The big idea: An input device turns a change in the world into an electrical signal the rest of the circuit can act on.
A switch gives a digital signal — on or off. Most sensors give an analogue one, usually as a changing resistance, which has to become a voltage before anything can use it.
Each input drawn as what it senses and what it gives — then the voltage divider that makes any of them usable.
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| Device | Senses | Gives | Where you meet it |
|---|---|---|---|
| Switch (digital) | Being pressed, or something reaching a position | On or off — nothing in between | A doorbell, a fridge door, a limit switch on a machine |
| LDR (analogue) | Light falling on it | A resistance that FALLS as light rises — hundreds of ohms in sunlight, megohms in the dark | Street lights, camera exposure, a phone dimming its screen |
| Thermistor (analogue) | Temperature | A resistance that changes with heat; in the common type it falls as it warms | An oven, a kettle, a fridge, a 3D printer hot end |
| Humidity sensor (analogue) | Water vapour in the air | A changing resistance or capacitance, often converted to a digital reading inside the sensor itself | A bathroom extractor fan, a greenhouse, a dehumidifier |
| Microphone (analogue) | Sound — changing air pressure | A small alternating voltage, a few millivolts, which almost always needs amplifying | A smart speaker, a baby monitor, noise measurement |
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The voltage divider, in three steps
Put the sensor in series with a fixed resistor
Across the supply, so the two share the supply voltage between them in proportion to their resistances.
Take the signal from the junction
As the sensor's resistance changes, its share of the supply changes — so the junction voltage moves. That voltage is the signal.
Choose the fixed resistor to match
Make it roughly equal to the sensor's resistance at the value you want to detect. That puts the junction near mid-supply, where the voltage change per unit of sensing is largest.
Which way round matters: Swapping the sensor and the fixed resistor inverts the signal: the same LDR divider can give a voltage that rises in the dark or one that rises in the light.
A design question that says "switch on at dusk" is testing exactly that.
How this is tested — explaining the purpose of analogue and digital input components. It comes up two ways:
Paper 1 — multiple choice
- Choose the sensor suited to a stated job.
- Identify whether a named input is analogue or digital.
Paper 2 — analysing a product
- Explain how a named sensor produces a usable signal.
- Justify a choice of input device for a given product.
The trap: Saying a sensor "detects light" and stopping. Say what it GIVES — a resistance that falls as light rises — and how that becomes a voltage.
A bathroom extractor fan is to run automatically when someone showers. Explain a suitable input device and how its signal is made usable.
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