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Computer Output Devices


Revision Points: (Full course and Short Course)

Candidates are expected to:

Monitors:

The computer monitor (also know as a VDU or Visual Display Unit) is the most common computer output device.  For desktop computers, cathode ray monitors (CRT) are gradually being replaced by flat-screen monitors such as the Liquid Crystal Displays (LCD) and Thin Film Transistor (TFT) displays used with laptop computers because they use less power and take up less space. [Further information on monitors]

Suitable uses - displaying text, images and numbers in full colour.

Advantages - relatively cheap and reliable, can display text and graphics in a wide range of colours. They are also quiet and do not waste paper.

Disadvantages - no permanent copy to keep and unsuitable for users with visual problems.


Printers:

Printers produce a printed 'hard copy' on paper.  Different printers are suited to different purposes and have a range of purchase and running costs.  [Further notes on Colour printing]

Printers can be divided up into three main types.

1 - Dot matrix printers:

How it works:  A magnified letter produced by a dot matrix printer (draft quality)The print head travels from side to side across the paper and is made up of numerous pins which are pushed out to form the shape of each character

The pins hit an ink ribbon against the paper so the characters are printed out.

The paper is usually continuous with holes down each side and perforations so the pages can be easily separated by tearing.

Because the pins make an impact against the paper the characters can be printed through several layers of self-carbonating paper to produce duplicate copies.

An animation showing how head of a 21 pin dot-matrix printer moves across the paper, firing the pins that strike the ink against the paper.

A typical dot matrix printer
Typical multi-part continuous paper used with dot matrix printers

Suitable uses:
  • Limited to situations where duplicate copies are needed and the quality is not too important.
  • Typical uses might be in warehouses where duplicate copies of orders need to produced quickly and cheaply.
Disadvantages:
  • The printing quality is low - these printers produce low to medium quality black and white printing and can only print low resolution graphics.
  • Because of the impact of the pins against the paper, these printers can be quite noisy.
Advantages:
  • The purchase cost is low and the running costs are very low.
  • They can print fairly quickly, particularly if you remember that multiple copies are being printed in one print run.
  • They are robust and can operate in harsh environments.
  • If several sheets of self-carbonating paper are placed into the printer then the impact will produce duplicate copies.

 2 - Ink-jet printers:

How it works:
  • The print head contains tiny nozzles through which different coloured inks can be sprayed onto the paper to form the characters or the graphic images.
  • The ink is forced out by heat or by tiny piezoelectric crystals which change shape when an electric current is applied across them.
Suitable uses:
  • A popular choice for home use where small amounts of printing are done and photographic quality colour printing is needed.
Disadvantages:
  • The ink cartridges can be expensive so running costs can be high.
  • The printing speed is slow compared to a laser printer. 
Advantages:
  • These printers are relatively inexpensive and produce high quality black and white or photographic quality borderless colour printing. 

3 - Laser printers:

How it works:
  • These print individual pages and work in a similar way to photocopiers.
  • A drum is charged to match the image and powdered ink (toner) sticks to the surface.  The toner is then transferred to the paper and fixed by heat and pressure.
  • A school or business printer would have a typical speed of 10 to 20 pages per minute (ppm).
Suitable uses:
  • Common wherever fast, high quality printing is required.
Disadvantages:
  • Non-colour laser printers are more expensive than ink-jet printers (but the difference is narrowing).
  • Colour laser printers are considerably more expensive. (but their speed and high quality output means they are becoming more popular).
Advantages:
  • They are quiet and fast and produce high quality printouts.
  • Running cost are low because although toner cartridges are expensive to replace, they last a long time.

Plotters:

How it works:
  • A drum plotter prints by moving a pen sideways over the surface of a sheet of paper.
  • One high precision motor moves the pen from side to side.
  • Another high precision motor moves the paper backwards and forwards.
  • An electromagnet lifts and drops different coloured ink pens onto the paper to draw lines.
 
  • A flat-bed plotter uses two high precision motors, one to move the pen in the X direction and one to move it in the Y direction.  The paper does not move.
Suitable uses:
  • Plotters are restricted to line drawing and can only create a solid region of colour by drawing a number of close, regular lines.
  • Plotters are often used in science and engineering applications for drawing building plans, printed circuit boards and machine parts.
A drum plotter printing out an engineering drawing
Advantages:
  • They are accurate and can produce far larger printouts than standard printers.
Disadvantages:
  • Slow and relatively expensive compared to printers.
  • They cannot print raster (photographic) images.
  • They can only fill solid blocks of colour using closely hatched lines.

Braille printer - by converting text into the Braille code, this printer produces patterns of raised dots on paper for use by the blind.


Electrical Transducers:

A electrical transducer converts an electrical current into an output such as light, sound or movement.

Speakers:

These transducers convert an alternating electrical current into sound. 

  • Suitable uses - They can output music as well as the spoken word. Useful for blind users where text or figures can be spoken by the computer. A speaker and computer-synthesised voice can be used by automated systems to read information such as telephone numbers or traffic updates.
  • Advantages - Cheap and widely available.  Capable of producing very high quality sound.
  • Disadvantages - they are an analogue device so the digital sound signal has to be converted, usually by a sound card.

Lights:

A typical microprocessor controlled security lightLight Bulbs: These use too much current to be powered directly by a computer but they can easily be switched on using a relay which is an an electronic switch.  In a relay a small current controls an electromagnetic switch which can turn on or off the much larger current which powers the light bulb.

  • Suitable uses - Light bulbs are used with relays and control systems in signalling, automatic lighting and burglar alarms.
  • Advantages - Cheap and widely available.
  • Disadvantages - They cannot be powered directly from a computer so need a relay to control them.

LED's in useLight-emitting diodes (LED's): These convert a small electrical current directly into light and can be powered directly by a computer. Today's LEDs can be found in just about every colour of the spectrum including invisible infra-red.

  • Suitable uses - LED's are commonly used to indicate various events such as 'power on' or 'hard disk in operation' and to monitor other control applications.  Infra-red LED's are used to transmit the data from a remote control to the receiver.
  • Advantages - Cheap, widely available and very reliable.  LED's use a tiny current so can be powered directly from a computer.
  • Disadvantages - None.

Motors:

Motors are not usually powered directly from a computer as they use too much current.  They are usually controlled through a relay which is an an electronic switch.  In a relay a small current controls an electromagnetic switch which can turn on or off the much larger current which powers the motor.

  • A typical relaySuitable uses - In control technology, a computer can be programmed to turn motors on and off using relay switches.  Examples include electronic doors and windows, cooling fans etc.  Motors are used in the automobile industry to move robot arms that spray body shells or assemble and in electronics manufacturing to assemble delicate electronic components on a printed-circuit board.
  • Advantages - Cheap, widely available and very accurate.
  • Disadvantages - They cannot be powered directly from a computer so need a relay to control them.

Buzzers:

These cheap transducers change a current directly into a simple sound.  They use a low current so can be powered directly from a computer.

  • Suitable uses - Often used as alarms or warnings in computer control systems.
  • Advantages - Cheap and widely available.  They use a tiny current so can be powered directly from a computer.
  • Disadvantages - Limited volume and they can only produce a limited range of sounds.

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