How do input sensors detect conditions, and how does a voltage divider turn a change into a voltage?
Input subsystems: switches, the light-dependent resistor, the thermistor, and the voltage divider that turns a sensor's resistance change into a voltage signal.
A CCEA GCSE Technology and Design answer on input subsystems: switches, the light-dependent resistor (LDR) and the thermistor, and how a voltage divider (potential divider) turns a change in a sensor's resistance into a changing voltage signal for the process stage.
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What this dot point is asking
CCEA wants you to know the main input subsystems - switches, the light-dependent resistor (LDR) and the thermistor - and to understand the voltage divider (potential divider) that turns a sensor's resistance change into a voltage signal for the process stage. Inputs are how a system senses the world.
The answer
Switches as inputs
The simplest input is a switch, which is either open (no current) or closed (current flows). Push-to-make switches, toggle switches, reed switches (operated by a magnet) and tilt switches all provide a clear on/off input to a system.
Sensors: the LDR and the thermistor
These sensors do not directly give a voltage - they change resistance. To make a useful signal, the change in resistance must be turned into a change in voltage, which is what the voltage divider does.
The voltage divider
If one of the resistors is a sensor (an LDR or thermistor), its changing resistance changes how the voltage is shared, so the output voltage changes with light or temperature. That changing voltage is the signal passed to the process stage.
Designing the divider for the right response
Worked example: a dark-sensing input
Examples in context
- Example 1. A street light
- An LDR in a voltage divider gives a rising output as it gets dark, which switches the lamp on automatically at dusk.
- Example 2. A fire/heat alarm
- A thermistor in a divider gives a changing output as the temperature rises, triggering an alarm above a set temperature.
- Example 3. A fridge door light
- A reed switch (input) closes when the door opens, switching on the light - a simple switch input rather than a sensor.
Being able to describe the sensors and explain the voltage divider lets you answer both the "how does an LDR work" and the divider-calculation questions.
Try this
Q1. How does the resistance of an LDR change as it gets darker? [1 mark]
- Cue. Its resistance increases (it is high in darkness).
Q2. What does a thermistor sense? [1 mark]
- Cue. Temperature (its resistance changes with temperature).
Q3. What is a voltage divider used for in an input subsystem? [2 marks]
- Cue. To turn a sensor's change in resistance into a changing output voltage for the process stage.
Q4. In a voltage divider across 9 V, 4 V appears across one resistor. What is the voltage across the other? [1 mark]
- Cue. (the two add up to the supply).
Q5. Give one use of a reed switch as an input. [1 mark]
- Cue. Detecting a door or window opening or closing (operated by a magnet).
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA style4 marksExplain how a light-dependent resistor (LDR) works and give one use.Show worked answer →
An LDR is a sensor whose resistance changes with light level (1). In bright light its resistance is low; in darkness its resistance is high (1).
It is used as the input to a circuit that detects light or dark (1), for example switching a street light on automatically when it gets dark (1).
CCEA style4 marksA voltage divider has a 10 kohm fixed resistor and a thermistor. The 9 V supply is shared so that 3 V appears across the thermistor. Calculate the voltage across the fixed resistor and explain what happens as the thermistor warms up.Show worked answer →
The voltages across the two components add up to the supply, so the fixed resistor has across it (1, 1).
As the thermistor warms up, its resistance falls (1). A smaller share of the voltage then appears across it and more across the fixed resistor, so the output voltage of the divider changes with temperature, which the process stage can detect (1).
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Sources & how we know this
- CCEA GCSE Technology and Design specification — CCEA (2017)