How do LDRs and thermistors turn light and temperature into a changing voltage?
Sensing subsystems: light-dependent resistors and thermistors, their resistance behaviour, and building light and temperature sensing circuits with potential dividers.
An Eduqas GCSE Electronics answer on sensing subsystems: how a light-dependent resistor and an NTC thermistor change resistance, and how to build light and temperature sensing circuits with potential dividers, including choosing which way round to put the sensor.
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What this dot point is asking
Eduqas wants you to describe how a light-dependent resistor (LDR) and a thermistor change resistance with light and temperature, and to build sensing circuits by putting one of them into a potential divider so that the output voltage tracks the physical quantity. Choosing which way round to connect the sensor decides whether the output rises or falls with the input.
The answer
The light-dependent resistor
The thermistor
Building a sensing circuit
Choosing which way round
Examples in context
LDR and thermistor sensing circuits are the input subsystem of a huge range of products: automatic porch and street lights, dusk-to-dawn switches, light meters, thermostats, fire and overheat alarms, and frost protection. The divider output feeds a comparator (often with a variable resistor setting the trigger level), whose digital output drives a transistor and an output transducer. These sensing circuits are a favourite context for exam questions and a common choice for the non-exam assessment project.
Try this
Q1. State what happens to an LDR's resistance as the light level rises. [1 mark]
- Cue. It decreases (high in the dark, low in bright light).
Q2. An NTC thermistor is in a divider with a fixed resistor, output across the fixed resistor. As it gets hotter, what happens to the output and why? [2 marks]
- Cue. The output rises; the thermistor's resistance falls, so the fixed resistor takes a larger share of the supply.
Q3. Why is a variable resistor often used as the partner to the sensor in a sensing circuit? [1 mark]
- Cue. To set/adjust the switching threshold (how dark or hot it must be before the system reacts).
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20204 marksA potential divider uses a light-dependent resistor (LDR) as the top resistor and a fixed resistor as the bottom resistor, with the output taken across the fixed resistor. Describe and explain how the output voltage changes as the light level increases.Show worked answer →
As the light level increases, the LDR's resistance decreases (1 mark) because more light frees more charge carriers in the semiconductor.
With the LDR (top) resistance falling, the fixed bottom resistor takes a larger share of the supply, so the output voltage across it increases (1 mark).
Reasoning via the divider equation (1 mark): as falls, the denominator falls, so the fraction and the output rise.
So bright light gives a high output and darkness gives a low output (1 mark). Markers reward the resistance change, the divider reasoning, and the correct direction of the output change.
Eduqas 20212 marksState what happens to the resistance of an NTC thermistor when it gets hotter, and name one use of a thermistor sensing circuit.Show worked answer →
Resistance change (1 mark): an NTC (negative temperature coefficient) thermistor's resistance decreases as its temperature increases.
Use (1 mark): any sensible temperature-sensing application, for example a thermostat or temperature alarm, a fire or overheat detector, a fridge or oven temperature controller, or frost protection.
Markers reward "resistance decreases as temperature rises" and one valid temperature-sensing use.
Related dot points
- Potential dividers: the potential-divider equation, choosing resistor values for a target output voltage, and the effect of loading the output.
An Eduqas GCSE Electronics answer on potential dividers: how two series resistors split the supply, the potential-divider equation, choosing resistor values for a target output voltage, and how connecting a load changes the output.
- Fixed and variable resistors: preferred (E-series) values, the resistor colour code, tolerance, and variable resistors (potentiometers and rheostats).
An Eduqas GCSE Electronics answer on fixed and variable resistors: reading the four-band colour code, preferred E-series values and tolerance, and how variable resistors are used as potentiometers and rheostats.
- Comparators: comparing two voltages, the reference set by a potential divider, the digital output, and using a comparator to make a sensing system switch at a threshold.
An Eduqas GCSE Electronics answer on comparators: how a comparator compares two input voltages and switches its output high or low, setting the reference with a potential divider, the digital nature of the output, and combining a sensor divider with a comparator to switch a circuit at a chosen threshold.
- Transistor switching: the bipolar transistor as a switch, cut-off and saturation, current gain, and choosing the base resistor to drive a load.
An Eduqas GCSE Electronics answer on using a bipolar transistor as a switch: the cut-off and saturation states, current gain relating collector and base current, and choosing the base resistor so a small input current turns on a larger load current.
Sources & how we know this
- WJEC Eduqas GCSE (9-1) Electronics specification (C490) — WJEC Eduqas (2017)