IGCSE Physics / Electrical resistance
Light-dependent resistors and thermistors
Edexcel International GCSE Physics (4PH1), chapter 8: Electrical resistance, specification point 2.11. Free lesson.
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An LDR changes resistance with light, while a thermistor changes resistance with temperature. In sensing circuits, these changes affect the current and voltage, allowing devices to switch on or off automatically.
How an LDR responds to light
A light-dependent resistor (LDR) changes its resistance according to the light falling on it. Its resistance is high in the dark and falls as light intensity increases. Light intensity is measured in lux. Resistance is measured in kilohms (kΩ). So, brighter light means lower resistance. Darkness means higher resistance.
How a thermistor responds to temperature
A thermistor changes its resistance with temperature. The common type has a high resistance when cold, and its resistance falls as it gets hotter. Temperature is measured in degrees Celsius (°C). Resistance is measured in ohms (Ω). For both components, more light or more heat means less resistance, but name the component and the condition when describing the change.
How a thermistor sensing circuit works
A thermistor can be used as a sensor in a single series loop with a battery and a fixed resistor. The battery provides a fixed supply voltage. An ammeter measures the current in the loop, while a voltmeter measures the voltage across the fixed resistor. That voltage can be used to switch on an alarm. When the thermistor gets hotter, its resistance falls. The total circuit resistance falls, so the current rises because the supply voltage stays the same. The voltage across the fixed resistor rises because V = I × R. A large enough voltage can switch on the alarm.
Where LDRs and thermistors are used
Both components are used in sensing circuits that switch something on or off. An LDR responds to light, while a thermistor responds to temperature. A street light uses an LDR and switches on when it gets dark. LDRs can also be used in automatic night lights, such as one in a bedroom. A fire alarm uses a thermistor to sense a rise in temperature, for example in a kitchen. A thermostat uses a thermistor to control heating in a home, switching it on when it is cold and off when it is warm.
Explaining what happens as it gets dark
For a street lamp circuit with an LDR in series with a fixed resistor, follow the changes from light to resistance to current. The total resistance increases as it gets darker, so the current falls.