Input Devices
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.
- A light source — an LED or a laser — is shone onto the barcode.
- The light is reflected back. Dark bars reflect less light; light bars reflect more.
- Sensors (photodiodes) detect the reflected light and produce a varying electrical signal.
- The signal is converted into a binary number.
- 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.
- The stock level for that item is automatically reduced by one.
| Advantages | Disadvantages |
|---|---|
| Much faster than typing the code in | The barcode can be creased, dirty or damaged and then will not scan |
| Fewer errors, because no one has to read and retype a number | Holds only a small amount of data — typically a 13-digit number |
| Stock control is updated automatically and in real time | Useless 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.
- A camera — usually in a smartphone or tablet — captures an image of the code.
- 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.
- The pattern of light and dark squares is decoded into binary and then into the data it represents.
- 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 payments4. Digital Camera
- Light enters through the lens, which focuses it onto an image sensor (a CCD or CMOS chip).
- The sensor's surface is divided into millions of tiny light-sensitive elements — the pixels.
- Each pixel produces an electrical charge proportional to the amount of light falling on it.
- An analogue-to-digital converter turns each charge into a binary value.
- 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
- Pressing a key completes a circuit beneath that key.
- The keyboard's processor detects which circuit in the key matrix has closed.
- It looks the position up in a character map to find the matching character code (such as ASCII).
- That code is sent to the computer, which displays or stores the character.
| Advantages | Disadvantages |
|---|---|
| Reliable, and needs no training to use for basic entry | Slow for large volumes of data compared with automatic methods |
| Text can be entered and corrected directly | Typing mistakes introduce errors that automatic input would not |
| Almost universally available | Prolonged use is associated with repetitive strain injury (RSI) |
6. Optical Mouse
- An LED underneath illuminates the surface below the mouse.
- A tiny sensor captures thousands of images of the surface every second.
- A processor compares consecutive images to work out the direction and distance moved.
- 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
- Sound waves reach a diaphragm and make it vibrate.
- The vibration is converted into a varying electric current — an analogue signal whose shape follows the sound wave.
- An ADC samples that signal at regular intervals and converts each sample into binary.
- 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.
- The document is placed face down on the glass.
- A bright light illuminates it while a scan head moves along the length of the document.
- Mirrors and lenses direct the reflected light onto a CCD.
- 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 airports9. 3D Scanner
A 3D scanner captures the shape of a solid object rather than a flat surface.
- The object is scanned using lasers, white light, or X-rays / CT scanning for medical use.
- The scanner records the position of a very large number of points on the object's surface, measuring its x, y and z dimensions.
- Software builds those measurements into a 3D model.
- The model can be edited, stored, or sent to a 3D printer to produce a physical copy.
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
Capacitive
Infrared
| Resistive | Capacitive | Infrared | |
|---|---|---|---|
| Detects | Pressure | Change in electrostatic field | Broken light beams |
| Cost | Cheapest | Medium | Most expensive |
| Durability | Poor — the top layer wears and scratches | Good — a solid glass surface | Good |
| Multi-touch | No | Yes | Yes |
| Gloved hand or stylus | Works | Does not work with an ordinary glove or plain stylus | Works with anything |
| Visibility in sunlight | Poorer — multiple layers reduce clarity | Good | Good |
| Weakness | Not durable, single touch only | Needs a conductive touch | Cost; dirt or strong light can interfere with the beams |