LDRs and thermistors (AQA GCSE Physics): Revision Notes
LDRs and thermistors
What are LDRs and thermistors?
LDRs and thermistors are special types of resistors that belong to a category called variable resistors. Unlike ordinary resistors that have a fixed resistance value, these components have a unique characteristic: their resistance changes when conditions around them change.
LDR stands for light-dependent resistor. As the name suggests, its resistance changes with the amount of light falling on it.
Thermistors are temperature-sensitive resistors that change their resistance when the temperature of their surroundings changes.
Both LDRs and thermistors are incredibly useful in electronic circuits because they can act as sensors, detecting changes in their environment and converting these changes into electrical signals that circuits can respond to.
Light-dependent resistors
LDRs are photosensitive components that respond to changes in light levels around them. They are made from semiconductor materials that exhibit photoconductivity, meaning their electrical conductivity increases when light shines on them.
How LDRs work
The operation of an LDR is based on the photoelectric effect in semiconductor materials:
- In the dark: LDR has high resistance (typically several megohms)
- In bright light: LDR has low resistance (can drop to a few hundred ohms)
- The brighter the light gets, the lower the resistance becomes
Remember this pattern: Brightness UP = Resistance DOWN
This inverse relationship is the key to understanding how LDRs function in circuits.
Circuit symbol
The LDR symbol shows a resistor with arrows pointing towards it (representing light rays hitting the component).
Uses of LDRs
LDRs are commonly used in automatic lighting systems. For example, street lights often use this system to switch on automatically when darkness falls and switch off at dawn. They're also found in camera light metres, security systems, and garden lighting.
Thermistors
Thermistors are temperature-sensitive resistors that play a crucial role in temperature sensing and control applications. The word "thermistor" comes from "thermal resistor," indicating their primary function.
How thermistors work
Most commonly used thermistors are NTC (Negative Temperature Coefficient) types, which means:
- When cold: thermistor has high resistance
- When hot: thermistor has low resistance
- The hotter it gets, the lower the resistance becomes
Remember this pattern: Temperature UP = Resistance DOWN
This behaviour makes thermistors excellent temperature sensors for electronic circuits.
Circuit symbol
The thermistor symbol shows a resistor with a diagonal line through it, indicating its temperature-dependent nature.
Uses of thermistors
Thermistors are widely used in temperature control systems. They can be used as thermostats in heating systems, in fire alarms for detecting dangerous temperature rises, and in electronic devices to prevent overheating.
Circuit applications
Both LDRs and thermistors work effectively in circuits when combined with other components to create sensing and control systems. Understanding how they behave in practical circuits is essential for designing effective electronic systems.
Worked Example: LDR Circuit Operation
Consider an LDR connected in series with a fixed resistor and battery:
Setup: LDR (variable resistance) + 1kΩ fixed resistor + 9V battery
Operation:
- In darkness: LDR = 10MΩ, total resistance ≈ 10MΩ, current ≈ 0.9μA
- In bright light: LDR = 100Ω, total resistance = 1.1kΩ, current ≈ 8.2mA
Result: Current increases dramatically as light intensity increases
Worked Example: Thermistor Circuit Operation
Consider a thermistor in a temperature sensing circuit:
Setup: Thermistor (variable resistance) + 2.2kΩ fixed resistor + 5V supply
Operation:
- At 0°C: Thermistor = 50kΩ, current ≈ 0.1mA
- At 50°C: Thermistor = 500Ω, current ≈ 1.9mA
Result: Current increases significantly as temperature rises
Fire alarm application
A thermistor-based fire alarm demonstrates practical circuit application. When temperature rises above a preset level, the decreased thermistor resistance allows more current to flow, which can trigger a buzzer or alarm system when the current reaches a predetermined threshold value.
Using voltmeters
You can connect a voltmeter across an LDR or thermistor in a circuit to observe how the voltage drop changes with varying conditions. This voltage change directly relates to the resistance change, providing a clear demonstration of how these components respond to environmental changes.
By measuring voltage changes across these components, you can quantify their sensitivity and calibrate them for specific applications. This is particularly useful when designing sensor circuits that need to respond to specific light levels or temperatures.
Key Points to Remember:
- LDRs change resistance with light levels - more light means less resistance
- Thermistors change resistance with temperature - higher temperature means less resistance
- Both follow the pattern: stimulus UP = resistance DOWN
- Both can be used in switching circuits and control systems
- You can use voltmeters to measure how their resistance changes in circuits
- These components are essential for creating automatic control systems that respond to environmental changes