LDR and Thermistors

General physics

Light Dependent Resistors and Thermistors — Physics Lesson

🎯 Learning Objectives

Learning Objectives

Understand that the resistance of a light-dependent resistor (LDR) decreases as light intensity increases
Understand that the resistance of a thermistor decreases as temperature increases (negative temperature coefficient)
Analyze the behavior of LDRs and thermistors in electrical circuits
Apply knowledge of variable resistors to solve practical problems
Interpret characteristic graphs of LDRs and thermistors

🗣️ Language Objectives

Language Objectives

Use scientific terminology related to variable resistors accurately
Describe the relationship between environmental factors and resistance
Explain the working principles of sensors using appropriate scientific language
Interpret and describe graphical data showing resistance variations
Communicate experimental observations and conclusions clearly

📝 Key Terms

Key Terms

English Term	Russian Translation	Kazakh Translation
Light Dependent Resistor (LDR)	Фоторезистор	Жарыққа тәуелді резистор
Thermistor	Терморезистор	Терморезистор
Light Intensity	Интенсивность света	Жарық қарқындылығы
Temperature Coefficient	Температурный коэффициент	Температуралық коэффициент
Photoresistive	Фоторезистивный	Фоторезистивті
Semiconductor	Полупроводник	Жартылай өткізгіш
Sensor	Датчик	Датчик
Variable Resistor	Переменный резистор	Айнымалы резистор

🃏 Topic Flashcards

Interactive Flashcards

Practice with these flashcards to memorize key concepts about LDRs and thermistors.

📚 Glossary

Glossary

Light Dependent Resistor (LDR): A variable resistor whose resistance decreases as the intensity of light falling on it increases. Also known as a photoresistor.
Translation
Russian: Фоторезистор — переменный резистор, сопротивление которого уменьшается с увеличением интенсивности падающего на него света. Также известен как светозависимый резистор.
Kazakh: Жарыққа тәуелді резистор — оған түсетін жарық қарқындылығы артқан сайын кедергісі азаятын айнымалы резистор. Фоторезистор деп те аталады.
Thermistor: A temperature-sensitive resistor whose resistance varies significantly with temperature. Most commonly refers to NTC (Negative Temperature Coefficient) thermistors where resistance decreases as temperature increases.
Translation
Russian: Терморезистор — температурочувствительный резистор, сопротивление которого значительно изменяется с температурой. Чаще всего относится к NTC (отрицательный температурный коэффициент) терморезисторам, где сопротивление уменьшается с повышением температуры.
Kazakh: Терморезистор — температураға сезімтал резистор, оның кедергісі температурамен айтарлықтай өзгереді. Көбінесе NTC (теріс температуралық коэффициент) терморезисторларға қатысты, мұнда температура артқан сайын кедергі азаяды.
Light Intensity: The amount of light energy per unit area per unit time, typically measured in lux or watts per square meter.
Translation
Russian: Интенсивность света — количество световой энергии на единицу площади в единицу времени, обычно измеряется в люксах или ваттах на квадратный метр.
Kazakh: Жарық қарқындылығы — уақыт бірлігіндегі аудан бірлігіне келетін жарық энергиясының мөлшері, әдетте люкспен немесе шаршы метрге ватпен өлшенеді.
Negative Temperature Coefficient (NTC): A characteristic where the resistance of a material decreases as temperature increases, which is typical for semiconductors and thermistors.
Translation
Russian: Отрицательный температурный коэффициент (NTC) — характеристика, при которой сопротивление материала уменьшается с повышением температуры, что типично для полупроводников и терморезисторов.
Kazakh: Теріс температуралық коэффициент (NTC) — температура артқан сайын материалдың кедергісі азаятын сипаттама, бұл жартылай өткізгіштер мен терморезисторлар үшін тән.
Photoresistive Effect: The phenomenon where the electrical conductivity of a material increases when light is absorbed by the material.
Translation
Russian: Фоторезистивный эффект — явление, при котором электрическая проводимость материала увеличивается при поглощении света материалом.
Kazakh: Фоторезистивті әсер — материал жарықты сіңіргенде материалдың электрлік өткізгіштігі артатын құбылыс.
Semiconductor: A material whose electrical conductivity is between that of conductors and insulators, and can be altered by factors such as temperature, light, or electric field.
Translation
Russian: Полупроводник — материал, электрическая проводимость которого находится между проводниками и изоляторами и может изменяться под воздействием таких факторов, как температура, свет или электрическое поле.
Kazakh: Жартылай өткізгіш — электрлік өткізгіштігі өткізгіштер мен оқшаулағыштардың арасында болатын және температура, жарық немесе электр өрісі сияқты факторлармен өзгертілуі мүмкін материал.

📖 Theory: Light Dependent Resistors and Thermistors

Theory: Variable Resistors

Introduction to Variable Resistors

resistors are whose resistance changes in response to conditions. Two important types are Light Dependent Resistors (LDRs) and thermistors, which respond to light and temperature respectively.

Kazakh Translation

Айнымалы резисторлар — қоршаған орта жағдайларына жауап ретінде кедергісі өзгеретін компоненттер. Екі маңызды түрі — жарыққа тәуелді резисторлар (LDR) және терморезисторлар, олар сәйкесінше жарық пен температураға жауап береді.

Circuit symbols for LDR (left) and Thermistor (right)

Light Dependent Resistor (LDR)

Working Principle

An LDR is made from semiconductor materials such as cadmium sulfide (CdS). When light falls on the LDR, it provides energy to electrons, allowing them to move more freely and increase conductivity.

Kazakh Translation

LDR кадмий сульфиді (CdS) сияқты жартылай өткізгіш материалдардан жасалады. LDR-ға жарық түскенде, ол электрондарға энергия береді, олардың еркін қозғалуына және өткізгіштікті арттыруына мүмкіндік береді.

LDR construction showing serpentine track design

Key Characteristics of LDRs:

Dark Resistance: Very high (typically 1MΩ or more)
Light Resistance: Much lower (typically 1kΩ or less)
Response Time: Relatively slow (milliseconds to seconds)
Spectral Response: Most sensitive to visible light

Graph showing inverse relationship between light intensity and LDR resistance

Thermistors

Working Principle

Thermistors are made from metal oxides and exhibit a strong temperature dependence. As temperature increases, more electrons gain enough energy to participate in conduction, reducing resistance.

Kazakh Translation

Терморезисторлар металл оксидтерінен жасалады және күшті температуралық тәуелділік көрсетеді. Температура артқан сайын, көбірек электрондар өткізгіштікке қатысуға жеткілікті энергия алады, кедергіні азайтады.

Different types of thermistor construction

Types of Thermistors:

NTC (Negative Temperature Coefficient): Resistance decreases as temperature increases
PTC (Positive Temperature Coefficient): Resistance increases as temperature increases

Note: In this course, we assume all thermistors are NTC type.

Thermistor Resistance vs Temperature Graph

Graph showing exponential decrease in NTC thermistor resistance with temperature

Mathematical Relationships

For LDRs:

R ∝ 1/L^γ

Where R is resistance, L is light intensity, and γ is a constant (typically 0.5-0.9)

For NTC Thermistors:

R = R₀e^{β(1/T — 1/T₀)}

Where:

R = resistance at temperature T
R₀ = resistance at reference temperature T₀
β = material constant
T = absolute temperature (Kelvin)

Practice Questions

Question 1 (Easy):

What happens to the resistance of an LDR when you shine a bright torch on it?

Answer

The resistance of the LDR decreases significantly when bright light shines on it. This is because the light provides energy to electrons in the semiconductor material, increasing conductivity and therefore reducing resistance.

Question 2 (Medium):

An NTC thermistor has a resistance of 10kΩ at 20°C. Explain what would happen to its resistance if the temperature increased to 60°C.

Answer

The resistance would decrease significantly (typically to a few hundred ohms or less). This is because NTC thermistors have a negative temperature coefficient, meaning resistance decreases exponentially as temperature increases. The higher temperature provides more thermal energy to charge carriers, increasing conductivity.

Question 3 (Medium):

Compare the response time of LDRs and thermistors. Which one would be better for a smoke detector and why?

Answer

LDRs typically have faster response times than thermistors. LDRs would be better for smoke detectors because:
1. They respond quickly to changes in light intensity caused by smoke particles
2. Smoke blocks light immediately, causing rapid resistance change
3. Thermistors respond to temperature changes which occur more slowly in early fire stages
However, modern smoke detectors often use both optical (LDR-based) and ionization sensors for best performance.

Question 4 (Critical Thinking):

A student designs a temperature monitoring system using an NTC thermistor in a voltage divider circuit. The thermistor is connected to the positive supply and a fixed resistor to ground. Analyze how the output voltage would change as temperature increases and suggest how this could be used to trigger an alarm system.

Answer

Analysis:
As temperature increases → thermistor resistance decreases → voltage across thermistor decreases → voltage across fixed resistor (output) increases.

Circuit behavior:
V_out = V_supply × R_fixed/(R_thermistor + R_fixed)
As R_thermistor decreases, V_out increases.

Alarm system:
Use a comparator circuit to trigger an alarm when V_out exceeds a preset threshold voltage corresponding to the critical temperature. This provides a simple, reliable temperature monitoring system.

🧠 Memorization Exercises

Exercises on Memorizing Terms

Exercise 1: Fill in the Blanks

The resistance of an LDR _______ as light intensity increases.
A thermistor with negative temperature coefficient has resistance that _______ as temperature increases.
LDRs are made from _______ materials like cadmium sulfide.
The circuit symbol for a thermistor shows a resistor with a _______ line through it.
In darkness, an LDR has _______ resistance compared to bright light conditions.

Answer

1. decreases
2. decreases
3. semiconductor
4. diagonal
5. higher/very high

Exercise 2: True or False

LDRs respond faster to changes than thermistors. ____
All thermistors have positive temperature coefficients. ____
An LDR has maximum resistance in bright sunlight. ____
Thermistors are commonly used in temperature sensors. ____
The resistance change in both LDRs and NTC thermistors is linear. ____

Answer

1. True
2. False (We assume NTC — negative temperature coefficient)
3. False (minimum resistance in bright light)
4. True
5. False (both show non-linear relationships)

Exercise 3: Match the Applications

Match each device with its most suitable application:

Devices:

LDR
NTC Thermistor
LDR
NTC Thermistor

Applications:

Car engine temperature gauge
Automatic street lighting
Digital camera light meter
Fire alarm system

Answer

1-B: LDR → Automatic street lighting
2-A: NTC Thermistor → Car engine temperature gauge
3-C: LDR → Digital camera light meter
4-D: NTC Thermistor → Fire alarm system

📺 Educational Video

Video Tutorial: LDRs and Thermistors

Additional Resources:

🔬 Problem Solving Examples

Worked Examples

Example 1: LDR in a Voltage Divider Circuit

Problem: An LDR with resistance varying from 1MΩ (dark) to 1kΩ (bright light) is connected in series with a 10kΩ resistor to a 9V supply. Calculate the output voltage in both light and dark conditions.

🎤 Audio Solution

Detailed Solution with Pronunciation

Dark conditions: (pronounced: dark kon-DISH-ens)

R_LDR = 1MΩ = 1,000,000Ω

Using voltage divider: V_out = V_supply × R₂/(R₁ + R₂)

V_out = 9V × 10,000/(1,000,000 + 10,000)

V_out = 9 × 10,000/1,010,000 = 0.089V ≈ 0.09V

Bright conditions: (pronounced: bright kon-DISH-ens)

R_LDR = 1kΩ = 1,000Ω

V_out = 9V × 10,000/(1,000 + 10,000)

V_out = 9 × 10,000/11,000 = 8.18V

📝 Quick Solution

Brief Solution

Given: V = 9V, R_fixed = 10kΩ

R_LDR(dark) = 1MΩ, R_LDR(light) = 1kΩ

Dark: V_out = 9 × 10k/(1M + 10k) = 0.09V

Light: V_out = 9 × 10k/(1k + 10k) = 8.18V

Range: 0.09V to 8.18V

Example 2: Thermistor Temperature Sensing

Problem: A thermistor has resistance of 5kΩ at 25°C and 1kΩ at 100°C. It’s connected in series with a 2.2kΩ resistor to a 5V supply. Calculate the output voltages at both temperatures.

🎤 Audio Solution

Detailed Solution with Pronunciation

At 25°C: (pronounced: twenty-five degrees SEL-see-us)

R_thermistor = 5kΩ = 5,000Ω

V_out = 5V × 2,200/(5,000 + 2,200)

V_out = 5 × 2,200/7,200 = 1.53V

At 100°C: (pronounced: one hundred degrees SEL-see-us)

R_thermistor = 1kΩ = 1,000Ω

V_out = 5V × 2,200/(1,000 + 2,200)

V_out = 5 × 2,200/3,200 = 3.44V

📝 Quick Solution

Brief Solution

Given: V = 5V, R_fixed = 2.2kΩ

R_therm(25°C) = 5kΩ, R_therm(100°C) = 1kΩ

25°C: V_out = 5 × 2.2k/(5k + 2.2k) = 1.53V

100°C: V_out = 5 × 2.2k/(1k + 2.2k) = 3.44V

Sensitivity: 1.91V change for 75°C change

🔬 Investigation Task

Interactive Simulation

Use this PhET simulation to investigate how LDRs and thermistors behave in circuits:

Investigation Questions:

How does the current through an LDR change when you increase the light intensity?
What happens to the voltage across a thermistor when temperature increases in a voltage divider circuit?
How would you design a circuit to turn on an LED when it gets dark?
What type of thermistor would be best for a fire alarm system?

Brief Answers

1. Current increases as LDR resistance decreases with higher light intensity
2. Voltage across thermistor decreases as its resistance decreases with temperature
3. Use LDR in voltage divider to trigger transistor/relay when voltage threshold is reached
4. NTC thermistor — resistance drops rapidly with temperature rise, giving early warning

👥 Group/Pair Activity

Collaborative Learning Activity

Work with your partner or group to complete this interactive quiz about LDRs and thermistors:

Discussion Points:

Compare the advantages and disadvantages of LDRs vs photodiodes for light sensing
Discuss why NTC thermistors are more common than PTC thermistors in temperature sensing
Brainstorm creative applications for LDRs and thermistors in everyday devices
How would you calibrate a thermistor for accurate temperature measurement?

Practical Investigation Ideas:

Test LDR resistance using a multimeter under different lighting conditions
Measure thermistor resistance at different temperatures (ice water, room temperature, warm water)
Build a simple light-activated switch circuit
Create a temperature monitoring system

✏️ Individual Assessment

Structured Questions — Individual Work

Question 1 (Analysis):

An automatic street lighting system uses an LDR connected to a control circuit. The LDR has a resistance of 500Ω in daylight and 1MΩ in darkness.

Explain why the resistance of the LDR changes with light intensity.
Calculate the ratio of dark resistance to light resistance.
Design a voltage divider circuit using a 12V supply that gives 2V output in daylight and 10V output in darkness.
Suggest how this voltage difference could be used to control street lamps.

Answer

a) LDRs are made from semiconductor materials. Light provides energy to electrons, freeing them to conduct electricity. More light = more free electrons = lower resistance.
b) Ratio = 1MΩ/500Ω = 2000:1
c) Use fixed resistor = 2.18kΩ in series with LDR
Daylight: V_out = 12 × 2.18k/(500 + 2.18k) = 2.0V
Darkness: V_out = 12 × 2.18k/(1000k + 2.18k) ≈ 10V
d) Use comparator circuit with threshold ~6V to switch relay/transistor controlling street lamps

Question 2 (Synthesis):

A greenhouse monitoring system needs to control both heating and lighting based on temperature and light levels. Design a system using both thermistors and LDRs.

Draw a block diagram showing how sensors connect to control systems.
Explain the logic for when heating should activate based on thermistor readings.
Describe how LDR readings would control artificial lighting.
Suggest how the two systems could interact to optimize plant growth.
Identify potential problems and propose solutions.

Answer

a) Sensors → Signal conditioning → Microcontroller → Output drivers → Actuators (heaters/lights)
b) Thermistor resistance decreases with temperature. When resistance exceeds threshold (low temp), activate heating
c) LDR resistance increases in low light. When resistance exceeds threshold, activate grow lights
d) Coordinate systems: reduce artificial lighting when heating is on to prevent overheating; adjust thresholds seasonally
e) Problems: sensor drift, false triggers, power consumption. Solutions: sensor calibration, hysteresis, energy-efficient actuators

Question 3 (Evaluation):

Compare LDRs and photodiodes for light sensing applications. Consider response time, sensitivity, cost, and temperature stability.

Create a comparison table highlighting key differences.
Recommend which sensor to use for: (i) camera light meter, (ii) security motion detector, (iii) solar tracker.
Justify your recommendations with scientific reasoning.
Discuss how temperature changes might affect each sensor type.

Answer

a) LDR: slower response, high sensitivity, low cost, temperature dependent | Photodiode: fast response, moderate sensitivity, higher cost, more stable
b) (i) Photodiode — needs fast response for moving subjects (ii) Photodiode — needs fast response for motion detection (iii) LDR — slow response acceptable, cost-effective
c) Camera needs millisecond response for exposure control; security needs fast detection; solar tracking is gradual
d) LDRs: resistance varies with temperature affecting calibration; Photodiodes: more stable, predictable temperature coefficient

Question 4 (Application):

A digital thermometer uses an NTC thermistor with the relationship R = 5000e^0.03(25-T) where R is in ohms and T is in °C.

Calculate the thermistor resistance at 0°C, 25°C, and 50°C.
If connected in a voltage divider with a 2kΩ resistor and 5V supply, find output voltages.
Determine the sensitivity (V/°C) at room temperature (25°C).
Explain why NTC thermistors are non-linear and discuss implications for measurement.

Answer

a) At 0°C: R = 5000e^0.75 = 10.6kΩ; At 25°C: R = 5000Ω; At 50°C: R = 5000e^-0.75 = 2.36kΩ
b) V_out = 5 × 2k/(R + 2k): At 0°C = 0.79V; At 25°C = 1.43V; At 50°C = 2.30V
c) Sensitivity ≈ (2.30-0.79)/(50-0) = 0.03V/°C at mid-range
d) Exponential relationship due to semiconductor physics. Requires linearization or lookup tables for accurate measurement

Question 5 (Critical Thinking):

A student observes that an LDR’s response to light changes is not instantaneous. Design and describe an experiment to measure the response time of an LDR and analyze factors that might affect this response.

Design an experimental setup to measure LDR response time.
Predict what factors might affect the response time.
Explain the physical mechanisms causing the delay.
Suggest how response time could be improved in practical applications.
Compare this to other light sensors and discuss trade-offs.

Answer

a) Setup: LDR in voltage divider → oscilloscope, controlled light source (LED with switch), measure voltage change vs time
b) Factors: material properties, temperature, light intensity change magnitude, LDR physical size
c) Delay due to: charge carrier generation/recombination time, thermal effects, material capacitance
d) Improvements: smaller sensing area, better materials, cooling, signal conditioning circuits
e) Photodiodes faster but more expensive; phototransistors offer amplification; trade-off between speed, sensitivity, cost

🔗 Additional Resources

Useful Links and References

📚 Study Materials:

🎥 Video Resources:

🧮 Practice Tools:

📱 Mobile Apps:

🤔 Lesson Reflection

Reflection Questions

Think about your learning today:

💡 Understanding:

How do LDRs and thermistors differ from regular fixed resistors?
Can you explain why both LDRs and NTC thermistors show decreasing resistance with their respective stimuli?
What connections can you make between semiconductor physics and sensor behavior?
How has your understanding of variable resistance changed?

🎯 Application:

How would you design a simple automatic night light using an LDR?
What sensor would you choose for a car engine temperature warning system and why?
How could you combine LDRs and thermistors in a single device?
Which activities helped you understand sensor behavior best?

🔄 Next Steps:

What other types of sensors would you like to learn about?
How confident do you feel about analyzing sensor circuits?
What practical applications of these sensors interest you most?
What questions do you still have about sensor physics?

📝 Self-Assessment Scale (1-5):

Rate your confidence in:

Understanding LDR behavior: ___/5
Understanding thermistor behavior: ___/5
Analyzing sensor circuits: ___/5
Choosing appropriate sensors for applications: ___/5
Explaining sensor physics: ___/5

🎯 Learning Goals Achieved:

☐ I understand that LDR resistance decreases with increasing light intensity
☐ I understand that NTC thermistor resistance decreases with increasing temperature
☐ I can analyze simple sensor circuits
☐ I can suggest appropriate applications for each sensor type
☐ I can explain the physical principles behind sensor operation