3.2 Input and output devices

Test yourself on Input and output devices 12 questions — drag-to-order, code completion, matching and multiple choice.
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Input Devices

Watch this lesson Video 3.2.1 · 11:37 · How each device turns something physical into binary — and why the order of the steps is the answer

1. What Counts as an Input Device

An input device is any hardware that puts data into a computer system. The pattern is the same for all of them: something physical in the real world — light, pressure, sound, movement — is measured and converted into the binary the processor can work with.

Exam questions on this section rarely ask what a device is for. They ask how it works. So for each device below, learn the sequence of steps, not just the description.

2. Barcode Scanner

A barcode is a series of dark and light lines of varying thickness that represent a number.

How it works
  1. A light source — an LED or a laser — is shone onto the barcode.
  2. The light is reflected back. Dark bars reflect less light; light bars reflect more.
  3. Sensors (photodiodes) detect the reflected light and produce a varying electrical signal.
  4. The signal is converted into a binary number.
  5. That number is used to look the item up in a database, which returns its price and description — the price is not stored in the barcode itself.
  6. The stock level for that item is automatically reduced by one.
Supermarket checkoutsLibrariesParcel tracking
AdvantagesDisadvantages
Much faster than typing the code inThe barcode can be creased, dirty or damaged and then will not scan
Fewer errors, because no one has to read and retype a numberHolds only a small amount of data — typically a 13-digit number
Stock control is updated automatically and in real timeUseless without the database it points into

3. QR Code Scanner

A QR (Quick Response) code is a matrix of black and white squares. Because it stores data in two dimensions rather than one, it holds far more than a barcode — thousands of characters rather than a dozen or so digits.

How it works
  1. A camera — usually in a smartphone or tablet — captures an image of the code.
  2. Software locates the three large squares in the corners, which tell it the code's position and orientation. This is why a QR code can be read upside down or at an angle.
  3. The pattern of light and dark squares is decoded into binary and then into the data it represents.
  4. The device acts on that data — typically opening a web address.

QR codes include built-in error correction, so they still scan correctly when part of the code is dirty, damaged or covered by a logo.

Web links in advertisingEvent ticketsMobile payments
The security point examiners like: a QR code is not human-readable, so you cannot tell where it leads before scanning it. A malicious code can send a user to a fake site designed to steal login or payment details, or trigger a download. That unreadability is a genuine disadvantage, not just a curiosity.

4. Digital Camera

How it works
  1. Light enters through the lens, which focuses it onto an image sensor (a CCD or CMOS chip).
  2. The sensor's surface is divided into millions of tiny light-sensitive elements — the pixels.
  3. Each pixel produces an electrical charge proportional to the amount of light falling on it.
  4. An analogue-to-digital converter turns each charge into a binary value.
  5. The camera's processor assembles the values into an image file, usually compressing it, and writes it to a memory card.

The number of pixels is the camera's resolution. More pixels means more detail — and a larger file.

5. Keyboard

How it works
  1. Pressing a key completes a circuit beneath that key.
  2. The keyboard's processor detects which circuit in the key matrix has closed.
  3. It looks the position up in a character map to find the matching character code (such as ASCII).
  4. That code is sent to the computer, which displays or stores the character.
AdvantagesDisadvantages
Reliable, and needs no training to use for basic entrySlow for large volumes of data compared with automatic methods
Text can be entered and corrected directlyTyping mistakes introduce errors that automatic input would not
Almost universally availableProlonged use is associated with repetitive strain injury (RSI)

6. Optical Mouse

How it works
  1. An LED underneath illuminates the surface below the mouse.
  2. A tiny sensor captures thousands of images of the surface every second.
  3. A processor compares consecutive images to work out the direction and distance moved.
  4. That movement data is sent to the computer, which moves the on-screen pointer to match.

Compared with the older mechanical ball mouse it has no moving parts to collect dirt, so it needs no cleaning, and it is more accurate. It will not track well on glass or a plain, featureless glossy surface, because there is no detail for it to compare between images.

7. Microphone

How it works
  1. Sound waves reach a diaphragm and make it vibrate.
  2. The vibration is converted into a varying electric current — an analogue signal whose shape follows the sound wave.
  3. An ADC samples that signal at regular intervals and converts each sample into binary.
  4. The binary data is stored or transmitted.

The sampling rate (how often samples are taken) and the sampling resolution (how many bits per sample) determine how closely the digital version matches the original sound.

8. 2D Scanner

A 2D scanner, usually a flatbed, digitises a flat document or photograph.

How it works
  1. The document is placed face down on the glass.
  2. A bright light illuminates it while a scan head moves along the length of the document.
  3. Mirrors and lenses direct the reflected light onto a CCD.
  4. The CCD converts the light into electrical signals, which are digitised into a bitmap image.

The result is a picture of the page. To turn a scanned page of text into text that can be edited, OCR (optical character recognition) software must process the image and identify the characters in it.

Digitising documentsScanning photographsPassport checking at airports

9. 3D Scanner

A 3D scanner captures the shape of a solid object rather than a flat surface.

How it works
  1. The object is scanned using lasers, white light, or X-rays / CT scanning for medical use.
  2. The scanner records the position of a very large number of points on the object's surface, measuring its x, y and z dimensions.
  3. Software builds those measurements into a 3D model.
  4. The model can be edited, stored, or sent to a 3D printer to produce a physical copy.
Medical prosthetics and dentistryReverse engineering partsArchaeologyAnimation and CGI

10. Touch Screens

A touch screen is both an input and an output device. The syllabus names three technologies, and they are distinguished by what physically happens when you touch the screen.

Resistive

Two transparent conductive layers are separated by a thin gap. Pressing the flexible top layer makes it touch the bottom layer, completing a circuit at that point. The change in voltage is measured and used to calculate the coordinates of the touch.

Capacitive

A glass layer is coated with a transparent conductor holding a uniform electrostatic charge. A bare finger conducts electricity, so touching the screen changes the electrostatic field at that point. The change is measured and the coordinates calculated.

Infrared

Emitters and detectors around the edge of the screen create an invisible grid of infrared beams just above the surface. Touching the screen breaks a beam horizontally and a beam vertically, and the coordinates come from which two beams were interrupted. (Some infrared screens detect the warmth of a finger instead.)
ResistiveCapacitiveInfrared
DetectsPressureChange in electrostatic fieldBroken light beams
CostCheapestMediumMost expensive
DurabilityPoor — the top layer wears and scratchesGood — a solid glass surfaceGood
Multi-touchNoYesYes
Gloved hand or stylusWorksDoes not work with an ordinary glove or plain stylusWorks with anything
Visibility in sunlightPoorer — multiple layers reduce clarityGoodGood
WeaknessNot durable, single touch onlyNeeds a conductive touchCost; dirt or strong light can interfere with the beams

11. Exam Focus

Describe the process, in order. "A barcode scanner scans the barcode" earns nothing. Light source → reflection differs between dark and light bars → sensor detects it → converted to binary → database lookup. The marks are in the sequence.
The price is not in the barcode. The barcode holds a number that identifies the product; the price comes from the database record that number points to. This matters — it is why a shop can change prices without reprinting labels.
Choose touch screen technology from the scenario. Outdoors or industrial, with gloves → resistive or infrared. A phone needing pinch-to-zoom → capacitive. A large interactive whiteboard → infrared. Justify by the named property, not by "it is better".

Output Devices

Watch this lesson Video 3.2.2 · 10:36 · Screens, projectors, the three printers and the speaker — each one as a sequence you can write down

1. What Counts as an Output Device

An output device takes binary data from the computer and turns it into a form a human can perceive — an image, a sound, a printed page — or into a physical action. As with input devices, exam questions ask how the device works, so learn the sequence of steps.

2. LCD Displays and LED Backlights

An LCD (liquid crystal display) screen is made of a grid of liquid crystal cells. Applying a voltage twists the crystals so that they either block light or let it through, which is how each pixel is turned dark or bright.

The crucial point: liquid crystals do not produce any light of their own. They only block or pass it. So an LCD screen must have a backlight behind it.

That backlight is what has changed over time:

  • Older LCD screens used CCFL (cold cathode fluorescent lamp) backlights.
  • Modern LCD screens use LEDs as the backlight instead.

This is why a television sold as an "LED screen" is really an LCD screen with an LED backlight — the picture is still produced by liquid crystals.

Why LED backlights replaced CCFL
Lower power consumption, so cheaper to run and better for battery devices
Brighter, with better contrast and colour
Light up instantly — no warm-up time
Physically much thinner and lighter
Longer working life
Contain no mercury, unlike CCFL, so less environmentally harmful to dispose of

3. Light Projectors

Both types throw an image onto a wall or screen, but they form it in completely different ways.

LCD Projector

  1. A bright light is shone through a prism or set of mirrors, splitting it into red, green and blue.
  2. Each colour is passed through its own small LCD panel, which forms that colour's part of the image.
  3. The three coloured images are recombined and sent out through a lens onto the screen.

DLP Projector

  1. A DMD chip (digital micromirror device) carries millions of microscopic mirrors — one per pixel.
  2. Each mirror tilts towards or away from the light source thousands of times per second. The proportion of time it spends tilted towards the light sets that pixel's brightness.
  3. Colour is added by a spinning colour wheel placed between the lamp and the chip (high-end models use three chips instead).
  4. The light reflected from the mirrors passes through the lens onto the screen.
LCD projectorDLP projector
Sharper image and richer colour saturationHigher contrast ratio — deeper blacks
Pixels can be visible as a faint grid ("screen door" effect)Smoother image, no visible pixel grid
Filters need cleaning or replacingMore reliable and longer lamp life
Contrast is lowerSome viewers see a brief "rainbow effect" from the colour wheel

4. Inkjet Printer

How it works
  1. A sheet is fed in and held in position by rollers.
  2. The print head moves across the paper, spraying extremely small droplets of ink.
  3. The droplets are produced either by thermal bubble — ink is heated so a bubble forms and forces a droplet out of the nozzle — or by piezoelectric action, where a crystal changes shape when a charge is applied and pushes ink out.
  4. The rollers advance the paper slightly and the head makes the next pass, until the page is complete.
AdvantagesDisadvantages
Excellent quality for photographs and colour imagesSlow, especially for long documents
Cheap to buyCartridges hold little ink and are expensive to replace
Small, quiet, suited to home useWet ink can smudge, and nozzles clog if unused

5. Laser Printer

A laser printer prints a whole page at a time using dry powder toner and static electricity.

How it works
  1. A rotating drum is given a positive electrical charge across its whole surface.
  2. A laser beam scans the drum, reversing the charge at every point where the image is to appear, leaving an invisible electrostatic image.
  3. Negatively charged toner is attracted to the reversed-charge areas of the drum, sticking only where the image is.
  4. The paper is given a charge and passes the drum, pulling the toner off it onto the page.
  5. A fuser applies heat and pressure, melting the toner so it bonds permanently to the paper.
  6. The drum's charge is removed, ready for the next page.
AdvantagesDisadvantages
Very fast, and faster still on long print runsExpensive to buy, particularly colour models
Low cost per page; toner lasts far longer than inkToner cartridges are costly to replace
Output is dry and cannot smudgeProduces heat and ozone; bulkier than an inkjet

6. 3D Printer

A 3D printer performs additive manufacturing: it builds a solid object up in very thin layers, rather than cutting material away.

How it works
  1. A design is produced in CAD software or captured with a 3D scanner.
  2. Software slices the model into hundreds or thousands of horizontal layers.
  3. In direct 3D printing, a print head moves left-right, front-back and up-down, depositing material — often molten plastic — one layer at a time, each layer setting before the next is added.
  4. In binder 3D printing, the printer makes two passes per layer: it lays down a layer of fine powder, then sprays a binding glue onto just the parts that are to become solid.

Materials include plastics, resin, powdered metal and ceramic powder.

Prosthetic limbsDental and medical implantsPrototypesAerospace and car partsReplacement parts for old machinery
AdvantagesDisadvantages
Can make shapes that are impossible to cut or mould in one pieceSlow — a single object can take many hours
One-off custom items, such as a prosthetic fitted to one patient, become affordableEquipment and materials are expensive
Less waste than machining material awayLimited range of materials, and finished strength can be lower
Parts can be made on site instead of shippedRaises the possibility of manufacturing dangerous or counterfeit items

7. Speaker

How it works
  1. A DAC (digital-to-analogue converter) turns the stored binary sound data into a varying electric current.
  2. The signal is amplified.
  3. The current passes through a coil of wire attached to a cone, sitting inside a magnetic field.
  4. The varying current makes the coil — and so the cone — move back and forth.
  5. The moving cone makes the surrounding air vibrate, producing sound waves.

8. Actuators

An actuator is an output device that produces a physical action rather than an image or a sound — a motor, a valve, a heater, a buzzer. Actuators are how a computer changes something in the physical world, and they are covered in detail in the Sensors and Actuators lesson in this section.

9. Exam Focus

An "LED screen" is an LCD screen. The liquid crystals form the image; the LEDs are only the backlight. If a question asks why LED backlights replaced CCFL, answer on power, thickness, brightness, warm-up time, lifespan and mercury.
Laser printers work by charge, not by heat alone. The sequence is: charge the drum → laser reverses the charge to draw the image → toner sticks to the charged image → paper attracts the toner → the fuser melts it on. Heat only appears at the last step.
Match the printer to the scenario. High-quality photos or occasional home use → inkjet. High volume, speed, low cost per page → laser. A physical object from a design → 3D printer. Justify with the named property.

Sensors and Actuators

Watch this lesson Video 3.2.3 · 9:07 · Fourteen sensors, the ADC, and the control loop that has to repeat

Sensors and Actuators

Syllabus Topic 3.2: Interacting with the Real World

1. The Analogue Problem

Physical properties in the real world (light, heat, pressure) are Analogue—they are continuous and constantly changing. However, the Microprocessor is a Digital device—it only understands discrete $1$s and $0$s.

ADC
Analogue-to-Digital Converter

Converts sensor signals for the CPU.

➤ CPU ➤
DAC
Digital-to-Analogue Converter

Converts CPU commands for Actuators.

An ADC is always needed on the input side, because every sensor produces an analogue reading and the processor cannot accept one. On the output side, a DAC is needed only when the actuator requires a varying analogue signal — a motor whose speed must be adjusted, for example. An actuator that is simply switched on or off, such as a buzzer or a heater, can be driven straight from a digital output.

2. Common Sensors (Input)

A sensor is an input device that continually measures a physical property and sends the readings to a microprocessor. It measures — it does not decide anything.

SensorWhat it measuresTypical application
TemperatureHow hot or cold something isCentral heating, ovens, greenhouses, car engines
LightIntensity of light falling on itAutomatic street lights, phone screen brightness
PressureForce applied to a surfaceElectronic scales, security mats, tyre monitors
ProximityHow close an object isCar parking sensors, automatic doors
InfraredInfrared radiation, or a broken infrared beamBurglar alarms, motion detectors, remote controls
HumidityWater vapour present in the airGreenhouses, museums, air conditioning
MoistureWater content in soil or a materialAutomatic irrigation, damp detection in buildings
pHAcidity or alkalinity of a liquidSoil monitoring, swimming pools, fish tanks
GasPresence or concentration of a specific gasCarbon monoxide alarms, air-quality monitoring
FlowRate at which a liquid or gas is movingWater meters, medical ventilators, fuel systems
LevelDepth or height of a liquidFuel tanks, water treatment, washing machines
Magnetic fieldPresence and strength of a magnetic fieldDoor and window contacts in alarm systems
AcousticSound or vibrationDetecting broken glass, leaks in pipes
AccelerometerAcceleration, and change of tilt or orientationRotating a phone screen, airbag deployment, fitness trackers

3. Actuators (Output)

An Actuator is an output device that carries out a physical action based on commands from the microprocessor.

Valves: Controls the flow of liquids or gases by opening/closing.
App: Automated irrigation, Central heating systems.
Motors: Creates rotational movement.
App: Washing machines, Robot arms, Fans.
Beepers / Buzzers: Produces a sound signal.
App: Microwave "finish" chime, Alarms.
Heaters: Converts electrical energy into heat.
App: Smart thermostats, Automated greenhouses.
Pumps: Moves a liquid or gas through a system.
App: Irrigation, central heating, fish tank filters.
Lamps / Lights: Provides illumination or a visual warning.
App: Automatic street lighting, warning indicators.

4. The Role of the Microprocessor

The microprocessor is the decision maker. The sensor measures, the actuator acts, and the microprocessor is the only part that compares anything. It runs a loop that repeats continuously:

  1. Receives Digital data from the ADC.
  2. Compares this data against a Pre-set Value stored in memory.
  3. If the condition is met, it sends a signal to switch the actuator on or off — through a DAC if that actuator needs an analogue signal.
  4. The process repeats, taking new readings continuously.

That last step is not a detail. A system that checked once would not be a control system; it is the constant re-reading that keeps the property within range.

Worked example: a greenhouse kept at 25°C
  1. The temperature sensor continually measures the air temperature and sends an analogue reading.
  2. The ADC converts each reading to digital so the microprocessor can accept it.
  3. The microprocessor compares the reading with the stored pre-set value of 25°C.
  4. If the temperature is below 25°C, it sends a signal to switch the heater on. If it is above, it switches the heater off and may open a vent using a motor.
  5. The whole cycle repeats indefinitely, so the temperature is held near the pre-set value.
⚠️ Exam Tips:
  • Always state that "data from the sensor is converted to digital using an ADC so the microprocessor can understand it." This is a standard mark-scheme requirement.
  • Say that the readings are taken continuously and the process repeats. Describing a single check does not describe a control system.
  • The sensor does not decide anything and cannot switch a device on. It only measures and sends data. The microprocessor makes every comparison and every decision.
  • Name the pre-set value the reading is compared against — questions frequently award a mark for it specifically.