A sensor lets a computer notice what is happening around it: how hot, how bright, how wet or how close. Learn the fourteen sensors in the syllabus, what each one measures and how to choose the right one for a job.
A sensor is an input device that measures a physical quantity (such as temperature, light or pressure) in its surroundings and sends the reading to a processor. Most sensors produce an analogue signal, which an analogue-to-digital converter (ADC) turns into digital data. Sensors are used in monitoring systems (record and warn) and control systems (the processor compares readings with stored values and makes an actuator act). Sensors only measure: they never make decisions or act on their own.
People can check a temperature now and then; a sensor can check it every second, day and night, without mistakes or getting tired. That is why sensors are used.
Never write "the sensor turns on the heater" or "the sensor decides". The sensor only sends readings; the microprocessor compares and decides; the actuator acts.
For each sensor, know what it measures (the type of data it captures) and a typical use.
| Sensor | Data captured | Typical use |
|---|---|---|
| Acoustic | Sound level / noise | Burglar alarm detecting breaking glass; noise monitoring |
| Accelerometer | Acceleration, movement and tilt | Phone screen rotation; car airbag crash detection |
| Flow | Rate of flow of a liquid or gas | Water supply pipes; fuel flow; breathing monitors |
| Gas | Level of a particular gas (e.g. CO2, carbon monoxide) | Carbon monoxide alarm; greenhouse CO2 control |
| Humidity | Amount of water vapour in the air | Greenhouses; museums and art galleries |
| Infra-red | Infra-red radiation (heat from bodies) or a broken infra-red beam | Intruder alarms; automatic doors and taps |
| Level | How high a liquid (or solid) is in a container | Fuel tanks; flood warnings; tank filling |
| Light | Light intensity (brightness) | Street lights switching on; automatic screen brightness |
| Magnetic field | Changes in a magnetic field | Phone compass; detecting doors and windows opening; vehicle detection |
| Moisture | Water content of soil or another material | Automatic plant watering |
| pH | Acidity or alkalinity | Soil in greenhouses; swimming pools; rivers and pollution checks |
| Pressure | Pressure or force, e.g. weight on a surface or air pressure | Burglar alarm pressure pads; weather stations; car tyres |
| Proximity | How close an object is (without touching it) | Car parking sensors; phone screen off near your ear |
| Temperature | Temperature | Central heating; ovens; greenhouses |
Moisture and humidity are different. Moisture is water in the soil (or a material); humidity is water vapour in the air.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is a sensor?
Which sensor measures the water vapour in the air?
Why is an ADC often needed with a sensor?
In a control system, which part decides whether the heater turns on?
Exam questions describe a system and ask for the most suitable sensors. Match each thing that must be measured to one sensor, and say why.
Answer with sensor names from the syllabus list. "Motion sensor" or "heat sensor" may not earn the mark: say infra-red or temperature.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 1 standard.
Which sensor would be most suitable to check how acidic the water in a swimming pool is?
A smartphone turns its screen sideways when it is rotated. Which sensor detects this?
Which sensor is most suitable for detecting carbon monoxide in a kitchen?
Street lights switch on automatically at dusk. Which sensor is used?
This skill is now recorded on your profile. Keep the chain going.