Thermistors and LDRs
Overview
Thermistors and LDRs are variable-resistance semiconductor devices used in sensing and control systems.
Their resistance changes when environmental conditions change.
- thermistor responds to temperature
- LDR responds to light intensity
They are commonly used together with Potential Divider circuits to produce changing output voltages.
This topic extends ideas from:
Definition
Thermistors and LDRs are semiconductor devices whose resistance changes with an environmental condition rather than remaining fixed.
Why It Matters
These devices allow circuits to convert:
- temperature change into a resistance change
- light change into a resistance change
- a resistance change into a measurable output voltage in a potential divider
They are important because they connect basic circuit theory to automatic sensing and control.
Variable-Resistance Devices
Unlike a fixed resistor, these components do not have constant resistance.
Their resistance depends on external conditions.
| Device | Controlled By |
|---|---|
| Thermistor | temperature |
| LDR | light intensity |
Thermistors
Definition
A thermistor is a temperature-sensitive resistor.
Most syllabus examples use the NTC thermistor.
NTC means negative temperature coefficient.
NTC Thermistor Behaviour
As temperature increases:
- resistance decreases
Reason, qualitatively:
- more charge carriers become available inside the semiconductor
Resistance-Temperature Graph
Typical shape:
- high resistance at low temperature
- falls as temperature rises
Uses
- thermostats
- temperature sensors
- circuit protection
Comparison with Metal Resistor
For a metal resistor:
- resistance usually increases with temperature
For an NTC thermistor:
- resistance decreases with temperature
LDRs
Definition
An LDR, or light-dependent resistor, is a resistor whose resistance changes with light intensity.
LDR Behaviour
As light intensity increases:
- resistance decreases
This happens because incident light increases the number of available charge carriers.
Resistance-Light Trend
Typical shape:
- high resistance in darkness
- lower resistance in bright light
Uses
- automatic street lights
- alarm circuits
- camera exposure systems
Potential Divider Applications
Thermistors and LDRs are often placed in Potential Divider circuits.
In such circuits:
- the component resistance changes with the environment
- the output voltage across one component changes
- the circuit can be used for sensing or automatic switching
Thermistor Control Circuits
Possible uses:
- temperature alarms
- automatic fan control
- thermostat systems
LDR Switching Circuits
Possible uses:
- automatic night lighting
- burglar alarms
- light-activated switches
Exam questions often ask whether output voltage rises or falls when the variable resistance changes, so track the circuit carefully instead of guessing.
Key Representations
Thermistor Trend
For the usual NTC thermistor:
LDR Trend
As light intensity increases:
Sensor Divider Idea
In a potential divider, changing sensor resistance changes the voltage share across each component, so the output voltage depends on both:
- the sensor trend
- where the output is measured
Common Exam Traps
1. Reversing Thermistor Trend
For the usual NTC thermistor:
- higher temperature means lower resistance
2. Reversing LDR Trend
Higher light intensity means lower resistance.
3. Confusing Resistance with Output Voltage
A changing resistance does not by itself tell you whether output voltage rises or falls.
You must check:
- where the output is measured
- which component the voltage is taken across
- how the potential divider shares voltage
Summary Table
| Device | Input Change | Resistance Change | Typical Use |
|---|---|---|---|
| NTC Thermistor | temperature increases | resistance decreases | temperature sensing / control |
| LDR | light intensity increases | resistance decreases | light sensing / switching |