The big idea: A digital system senses its environment with a sensor chosen for what actually needs measuring — which is often not the obvious quantity.
A bathroom fan should sense humidity, not light. A car park barrier should sense distance, not weight. Getting that right is most of the mark.
What each sensor senses and what it gives — then the divider that makes any of them readable.
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| Sensor | Senses | Typical use |
|---|---|---|
| Accelerometer | Motion, orientation and sudden deceleration | Screen rotation, step counting, airbag triggering, a bicycle light that brightens under braking |
| Ultrasonic | Distance or proximity, by timing an echo | Parking sensors, tank level, a robot avoiding a wall, a hand under a tap |
| Photoresistor (LDR) | Light level | Street lights, screen brightness, a camera exposure meter |
| Voltage or resistance probe | Moisture, by how well the material conducts | Soil moisture, a leak detector, a washing-machine load sensor |
| Hygrometer | Humidity, and usually air temperature with it | A bathroom fan, a greenhouse, a dehumidifier, a weather station |
| Pressure sensor | Barometric or applied pressure | Altitude in a watch, weather prediction, a blood-pressure monitor, a load cell |
| Microphone | Sound — changing air pressure | A smart speaker, a baby monitor, noise measurement, a clap switch |
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Four things to settle after choosing the sensor
Analogue or digital output?
A switch gives one bit. Most sensors give a changing resistance or voltage, and many modern ones do the conversion inside and send a digital reading.
Does it need a divider?
A resistive sensor does: in series with a fixed resistor, so its changing resistance becomes a changing voltage at the junction.
Does it need amplifying?
A microphone gives millivolts and a load cell microvolts, so an op-amp stage sits between them and the converter.
Where does it physically go?
A thermistor measuring room air must not be inside a warm case; a humidity sensor must be in the moving air, not behind a panel. Placement decides accuracy more often than the sensor does.
The mistake that looks like a sensor fault: A sensor in the wrong PLACE reads the wrong thing perfectly.
A thermostat above a radiator switches the heating off while the room is still cold. A light sensor behind glass in direct sun never sees dusk. Neither is a component failure, and neither is fixed by a better sensor.
How this is tested — determining sensors to collect and input information into a digital system. It comes up two ways:
Paper 1 — multiple choice
- Choose the sensor for a stated job.
- Identify what a named sensor measures.
Paper 2 — analysing a product
- Justify the sensor chosen for a named product.
- Explain how a sensor's signal is made usable and where it must sit.
The trap: Choosing the obvious quantity rather than the useful one. A bathroom fan should sense humidity; a hand dryer should sense reflection, not weight.
A public washroom tap is to run only while hands are beneath it. Justify the sensor you would specify, and say where it must sit.
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