly
move from the valence band to the conduction band.
TYPES OF LDRS
Based on light sensitivity
1. Visible Light LDRS: These LDRs are sensitive to visible light
wavelengths, decreasing resistance as light intensity increases. They
are used in automatic lighting systems, photography light meters, and
light-sensitive circuits.
2. Infrared (IR) LDRS: Designed for sensitivity to infrared light, they detect
IR radiation and find application in IR remote controls, security systems,
and industrial automation.
3. UV (Ultraviolet) LDRs: Sensing UV light, these LDRs are used in UV
index sensors, sterilization monitoring systems, and UV light exposure
meters.
Based on Materials
1. Intrinsic LDRS: These are photoresistors made from semiconductor
materials like cadmium sulfide (CdS) or cadmium selenide (CdSe),
which naturally exhibit light-sensitive properties by creating electron-hole
pairs
when exposed to light. They are used in light sensors and automatic
lighting systems.
2. Extrinsic LDRs: These semiconductor devices are intentionally doped
with impurities to enhance their lightsensitivity. They oer improved
performance in applications requiring precise light detection, such
asoptical communications and scientific instruments
APPLICATIONS OF LDRs
· Photography Light Meters: LDRs are employed in cameras and
photographic equipment to measure light intensity for proper
exposure settings.
· Security Systems: LDRs are utilized in security alarm sand
surveillance systems to detect changes in ambient light levels, triggering
alerts or activating cameras.
· Solar Panels and Chargers: LDRs assist in solar panel systems
by tracking sunlight intensity to optimize energy collection and
storage.
· Industrial Automation: LDRs are used in industrial automation for
light-sensitive tasks such as conveyor belt control, product sorting based
on color or brightness, and object detection
ADVANTAGES OF LDRs
• Sensitivity is High.
• Simple & Small devices.
• Easily used.
• Uses of LDR sensors in a variety of projects.
• Inexpensive.
• There is no union potential.
• The light-dark resistance ratio is high.
• Its connection is simple
DISADVANTAGES OF
LDRs
· The spectral response is limited.
· The best materials have limited temperature stability due to the hysteresis
ect.
· Is chemical reaction in stable materials.
· LDR Sensor is only used in situations when the light signal fluctuates
dramatically.
· It is not a particularly responsive tool.
· As soon as the operating temperature changes, it gives wrong results
EXPERIMENT
● AIM:
To study the variation of current through an LDR with the intensity of the
incident radiation.
● APPARATUS:
12v bulb, LDR, milliameter, battery(6v or 9v), a meter scale, bulb holder,
key
PROCEDURE:
The LDR was connected in series as shown in the circuit diagram. Initially,
the LDR was covered using the hand. The reading in the milliammeter
was measured to be very low due to high resistance in the LDR.
Then the reading in the milliammeter was noted with the room light alone.
Later the given bulb was kept at 10 cm, 20cm, 30cm, 40cm, 50 cm, 60cm.
70 cm, 80cm, 90cm, and 100 cm from the LDR right in front of it and the
corresponding readings were noted.
A graph was plotted with I along the Y axis and distance along the X axis. It
was observed that as the intensity of illumination of LDR increased its
resistance decreases and therefore current through the circuit increases.
The graph obeyed the inverse square law.
PRELIMINARY OBSERVATION:
Reading shown by the ammeter when LDR was covered-2mA
Least count of scale-0.1 cm
Least count of mA 0.01 MA
OBSERVATIONS
1) Small LDR
2) Medium LDR
3) Big LDR- Orange coloured
4) Big LDR- Grey coloured
RESULTS
1. The variation of current through LDR with intensity of incident radiation
was studied.
2. From the graph it was observed that as the intensity of illumination of
LDR increases its resistance decreases. Therefore current through the
circuit increases, and the graph obeyed the inverse square law.
PRECAUTIONS
1. If ordinary light is used the intensity of light decreases rapidly with
distance.
2. Use A if mA is not suitable.
SOURCES OF ERROR
1. There may not be a variation of current in the circuit if LDR resistance is
very high.
2. Avoid loose connections.
3. Avoid laser source because its intensity does not vary too much over
a large distance.
BIBLIOGRAPHY
1. Ncert Textbook
2. robocraze.com
3. en.wikipedia.org
4. www.electronicsforu.
