Thermistors and LDRs are input transducers whose resistance changes with environmental conditions.
Thermistor (NTC type)
An NTC (negative temperature coefficient) thermistor:
- Resistance decreases as temperature increases.
- At low temperature: high resistance.
- At high temperature: low resistance.
Uses: temperature sensors, thermostats, fire alarms, baby incubator temperature control.
Light-Dependent Resistor (LDR)
- Resistance decreases as light intensity increases.
- In darkness: very high resistance (MΩ).
- In bright light: low resistance (hundreds of Ω).
Uses: automatic street lights, camera light meters, security lights.
Potential divider sensing circuits
A potential divider splits the supply voltage between two resistors in series.
When a thermistor or LDR is one of the resistors, the voltage across each component changes with the environmental condition.
Temperature sensing example:
- Thermistor in series with a fixed resistor.
- As temperature rises, thermistor resistance drops.
- More voltage is dropped across the fixed resistor.
- This voltage change can trigger an alarm or switch.
Light sensing example:
- LDR in series with a fixed resistor.
- As light level drops, LDR resistance rises.
- More voltage is dropped across the LDR.
- This voltage change can switch on a light.
Common errors and how to correct them
- Saying thermistor resistance increases with temperature. For NTC thermistors (the IGCSE type), resistance DECREASES with temperature.
- Confusing which component the output voltage is taken across.
How to apply this in an exam
State how the resistance changes. Describe the effect on the voltage across each component in the potential divider. Explain how this triggers the required action.
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