Understanding the difference between input and output devices is probably the simplest concept in computer science. An input device is simple something which inputs information in to a computer. Therefore, for a device to be labelled an 'input device', data must travel from the device to the computer where it is used in some way.

Examples of input devices are:

An input device used to be defined as something which would enable a user to input information or control a system externally. However, many exam boards are now beginning to consider USB Storage Media and similar devices as input devices because they can technically input data in to a computer. Look out for this distinction in any questions which are asked in the exam.

The 2014/2015 IGCSE Computer Science specification requires that you also understand:

1) How some input devices work

2) How they are used in real life scenarios

The devices you need to know about are shown below.



A scanner allows you to take a copy of a physical document and store it in digital form on a computer. A bright light moves across the document. The light is reflected in to a light-sensitive electronic integrated circuit called a CCD (charge coupled device). From here the light is assembled in to digital code which represents which colour of light was in which part of the image. The data is put together to form the digital version of the scanned image.



When you speak, sound waves created by your voice carry energy toward the microphone. Inside the microphone, the diaphragm and a coil move back and forth when the sound waves hit them. The permanent magnet produces a magnetic field that cuts through the coil. As the coil moves back and forth through the magnetic field, an electric current flows through it. The electric current flows out from the microphone to an amplifier or sound recording device.



An optical mouse has a small red light at the bottom of it which you will notice if you turn it over. This small red light comes from an LED (light emitting diode). The light is used to recognise small patterns on the surface which the mouse is moving on. The light is sent to a CMOS sensor for analysis. If the mouse is moved, the CMOS sensor (complimentary metal-oxide semiconductor) will recognise this from the change in where the patters are on the surface. It will in turn send this information to the computer which will make the movement of the mouse cursor, or character in a video game.



When a key is pressed on a keyboard, two small pieces of conductive material are pushed together which cause an electrical signal to be sent to eh computer. The computer recognises which key was pressed by which circuit was completed. The computer then sends this information to the piece of software which is active at the time, for example the Word Processor.



An interactive whiteboard recognises when a person touches the screen and performs an action as a result of this. There are several types of whiteboard. The most common uses a resistive system. This means that there is a thin layer over the top of the board. When a certain part of the two layers are pressed together, the board recognises the position of this contact and sends this information to the computer. Other types use infrared signals which when broken, will send a signal to the computer. The board will recognise where the signal was broken and write or click in the appropriate place.

Instead of the ink in an inkjet printer, a laser printer uses what is called ‘toner’. This is a carbon passed powdered form of ink. As a laser beam scans back and forth across a ‘drum’ in the printer, static electricity is built up which draws the powdered ‘toner’ on to the paper.



A barcode reader uses light in the same way that a scanner does. It reads the thickness of the lines on the barcode and converts this in to a digital code. The code is then entered in to a database where it corresponds to a product. In a supermarket, one of the products would then be removed from the database to show that the product had been purchased.



QR code readers work like barcode readers except that they usually use the camera of the device that they are installed on rather than a CCD chip. The camera is used to recognise the mathematical properties of the shape on the QR code. This is then translated to code which corresponds to a website. The information is then sent to the browser and the website is launched.



Touchscreen is a general term used to describe any device which uses touch to control the software. There are two main types of touchscreen, resistive and capacitive. Resistive screens have a thin layer over the top of another layer of material. When a certain part of the two layers are pressed together, the screen recognises the position of this contact and sends this information to the software. Capacitive screens are more expensive. They rely on the human body being able to conduct small amounts of electrical current. Touching the screen distorts the electronic field and the device recognises where the interference was.



A digital camera uses a CCD (charge coupled device) much like a flatbed scanner. Light is transmitted in to the device where is analysed for brightness and colour. The information is sent to the software on the computer where it is decoded and converted in to individual pixels. Each pixel of contains details of how much of each colour was in this part of the light reading. The pixels are then put together to form an digital representation of the light which was captured by the CCD

1) Describe how a 2D scanner digitises an image of a document

2) Explain the process of a bar code reader scanning an item at a supermarket