Input and output devices

Input and output devices

1. Input Devices

   • Scanners (can be 2D or 3D/0
     • 2D scanners
       • Most common form of scanner, generally used to input hard-copy (paper) documents. The image is converted into an electronic form which can be stored in a computer
         • The cover is first raised, then the document is placed on a glass panel, and then the cover is closed
           • A bright light then illuminates the document - modern scanners use a type of xenon lamp which produces a very bright, white light
             • A scan head moves across the document until the whole page has been scanned. An image of the document is produced, which is then sent to a lens using a series of mirrors. The lens focused the document image
               • The focused image now falls onto a charge couple device (CCD), which consists of a number of integrated circuits etched into silicon. Essentially the CCD is made up of thousands of light sensitive elements (pixels). Each element creates an electric charge then light falls on it, meaning that the scanned image is now turned into an electronic form. Software produces a digital image from the electronic form
                 • Computers equipped with optical character recognition (OCR) software allow the scanned text from the document to be converted into a text file first, meaning the scanned image can now be edited and manipulated by importing it into a word processor. If the original document was a photo/image, an image file is created.
       • Used at airports to read passports
         • Make use of OCR technology to produce digital images which represent the passport pages. The OCR software can review these images, select the text part then automatically put the text into the correct fields of an existing database. The text can be stored in ASCII format
           • The 2-dimenisonal photograph in the passport is also scanned and stored as a jpeg image. The passenger's face is also photographed with a digital camera, and these two images are compared using face recognition software
     • 3D scanners
       • Scan solid objects and produce a 3D image. These scanners take images at several points along the x, y and z coordinates, and a digital image representing the solid object is formed.
         • The scanned images can be used in CAD (Computer Aided Design), or sent to a 3D printer to produce a working model of the scanned image
       • Used in computed tomographic (CT) scanners
         • Used to create a 3D image of a solid object, which is based on tomography technology. This technology basically builds up an image of the solid object through a series of very thin 'slices'. These 2D slices make up the representation of the 3D object. Each slice is built up by use of X-rays, radio frequencies or gamma imaging, although other methods exist. Each 'slice' is then stored as a digital image in the computer memory. The whole of the solid object is represented digitally in the computer memory
           • Depending on how the image is formed, the type of tomographic scanner can have different names, e.g X-rays - CT scanners - computerised tomography, radio frequencies - MRI - magnetic resonance imaging, gamma rays - SPECT - single photon emission computed tomography
   • Barcode readers/scanners
     • A barcode is series of dark and light parallel lines of varying thickness. The numbers 0-9 are represented by a unique series of lines. Various barcode methods for representing these digits exist, e.g UPC (Universal Product Code).
       • For UPC, the left-hand and right-hand sides of the barcode are separated by guard bars, each digit represented by bars of 1 to 4 blocks thick
         • When a barcode is scanned, the barcode is first read by a red laser or red LED (Light Emitting Diode)
           • Light is reflected back off the barcode; the dark areas reflect little or no light, allowing the bars to be red
             • The reflected light is read by sensors (photoelectric cells)
               • As the laser or LED light is scanned across the barcode, a pattern is generated which is converted into digital data - this allows the computer to understand the barcode
                 • The barcode number is looked up in the stock database (the barcode is known as the key field in the stock item record); this key field uniquely identifies each stock item
                   • When the barcode number is found, the stock item record is looked up
                     • The price and other stock item details are sent back to the checkout (or point of sale terminal (POS))
                       • The number of stock items in the record is reduced by 1 each time the barcode is read
                         • This new value for number of stock items is written back to the stock item record
                           • The number of stock items is compared to the re-order level; if it's less than or equal to this value, more stock items are automatically ordered
                             • Once an order for more stock items is generated, a flag is added to the record to stop re-ordering every time the stock item barcode is read
                               • When new stock items arrive, the stock levels are updated in the database
     • Used at retail checkouts
   • Quick response (QR) codes
     • Another type of barcode, made up of a matrix of dark squares on a light background. To make a comparison, normal barcodes can hold up to 30 digits; QR codes can hold over 7000 digits
       • Because of modern smartphones, QR codes can be scanned anywhere, which allows advertising of products on trains, buses, shopping malls and many other places. By using the built-in camera facility on mobile phones and downloading the appropriate app, it's possible to read the QR code, which may contain a website link or some form of advertising.
   • Digital cameras
     • Essentially replaced the more traditional camera that used film to record photos. The film required developing and then printing before the photographer could see the results, making them expensive to use, as unwanted photographs couldn't be deleted. Modern digital cameras link to a computer system via a USB port or by using Bluetooth. These cameras are controlled by a microprocessor which can automatically: adjust the shutter speed, focus the image, operate the flash, adjust the aperture size, adjust the size of the image and remove 'red eye' when flash is used.
       • The photograph is captured when light passes through the lens onto a light-sensitive cell. This cell is made up of tiny elements known as pixels; the number of pixels determines the size of the file used to store the photograph. The quality of the photograph depends on the type of lens used and lighting. Mobile phones can use same number of pixels as digital cameras, but have poor lens quality and limited memory
   • Keyboards
     • Most common method for data entry. Used as input devices for computers, tablets, mobile phones, etc. It's connected to the computer via a USB connection or by wireless connection. For tablets and mobile phones, the keyboard is virtual or a type of touchscreen technology. Each character on a keyboard has an ASCII value - each character pressed is converted into a digital signal, which the computer interprets
       • Relatively slow method of data entry and frequent use can lead to RSI (Repetitive Strain Injury)
   • Pointing devices
     • The selection of an application often requires the user to 'click' on an icon. Selection of the icon is usually done with a pointing device (such as a mouse or a trackball) or by using a touchscreen. The mouse is the most common pointing device, coming in various forms: the more traditional type with mechanical ball arrangement. connected to computer via a USB port; the more modern type that uses red LEDs to detect movement in the x, y direction - type of optical mouse or mice that use these technologies but are wireless
       • An area of desk space is required, often requiring a mouse mat as some surfaces, such as paper, prevent correct operation of the device. Depending on type of computer, the noise is fitted with one or two buttons to allow for selection and other functions. Many mouse designs have a scroll wheel to allow rapid movement up and down the screen. Trackerballs are more often used in an industrial environment such as a control room. As they don't need to move, the trackerball doesn't need any desk space or special surface. The operator is also less likely to suffer from injuries such as RSI
         • A ball on top of the trackeraball is moved to control a cursor on the screen. As with the mouse, buttons are used to select icons and to carry out other functions. Most laptop computers have a built-in touch pad which contains a tactile sensor that allows the user to control a cursor by simply moving a finger over the pad surface. Buttons to the left and right of the pad act in the same way as buttons on a mouse
   • Microphones
     • Used to input sound to a computer, e.g doing a voice over in a presentation, part of a voice recognition system, part of a speech recognition system, enabling a disabled person to communicate with a computer
       • Built into computer or are external devices connected through USB port or wireless connectivity. When a microphone picks up sound, a diaphragm vibrates producing an electric signal. This signal goes to a sound card and is converted into digital values and stored in the computer.
         • If a microphone's being used in a voice recognition system, the user's voice is detached and then converted into digital. A few words spoken produce a digital wave pattern, which is compared by software o wave patterns stored in memory to see if they match. If they match, the person has been correctly identified. Only certain words can be used, since the system is designed to recognise only a few key phrases. This technology can be used in security systems.
           • Speech recognition is a different and more complex technology - uses a microphone to input spoken words, but the software doesn't try to recognise the person talking. The spoken words are recognised and shown on a screen, input into a word processor or used in other applications. Accents and dialects can be an issue
   • Touchscreens
     • Very common form of input device. Allow simple touch to launch an application or to carry out functions of pointing devices. Mainly used in mobile phones. Three major types: capacitative, resistive and infra-red
     • Types
       • Capacitative
         • Made up of many layers of glass that act like a capacitor, creating electric fields between the glass plates in layers
           • When the top glass layer is touched, the electric current changes and the coordinates where the screen was touched is determined by an on-board microprocessor
       • Infra-red
         • Heat-sensitive
           • Uses glass as the screen material
             • Needs a warm object (e.g fingers) to carry out an input operation
         • Optical
           • Uses glass as the screen material
             • Uses an array of sensors (in grid format); point of contact based on which grid coordinate is touched
       • Resistive
         • Makes use of an upper layer of polyester (form of plastic) and a bottom layer of glass
           • When the top polyester layer is touched, the top and bottom layer complete a circuit
             • Signals are then sent out which are interpreted by a microprocessor, the calculations determine the coordinates of where the screen was touched
   • Sensors
     • Devices which read/measure physical properties. Real data is analogue in nature - it's constantly changing and doesn't have a discrete (finite) value. Analogue data usually requires some form of interpretation, e.g the temperature measurement on a mercury thermometer requires a user to look at height of mercury to work out temperature. The temperature can have an infinite number of values depending on precision of measurement.
       • Computers can't make sense of analogue quantities, so data needs to be converted to a digital format via an analogy to digital converter (ADC), which converts physical values into discrete digital values
         • When a computer is used to control devices like a motor/valve, a digital to analogue converter (DAC) is used, because these devices need analogue data to operate. An actuator is used in these control applications - it's an electromechanical device such as a relay, solenoid or motor.
     • Types
       • Temperature: controls a heating system, controls/monitors a chemical process. controls/monitors temperature in a greenhouse
       • Moisture/humidity: controls/monitors moisture levels in soil in a greenhouse. controls/monitors humidity levels in air in a greenhouse, monitors dampness levels in an industrial application
       • Light: switches street lighting on at night, off during day, monitors/controls light levels in a greenhouse, automatically switches a car's headlights on when it gets dark
       • Infra-red/motion: turns on windscreen wipers automatically, detects intruders in a burglar alarm system, counts people entering/leaving a building
       • Pressure: detects intruders in a burglar alarm system, weighs things, monitors/controls a process where gas pressure is important
       • Acoustic/sound: picks up noise levels in a burglar alarm system, detects noise of liquids dripping in a pipe
       • Gas (e.g O2, or CO2): monitors pollution levels in a river/air, measures CO2 and O2 levels in a greenhouse, checks for CO2 leaks in power stations
       • pH: monitors/controls alkalinity/acidity levels in soil in a greenhouse, monitoring pollution in rivers
       • Magnetic field: any application where detection of a magnetic field is required (e.g cell phones, CD players), anti-lock braking systems in cars
     • Sensors send signals to microprocessor/computer
       • Signals converted from analogue to digital with a ADC converter
         • Computer/microprocessor analyses data received by checking it against stored values
           • Monitoring
             • If new values are outside acceptable range , a warning message is sent to a screen or an alarm is activated
               • Microprocessor/computer has no effect on what's being monitored
             • E.g monitoring a patient's vitals in hospital, monitoring intruders in a burglar alarm system, checking temperature levels in a car engine
           • Controlling
             • If new data is outside the acceptable range, computer/microprocessor sends signal to valves, motors, etc
               • Output from system affects the next set of inputs
             • E.g turning street lights on at night, off at day, regulating temperature in a central heating/air conditioning system, changing traffic lights at a road junction
   • Interactive whiteboards
     • Devices that allow computer images to be displayed on a whiteboard using a digital projector. Also allow a user to write on the whiteboard, and the text or drawings produced are stored in an electronic format for later use. Also possible to run software on the whiteboard and users can interact by using their fingers on the whiteboard's surface

2. Output devices

   • Inkjet printers
     • Made up of: a print head - consists of nozzles which spray droplets of ink onto paper to form characters; an ink cartridge/cartridges - either a group of cartridges for each colour (blue, yellow, magenta) and a black cartridge or one single cartridge containing all 3 colours+black; a stepper motor and belt - moves print head assembly across page from side to side and a paper feed, which automatically feeds printer with pages
       • Thermal bubble: tiny resistors create localised heat, making the ink vaporise. The ink forms a tiny bubble, and as the bubble expands, some of the ink is ejected from the print head onto the paper. When the bubble collapses, a small vacuum is created allowing fresh ink to be drawn into the print head. This continues until the printing cycle finishes
         • Piezoelectric: a crystal is located at back of ink reservoir for each nozzle. The crystal is given a tiny electric charge, making it vibrate. This vibration forces ink to be ejected onto paper, whilst more ink is drawn in for further printing simultaneously
     • Data from document sent to printer driver
       • Printer driver ensures the data is in a format that the chosen printer can understand
         • Printer driver checks that chosen printer is available (not busy, offline, etc)
           • Data sent to printer and stored in a printer buffer
             • A sheet of paper then fed into main body of printer; a sensor detects if paper is available in paper feed tray - an error message is sent to computer if it's out of paper or there's a paper jam
               • As sheet of paper is fed through printer, print head moves from side to side across paper, printing text or image; four ink colours sprayed in exact amounts to produce desired colour.
                 • At end of each full pass of print head, paper is advanced slightly to allow next line to be printed; continues until whole page is printed
                   • If there's more data in printer buffer, whole process repeated until it's empty
                     • Once it's empty, printer buffer sends an interrupt to processor in computer; request for more data to be sent to printer
     • Best for one-off photos, or if only a few good quality pages are needed,, or colour printings are needed
   • Laser printers
     • Use dry powder ink rather than liquid ink, use static electricity, print whole page in one go, faster than inkjet printers with large print jobs
       • Data from document sent to printer driver
         • Printer driver checks data is in a format chosen printer can understand
           • Printer driver checks to ensure chosen printer is available
             • Data sent to printer and stored in printer buffer
               • Printing drum given a positive charge; as it rotates, a laser beam is scanned across it, removing positive charge in certain areas, leaving those areas negatively charged, which exactly matches text/images to be printed
                 • Drum then coated with positively charged toner (powdered ink), which sticks to negatively charged areas of drum
                   • Negatively charged sheet of paper rolled over drum
                     • Toner on drum sticks to paper to produce an exact copy of page sent to printer
                       • To prevent paper sticking to drum electric charge on paper is removed after one rotation of the drum
                         • Paper finally goes through a fuser (set of heated rollers); heat melts the ink so it fixes permanently to paper
                           • Discharge lamp removes all electric charge from drum, making it ready to print next page
     • Useful for high volume printing, have larger toner cartridges and larger paper trays. Produce high quality printouts
   • 3D printers
     • Primarily used in CAD (Computer Aided Design)
       • Can produce solid objects that work - it's built up layer by layer, using materials such as powdered resin, metal, paper or ceramic powder
         • Various types of 3D printers exist - can range from size of microwave oven to size of a small car
           • Use additive manufacturing (object built layer by layer), instead of subtractive manufacturing (removing material to make object)
             • Direct 3D printing: inkjet technology used - print head moves left to right, but it can also move up and down to build layers of object
               • Binder 3D printing: similar to direct 3D printing, but uses two passes for each layer - first pass sprays dray powder, then on second pass, a binder (type of glue) sprayed to form a solid layer
                 • Newer technology using layers and UV light to harden liquid polymers
     • Design made using CAD software
       • Finalised drawing imported into some special software that prepares it in a format which is understood by 3D printer
         • 3D printer set up to allow object to be made
           • Solid object built up layer by layer (typically 0.1mm thick), which can take several hours, depending on thickness of layers, material used and final size of object
             • Object removed from printer and then prepared, e.g some use a jelly-like support, which needs to be washed away by immersion in water. In many cases, object needs to be left to 'cure' for a few hours
     • Used to make prosthetic limbs, precise reconstruction surgery, wings and other parts for planes, fashion and art, making items no longer in production
   • 2D and 3D cutters
     • Work similarly to each other, but 3D cutter can recognise an object in x,y z direction rather than just x,y
       • 3D laser cutters can cut glass, crystal, metal, polymer and wood
         • Very complex designs can be cut, since they're controlled by computers and very sophisticated software
   • Loudspeakers/headphones
     • Sound is produced from a computer by passing digital data through a DAC, then through an amplifier, and sound then emerges from loudspeaker
       • Sound is produced by voltage differences vibrating a cone in speaker housing at different frequencies and amplitudes
         • Rate at which DAC translates digital output into analogue voltages is known as sampling rate
   • LCD and LED monitors
     • LCD (Liquid Crystal Display/Diode) technology
       • Front layer of monitor made up of liquid crystal diodes, which are grouped together in threes or fours - these are known as pixels (picture elements). Three colours that are grouped together use red, green and blue diodes. Some systems have a yellow diode, to make colours more vivid
         • LCD monitors are backlit using LED (Light Emitting Diode) technology, giving the image better contrast and brightness. Before LEDs, LCD monitors used a cold cathode fluorescent lamp (CCFL). CCFL uses two fluorescent tubes behind LCD screen, which supplies light source. When LEDs are used, a matrix of tiny LEDs is used behind the LCD screen. This is needed, because LCD itself doesn't emit light
           • Future LED technology makes use of Organic Light Emitting Diodes (OLEDs), which use organic materials to create flexible semi-conductors. Organic films are sandwiched between two charged electrodes (one a metallic cathode, the other a glass anode/0. When an electric field is applied to the electrodes, they give off light, so no backlighting required, allowing for very thin screens
     • Used for TVs, mobile phones, tablets, game consoles
   • Light projectors - used to project computer output onto larger screens or onto interactive whiteboards. Usually used in presentations and in multimedia applications
     • DLP (Digital Light Projectors)
       • Millions of micro mirror used on a small DLP (Digital Light Projector) chip
         • Number of micro mirrors and their arrangement determine resolution of digital image. When the micro mirror tilt towards the light source, they're ON, and tilting away, means they're OFF.
           • This creates a light or dark pixel on the projection screen. The micro mirror can switch on and off several thousand times a second, creating various grey shades - typically 1024 grey shades can be created (grey scale image)
             • A bright white light source (e.g from a xenon bulb) passes through a colour filter on its way to DLP chip, splitting it into primary colours red, green and blue. DLP can create over 16 million colours
               • ON and OFF states of micro mirror linked to colours of colour filter to produce coloured image
     • LCD projectors
       • Older than DLP
         • Powerful beam of white light generated from a bulb or LED inside projector body
           • This beam of light is then sent to a group of chromatic-coated mirrors; these reflect light back at different wavelengths
             • When the white light hits these mirrors, the reflected light has wavelengths corresponding to red, green and blue light components
               • These three different coloured light components pass through three LCD screens; these screens show the image to be projected as millions of pixels in a grey scale
                 • When the coloured light passes through the LCD screens, a red, green and blue version of the grey image emerges
                   • These images are then combined using a special prism to produce a full colour image - this final image consists of millions of colours (each shade of grey in the original image produces a different shade in each of the colour images)
                     • Finally, image pass through projector lens onto a screen