Input and Output Devices
Hardware · 4 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 3% of your exam marks.
Typically tested as 'give one use of…' or 'describe how this sensor works' questions.
A sensor is an input device that measures a physical property of the environment and converts the measurement into a digital value the computer can use.
Monitoring systems vs control systems
Two related uses of sensors:
- Monitoring system: the computer records and reports the readings, for example by showing them on a screen or raising an alert. It does not take action on its own. Example: a weather station that records temperature and humidity.
- Control system: the computer acts on the readings, switching an output device on or off (or sending a signal to an actuator) to keep some condition stable. Example: a thermostat that turns on a heater when the temperature falls below the target.
A control system usually contains a feedback loop: the output (the heater) changes the condition the sensor is measuring (the temperature), which changes the reading the sensor sends, which changes whether the heater stays on. Feedback loops are covered in topic 15.
Common sensors
| Sensor | Physical property | Where it is used |
|---|---|---|
| Acoustic | Loudness of sound | Noise pollution monitoring; security alarms; industrial machinery condition |
| Accelerometer | Acceleration, tilt, vibration | Smartphone orientation; car airbag deployment; pedometer step counting |
| Flow | Speed at which gas, liquid or powder is moving through a pipe | Water and gas meters; chemical pipelines |
| Gas | Presence of a specific gas (e.g. carbon monoxide, methane) | Home CO alarms; leak detection in industrial plants |
| Humidity | Water-vapour content of the air | Greenhouse climate control; museum preservation |
| Infrared | Heat radiation; or detecting whether an infrared beam is broken | Burglar alarms; thermal imaging cameras; automatic doors |
| Level | The height of a liquid in a tank | Petrol tank gauge; water tank; coolant level in a car |
| Light | Light intensity (brightness) | Automatic street lights; smartphone display auto-brightness; greenhouse blinds |
| Magnetic field | Strength and direction of a magnetic field | Anti-lock braking systems; reed switches on doors; compass apps |
| Moisture | Water content of soil or a material | Smart-irrigation systems; building damp detection |
| pH | Acidity or alkalinity of a liquid | Hydroponic farming; chemical process control; swimming-pool monitoring |
| Pressure | Force per unit area of a gas, liquid, or physical contact | Car tyre pressure monitoring; weather stations (barometric pressure); pipeline integrity |
| Proximity | Distance to a nearby object | Parking sensors when reversing; robotic obstacle avoidance; production-line counting |
| Temperature | How hot or cold something is | Central heating; ovens; refrigerators; weather stations; medical equipment |
Explaining how a sensor, microprocessor and actuator work together
What comes up: a scenario (robot, automated door, self-driving vehicle) asks you to explain how the sensor, microprocessor and actuator are used to control the system. Typically worth 4–6 marks.
Write (key mark points): (1) The sensor continuously sends digitised readings to the microprocessor. (2) The microprocessor compares the data to a stored value or threshold. (3) If the reading meets the condition, the microprocessor sends a signal to the actuator to take action (e.g. stop a motor, close a door). (4) If the condition is not met, no action is taken (or the system waits and checks again). (5) The process repeats continuously until the system is switched off.
Watch out: the mark scheme requires you to name all three components by role. Simply writing "the computer processes the data" without naming the microprocessor, or omitting the actuator, will lose marks.
Naming a sensor and explaining its use
What comes up: a scenario is given (e.g. a self-driving vehicle, a factory robot) and you are asked to name one suitable sensor and explain how it would be used. Usually worth 2 marks: one for the sensor, one for the matching use.
Write: state the sensor name and then explain what it detects and why that is useful in the scenario. Example pairs: proximity sensor — detects objects nearby, so the vehicle can slow down or stop before colliding; accelerometer — detects movement and tilt, so the system can adjust for uneven ground; light sensor — detects low light levels, so headlights or display brightness adjusts automatically.
Watch out: the mark scheme requires the sensor name AND a matching use that makes sense in the given scenario. Naming a sensor with a use that does not match the scenario gains no credit for the use mark.