1. The input-process-output model
Every computer system follows the same three-step pattern: data goes in, it is processed, results come out.
Input devices are hardware components that take data or commands from the outside world and feed them into the computer.
Output devices are hardware components that take processed results from the computer and present them to the user (or the environment) in some understandable form.
A computer with neither input nor output devices would be useless: it could process data, but no one could give it new data and no one could see what it had produced.
This topic covers:
- The most common general-purpose input devices (keyboards, mice, scanners, cameras, microphones).
- The most common general-purpose output devices (monitors, printers, speakers, actuators, 3D printers).
- Sensors: specialised input devices that measure physical properties of the environment.
- Two comparison topics: printer types (inkjet, laser, 3D) and touch-screen types (resistive, capacitive, infrared).
2. Common input devices
| Device | What it does | Typical uses |
|---|---|---|
| Keyboard | Lets the user enter text and commands by pressing labelled keys | Typing documents, data entry, software control |
| Mouse (optical / mechanical) | Tracks hand movement on a surface to move a cursor; left/right buttons let the user click and select | Pointing, clicking and selecting on a desktop interface |
| Microphone | Converts analogue sound waves in the air into a digital signal | Voice calls, voice recognition, recording, online meetings |
| Digital camera | Captures a 2D image (or a stream of images) as a digital file | Photography, video conferencing, webcams, smartphones |
| Barcode scanner | Reads the parallel-line pattern of a barcode and converts it to a number | Supermarkets, warehouses, libraries |
| QR code scanner | Reads the 2D dot pattern of a QR code and converts it to text or a URL | Smartphone apps, marketing, contactless menus |
| 2D scanner | Captures a flat 2D image of a document or photo | Digitising paperwork, archiving, photo editing |
| 3D scanner | Captures the 3D shape and dimensions of an object | Reverse engineering, design, medical imaging, custom prosthetics |
| Touch screen | Combines a display with a touch-sensitive overlay, allowing the user to point and tap on the screen itself | Smartphones, tablets, ATMs, kiosks, modern monitors |
| Sensor | Measures a physical property (light, temperature, motion, etc.) and converts it to a digital reading | Monitoring and control systems (see section 5) |
A useful test for "input vs output": ask whether information is going into the computer (input) or out of the computer (output). A touch screen is both: the touch part is input, the display part is output.
Common exam question
Input and output devices built into a phone or smartwatch
Question: Give or identify two input devices, and one or two output devices, that are built into a mobile telephone, smartphone or smartwatch (3–4 marks in total, one per device).
Asked in 3 of the 17 papers, with the input and output halves always set together, one mark per device. For input, all three schemes credit touchscreen, microphone, digital camera, button and sensor, and a named sensor such as an accelerometer is accepted. The two phone schemes add a biometric device, and the mobile telephone scheme also lists keyboard and keypad. For output, the credited devices are screen, speaker, LED (or light) and actuator (a motor, which is what makes the device vibrate).
Keep the halves apart: the touch layer you tap is the input device and the screen you look at is the output device, so "touchscreen" earns an input mark while "screen" earns an output mark. Speakers, LEDs and motors belong on the output lines.
Exam tip
Name the type of device, not a brand
The mark schemes give no marks for the brand name of a piece of hardware, so always name the type of device: write "biometric device" or "fingerprint scanner", not "Touch ID", and "headphones", not "AirPods".
3. Common output devices
| Device | What it does | Typical uses |
|---|---|---|
| Monitor (LCD / LED screen) | Displays text and images on a flat panel using liquid-crystal cells lit by LEDs or a backlight | Desktops, laptops, smart TVs, smartphones |
| Inkjet printer | Sprays tiny droplets of liquid ink onto paper | Home and small-office printing of colour photos and documents |
| Laser printer | Uses a laser to fuse fine powder (toner) onto paper with heat | Office printing of large volumes of documents |
| 3D printer | Builds a solid 3D object by depositing thin layers of plastic (or other material) one on top of another | Prototypes, custom parts, medical implants, education |
| Speakers / headphones | Convert a digital audio signal into analogue sound waves | Music, voice calls, alerts |
| Projector (LCD / DLP) | Throws an enlarged image of the screen onto a wall or projection screen | Classrooms, meeting rooms, home cinema |
| Actuator | Converts an electrical signal into physical motion (e.g. opening a valve, turning a motor, moving a robotic arm) | Robotics, factory automation, washing machines, automated systems |
An is the opposite of a . A sensor turns a physical property into a digital signal (input). An actuator turns a digital signal into a physical action (output). Together they are the core of every automated control system (topic 15).
Common exam question
Classifying a device as input, output or storage
Question: Tick whether a named device such as a keyboard or a printer is an input, memory, output or storage device, or circle the three output devices in a list of hardware (1–3 marks).
Asked in 2 of the 17 papers, one mark per device. A keyboard is an input device and a printer an output device. In the circling version the three output devices among ten names were actuator, printer and speaker; the other seven were input devices (keyboard, microphone, mouse, scanner, sensor) and storage media (DVD and SSD).
Place a sensor with the inputs: it sends readings into the computer. Place an actuator with the outputs: it receives a signal from the computer and produces movement, which is why a motor counts as an output device. Storage media such as a DVD or an SSD are neither, and when the options include memory and storage keep those two separate from the input and output pair.
4. Comparing printer types
The syllabus names three kinds of printer: inkjet, laser and 3D.
| Feature | Inkjet | Laser | 3D printer |
|---|---|---|---|
| How it works | Sprays liquid ink in tiny droplets through nozzles onto paper | Uses a laser to draw the page on a charged drum; toner sticks to the charged areas and is fused onto the paper by a hot roller | Deposits successive thin layers of melted plastic (or similar material) that stack up into a 3D shape |
| Output | 2D page (text or image) | 2D page (text or image) | 3D solid object |
| Material | Liquid ink in colour cartridges | Powdered toner (typically black plus 3 colours) | Filament of thermoplastic (PLA, ABS) or resin |
| Speed | Slow | Fast | Very slow (objects take hours) |
| Print quality | High-quality colour photos | Sharp text; colour quality depends on model | Layered surface; not as smooth as injection moulded |
| Cost per page / item | Higher (ink is expensive) | Lower for high volume | High for each object |
| Cost of the printer | Cheap | Mid-to-high | Mid-to-high for desktop models |
| Typical use | Home printing, photo printing, low-volume colour | Offices, schools, anywhere with high-volume monochrome printing | Prototyping, custom parts, education, medicine |
Why the three look so different
- Inkjet is cheap to buy but expensive to run because of the ink. Best for low-volume colour at home.
- Laser is more expensive to buy but cheaper per page because toner lasts longer. Best for offices that print thousands of pages a month.
- 3D printers do something completely different: they output three-dimensional objects rather than printed paper. The internal mechanism is unrelated to inkjet or laser.
5. Comparing touch-screen types
There are three main touch-screen technologies. The key differences are how each one detects a touch.
| Feature | Resistive | Capacitive | Infrared |
|---|---|---|---|
| How it detects touch | Two flexible conductive layers that press together when touched, completing a circuit at the touch point | A grid of conductors holds a tiny electrical charge; the human body changes the charge when a finger gets close, and the screen detects the change | A grid of infrared light beams crosses the screen surface; a finger or stylus breaks the beams and the screen detects where |
| Works with... | Almost anything: fingers, gloves, plastic stylus, fingernails | Bare skin only (or specially-conductive styluses); standard gloves and non-conductive objects will not register | Almost anything that can block the light beam (finger, gloved hand, stylus) |
| Multi-touch? | No (or only basic) | Yes | Yes |
| Sensitivity / accuracy | Slow, less responsive | Fast, smooth, very responsive | Fast and very accurate |
| Screen quality | Image looks slightly dimmer (the touch layers reduce light through) | Sharp and bright | Sharp and bright |
| Wear / durability | Soft layer can be scratched or wear out | Glass surface; very durable | Frame can collect dust and dirt |
| Cost | Cheap | More expensive | Most expensive |
| Where you see it | Older smartphones, ATMs, point-of-sale terminals, industrial control panels where gloves are worn | Modern smartphones, tablets, touch monitors | Large interactive displays, kiosks, electronic whiteboards |
Choosing between them
- For a modern smartphone, capacitive is the obvious choice: fast, accurate, supports pinch-to-zoom.
- For an ATM outside in winter, resistive is better: it works with gloved hands.
- For a large interactive whiteboard that needs to work with finger, pen, or hand, infrared is well suited.
6. Sensors
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 | Amount of liquid, gas or steam flowing 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 |
Common exam question
Naming a sensor and what it measures or is used for
Question: Identify a sensor a given system could use and how it would use it, state what a named sensor measures, or tick the most suitable sensor for a stated purpose (1–2 marks).
Asked in 3 of the 17 papers. Naming a sensor and its use earns one mark for the sensor and one for a matching use. For a self-driving tractor the scheme pairs an accelerometer with adjusting to uneven ground or detecting a crash, a proximity sensor with detecting obstacles or the end of the field, and a light sensor with knowing when to switch the headlights on. A use that does not match the sensor loses the second mark.
Asked what a sensor measures, name the quantity: a flow sensor measures the amount of liquid, gas or steam moving through a pipe or other environment. Asked for the most suitable sensor for a robot that steers round obstacles, pick proximity, not flow, pressure or level.
7. Choosing the right input or output device
A common exam question gives a scenario and asks which device best fits. The factors to consider:
- What is the task? Typing → keyboard. Drawing → graphics tablet. Pointing → mouse or touch screen. Capturing an image → camera or scanner. Producing printed pages → inkjet or laser printer. Producing a solid object → 3D printer.
- Who is the user? A small child may find a touch screen more intuitive than a keyboard. A visually impaired user may need a screen reader with speakers or headphones (output) and a Braille input device.
- Where is the device used? Outdoors → robust, weather-resistant. In a sterile environment (hospital, factory) → wipe-down or contactless. Public space → tamper-resistant.
- What is the budget? Inkjet printers are cheap to buy; laser printers are cheap to run for high volumes. Capacitive touch screens cost more than resistive but feel better. Trade off purchase cost against running cost and longevity.
Common exam question
Choosing an input or output device for a scenario
Question: Give or identify one or two appropriate input devices that would let a user or a system do a stated job, or an appropriate output device for a stated user (1–2 marks, one per device).
Asked in 2 of the 17 papers. Name a specific device that fits the job. For a library user entering a membership number at a self-checkout, the scheme credits a keyboard or keypad, touchscreen, mouse, microphone, digital camera, barcode scanner, QR code scanner or 2D scanner. For a system that reads train tickets automatically, the credited devices are a barcode scanner, QR code scanner or digital camera. For a blind user who cannot read a monitor, the output device is a speaker or headphones.
Match the device to the user: a machine checking tickets needs a code reader; a person entering a number can type, tap, scan or speak it; and a user who cannot see the screen needs sound.