Output Devices (2.2)

• Output devices


  Part of a control system where the computer is controlling a process and sends signals to these output devices
• Monitors (screens)

  • Cathode ray tube (CRT) monitor
  • LCD (or TFT) screen (TFT means 'thin film technology' for modern thin screens)
• Cathode ray tube (CRT) monitors are the least expensive type of monitor, although they are becoming increasingly rare as LCD monitors are now taking over
• LCD screens

  Made up of tiny liquid crystals that make up an array of pixels which are affected by changes in applied electric fields
• Modern LCD screens are backlit using light emitting diode (LED) technology
• LCD screens


  • So much smaller and lighter than CRT
  • Can be easily and safely wall mounted
  • If placed on a desk, take up very little space
  • Used in laptop, tablets and mobile phones
  • Produce less glare on the screen, make them more relaxing on the eye
• LED screen


  • Made up of tiny light emitting diodes (LEDs) that can be controlled in brightness to produce a vast range of colours
  • Used for large outdoor displays due to the brilliance of the colours produced
  • Recent advances have led to the introduction of OLED (organic LED) screens
• Touch screens

  Can work as both an input device and as an output device
• Multimedia projectors


  • Receive signals that can be either analogue or digital (most modern projectors only work with digital inputs)
  • The signal source is usually from a computer, television or DVD player
  • The source is magnified and projected onto a large screen
• Laser Printer


  • Produce very high-quality hard copy output
  • The print rate per page is very quick if a large number of pages are being printed
  • They rely on large buffer memories where the data for the whole document is stored before pages can be printed out
  • Used where low noise is required (for example, in an office)
  • If fast, high-quality, high-volume printing is required then laser printers are the best option
• Inkjet printer


  • Used to produce good-quality hard copies
  • Unlike laser printers, inkjet printers do not have large buffer memories, therefore printing is done a bit at a time
  • This is why printing is sometimes paused - the whole print job cannot be stored in the buffer, and it has to wait for the computer to send more data
• Dot matrix printer

  • A type of impact printer where a print head (made up of a matrix of pins) presses against an inked ribbon
  • They tend to be slow, noisy and the output is not that good compared to inkjet and laser printers
  • They are still useful where multi-part stationery (carbon copies) or continuous rolls of paper (rather than individual sheets) are being used
  • They also work well in dirty atmospheres (such as on a factory floor), unlike inkjet or laser printers
• (Graph) plotter

  • Although they print on paper, they work very differently to printers
  • Instead of toner or ink cartridges, plotters use a pen, pencil or marker pen to draw multiple continuous lines, rather than a series of dots like a printer
  • The size of the paper can be anything from A4 up to several metres
  • They produce vector graphic drawings and are often used in conjunction with CAD and CAM (computer aided manufacturing)
  • Plotters are being phased out, as wide-format inkjet printers are being produced at lower cost
• 3D Printing


  • 3D printers are primarily used in computer-aided design (CAD) applications
  • They are primarily based on inkjet and laser printer technology and can produce solid objects that actually work
  • The solid object is built up layer by layer using materials such as powdered resin, powdered metal, paper or ceramic
• 3D printing & CNC

  1. 3D printers use additive manufacturing (i.e. the object is built up layer by layer)

  2. Contrast to the more traditional method of subtractive manufacturing (i.e. removal of material to make the object)

  3. For example, making a statue using a 3D printer would involve building it up layer by layer using powdered stone until the final object was formed

  4. The subtractive method would involve carving the statue out of solid stone (i.e. removing the stone not required) until the final item was produced

  5. Similarly, CNC (computer-controlled machine – a type of lathe) removes metal to form an object; 3D printing would produce the same item by building up the object from layers of powdered metal
• Creating a solid object using 3D printers
• Speakers (or loudspeakers)


  • Output devices that produce sound
  • When connected to a computer system, digitised sound stored on a file needs to be converted into sound as follows:
  • The digital data is first passed through a digital to analogue converter (DAC) where it is changed into an electric current
  • This is then passed through an amplifier (because the current generated by the DAC will be very small) to create a current large enough to drive a loudspeaker
  • This electric current is then fed to a loudspeaker where it is converted into sound
• How a loudspeaker works

  1. When an electric current flows through the coil of wire that is wrapped around an iron core, the core becomes a temporary electromagnet

  2. A permanent magnet is also positioned very close to this electromagnet

  3. As the electric current through the coil of wire varies, the induced magnetic field in the iron core also varies

  4. This causes the iron core to be attracted or towards or repelled from the permanent magnet and as the current varies this will cause the iron core to vibrate

  5. Because the iron core is attached to a cone (made of paper or thin synthetic material), this causes the cone to vibrate, producing sound waves
• Actuator


  • A mechanical or electromechanical device such as a relay, solenoid or motor
  • Used when a computer is used to control devices such as a conveyer belt or a valve, to start/stop the conveyer belt or open/close the valve
  • They allow a computer to control physical devices that normally require analogue inputs
• Sensors and actuator in a washing machine
• Exercise 1 - Only one of the responses is correct. Choose one of the five options given.
• Exercise 2
• Exercise 3
• Exercise 4