LDRs and thermistors (AQA GCSE Physics Combined Science): Revision Notes
LDRs and thermistors
What are LDRs and thermistors?
LDRs and thermistors are special types of resistors whose resistance changes when conditions around them change. This unique property makes them incredibly useful for creating responsive electronic systems.
- LDR stands for Light-Dependent Resistor
- Thermistor is short for thermal resistor
Both components are essential in circuits where we want things to happen automatically when environmental conditions change. They act as sensors that can trigger different responses based on light levels or temperature changes.
Light-dependent resistors (LDRs)
How LDRs work
An LDR's resistance depends on how much light shines on it. This relationship is predictable and consistent:
- In the dark: resistance is very high (can reach several megaohms)
- In bright light: resistance becomes much lower (can drop to just a few hundred ohms)
The brighter the light gets, the lower the resistance becomes. This creates a smooth, continuous change rather than a simple on/off response.
Key rule for LDRs: Brightness UP → Resistance DOWN
This inverse relationship is fundamental to understanding how LDRs work in circuits.
What LDRs are used for
LDRs can automatically switch lights on when it gets dark, making them perfect for energy-efficient lighting systems. Common applications include:
- Street lights that turn on at night
- Security lights for home protection
- Automatic car headlights that activate in tunnels or at dusk
- Garden lighting systems
- Light-following robots and solar trackers
Practical Example: Street Light Control
Consider a street light with an LDR sensor:
- Daylight: LDR resistance = 10,000Ω, circuit current is low, light stays OFF
- Dusk: LDR resistance = 5,000Ω, circuit current increases slightly
- Dark: LDR resistance = 500Ω, circuit current is high, light switches ON
This automatic switching saves energy and requires no manual intervention.
Thermistors
How thermistors work
A thermistor's resistance changes with temperature. Most common thermistors are NTC (Negative Temperature Coefficient) types, which means:
- When cold: resistance is high
- When hot: resistance becomes lower
As temperature increases, the resistance decreases in a predictable pattern.
Key rule for thermistors: Temperature UP → Resistance DOWN
This relationship allows precise temperature measurement and control in electronic systems.
What thermistors are used for
Thermistors work exceptionally well as temperature sensors because they provide accurate, continuous readings. They can be found in:
- Thermostats for heating systems
- Temperature alarms in industrial equipment
- Electronic thermometers for medical use
- Car engine temperature monitoring
- Refrigerator and freezer controls
- 3D printer hot-end temperature regulation
Using LDRs and thermistors in circuits
LDR circuits
When you connect an LDR in a circuit, the current flow changes dramatically with light conditions:
- In bright light, more current flows (because resistance is low)
- In darkness, less current flows (because resistance is high)
You can measure this change with an ammeter to observe how the current varies with light intensity.
Thermistor circuits
When you connect a thermistor in a circuit, current flow responds to temperature changes:
- At high temperatures, more current flows (because resistance is low)
- At low temperatures, less current flows (because resistance is high)
This current variation can be used to trigger heating or cooling systems automatically.
Both LDRs and thermistors work with Ohm's Law: V = IR. As resistance (R) changes, either current (I) or voltage (V) will change proportionally, depending on your circuit design.
Combining both components
You can use LDRs and thermistors together in the same circuit to create sophisticated systems that respond to both light and temperature changes.
Practical Example: Smart Greenhouse Control
A greenhouse control system could combine both sensors:
Temperature Control:
- Cold morning: Thermistor resistance high → triggers heating system ON
- Hot afternoon: Thermistor resistance low → activates cooling fans
Light Control:
- Dark evening: LDR resistance high → turns on grow lights
- Sunny day: LDR resistance low → grow lights stay OFF
This creates a fully automated environment that maintains optimal growing conditions.
Key Points to Remember:
- LDRs: Light up → resistance down. Perfect for automatic lighting systems
- Thermistors: Heat up → resistance down. Great for temperature control
- Both change resistance smoothly and continuously, not just on/off like switches
- You can use ammeters and voltmeters to measure how much their resistance changes
- They're essential components for making automatic control systems that respond to environmental changes
- Combining both sensors creates powerful, responsive electronic systems