Input and Output Devices
Touch screens:
- Work off heat signals from your fingers.
- Very simplistic and easy to use, which makes them ideal to use for fast food self service, or for an employee entering an order at a pub.
Mouse:
- Used to have a silicon ball at the bottom to tell the mouse which direction the mouse was moving. This led to dirt buildup over time.
- We now have optical mice which work off infrared. A small light emitting diode (LED) shines light onto the desk, which is reflected back to a photocell next to the LED. This has a lens, which magnifies the light signal for better accuracy.
- The reflected light signal is analysed by a processor inside the mouse, which can work out that your mouse is moving and where it has moved to.
Microphone:
- When you speak or play music into a microphone, the microphone takes the sound waves and converts them into a voltage.
- As the sound waves vary, so the voltage varies. The microphone is connected to a computer's sound card. The sound card samples the microphone's voltage at intervals.
- How many times it does this in a second is known as the 'sample rate'. The time between samples is the 'sample interval'. The bigger the gap between taking a sample, (in other words the larger the sample interval), the lower the quality of the recording, although the benefit is a smaller file size.
- Each time the voltage is sampled, it is converted into a binary number by the sound card's Analogue to Digital Converter (ADC) and stored. If you store all of the digital samples, you end up with a sound file. This might be a song or a recording of your voice, for example.
Scanners:
- Typically, the image is placed on a flat screen or 'bed'.
- A cover is placed over the image.
- The image is divided up into sections or 'pixels' by the software. The user can tell the software what resolution to use (how many pixels per square centimetre to split the picture up into). The higher the resolution, the better the detail of the image but the bigger the file. Often, low resolution, smaller files will be perfectly adequate for most uses.
- A light is passed from one end of the flat bed to the other, so that it passes over the image and over each pixel.
- When the light hits each pixel, it gets reflected back. The intensity of the reflection depends on the colour at that pixel. Each pixel's information is stored.
- The information about all of the pixels is used by the software to reconstruct a bit map image of the whole picture.
- Because bit maps are large files, they are often compressed. This can be done by telling the software to save the image as a different file type that uses compression, such as GIF files or JPG files.
- Software tools either within the scanning software or within a drawing package allow the user to manipulate an image in various ways. These typically include allowing the user to 'crop' an image (select just a portion of an image), allowing the user to improve the detail of the image, allowing the adjustment of colours and allowing the user to add special effects such as making a photo image look antique.
Digital Cameras:
- These cameras do not store images on film. They store images digitally in memory. The images are then transferred to the computer. There are many points that could be made about digital cameras.
- The price of digital cameras has been steadily falling over recent years. (because of smartphone cameras)
- The amount of memory is a very important consideration with these cameras, as is the ability of a camera to add memory. This is because storing images is memory-intensive.
- Cameras often allow the user to select between high resolution and low resolution modes. If you use a high resolution mode, you will be able to take fewer pictures than low resolution mode.
- You can immediately view photos and re-take or overwrite them if they are not what you want.
- You can often add extra information easily, such as the date or information about the photo.
- Many cameras allow you to add special effects as you take the picture.
- Images can easily be combined into digital photo albums and distributed or emailed to friends.
- Once the picture has been taken and transferred to the computer, it can be opened in a drawing package and manipulated. Pictures can be cropped, colours changed and parts of the photo 'touched up', for example.
Dot Matrix Printer:
- These are relatively slow and noisy and the quality of the hard copy is relatively poor compared to ink-jets and laser printers. They were very common a few years back in the early days of computing.
- Their uses are far more limited now but they do have one particular advantage. The hard copy is made by pins striking paper, which is why they are also known as 'impact printers'.
- That means that identical copies can be made of a printout by using carbon paper between sheets of paper (as opposed to printing out two copies of something - there is a subtle difference).
- This system is used by credit card companies to produce actual copies of receipts when a customer makes a purchase.
Ink Jet Printers:
- This type of printer 'sprays' ink onto paper. Different ink-jet printers work in different ways.
- Canon printers, for example, heat up the ink so that it explodes in bubbles onto the paper. The characters are still made up of dots, but the dots are so small (of the order of 50 microns diameter - a human hair is about the same diameter) that you can't see them and tell them apart.
- The dots are accurately positioned together to create characters and images but of course, you can use the printer software for an ink-jet printer to order your printer to use more dots per inch (or DPI).
Laser Printers:
- These types of printers are in widespread use. A laser beam scans across a drum inside the printer to 'paint a pattern of static electricity corresponding to whatever it is you are printing out. The static electricity attracts powdered ink called toner onto the page and then a special unit bonds the toner onto the paper so it is permanently fixed.
Plotter:
- Plotters are widely used in some industries.
- Plotters are used to plot very large drawings such as those needed by engineers and designers whereas standard printers commonly print up to A4 (and sometimes A3).
- Plotters produce very high quality, very accurate, colour drawings.
- Plotters are relatively expensive compared to printers.
Visual Display Units (VDUs):
- Monitors are ideal for displaying data and information to users. They come in a range of sizes. Larger ones such as 21-inch screens, for example, would be ideal for engineers using Computer Aided Design software applications.
- 15-inch screens are perfectly acceptable for users using a range of generic applications. CRT (Cathode Ray Tube) monitors (similar to televisions) do take up a lot of space on a desk. Flat panel liquid crystal display screens, often referred to as TFT screens, save a lot of space by comparison. They are not quite as sharp as CRT screens, however, and are more expensive.
- TFT screens produce less radiation than CRT monitors. Excessive exposure to radiation is seen as a potential risk to computer users. They also use about half the power a CRT screen uses.
- If you multiply up the savings in power use in an organisation with thousands of computers, the cost-savings do become significant.
Speakers:
- Some applications such as burglar alarms, factory warning systems and monitoring equipment make use of audio output.
- Some applications also require sound, such as video-conferencing, using your computer to make phone calls, listening to DVDs or CDs and playing games.
Headphones:
- There are situations where a user wants to listen to sound in a public place but doesn't want to disturb others.
- For example, a user in a library might want to listen to CDs on a computer, or a telesales operator might need to concentrate on what a customer is saying.
- They work in the same way as speakers.
3D Printers:
- There are lots of short but very interesting videos you can watch to see the possibilities of this technology, which involves designing an object using some software and then sending it to a 3D printer to print out the actual object.
- It does this by printing out very thin layers of the object one at a time, and binding each layer with the previous one.
Optical Mark Recognition:
- Data sheets are prepared and people put marks in set places to indicate a choice.
- They are used, for example, in multiple-choice tests because the answers can be scanned in and a pupil's mark calculated by the computer - less work for the teacher. They can be used to capture answers to questions on a questionnaire, or to select numbers on a lottery ticket.
- Some applications are not suitable for OMR. For example, you wouldn't collect names using OMR because you would have to provide 26 places for a mark to be made for each letter in the name! They are really only suitable when a small number of choices are available.
Optical Character Recognition:
- A page of writing or data is first scanned on a scanner/camera. A very bright light is shone on the text and the white and dark space caused by the letters on the page reflect different amounts of light back. This is measured and used to produce a bit map picture of the writing.
- It is not yet a word processing document. Some OCR software is then run on the picture. This scans the picture looking for patterns that represent characters on a keyboard. As it finds these patterns, it transfers them to a text file.
- When the whole picture has been scanned, a text file will have been produced that can then be opened in a word processor or other applications.
Barcodes:
- Bar codes are made up of black and white striped lines. The lines represent data in coded form. The data held in a bar code can be retrieved by using a laser scanner. The data obtained can then be used to look up further information held on computer.
- Bar codes can be attached to libraries to speed up taking out and returning books. Membership systems that require members’ details to be retrieved could employ barcodes. Supermarkets could use them for stock control systems, to speed up the checkout process and to produce itemised receipts.
- Bar code systems have built in validation techniques that greatly reduce errors. The data scanned can be integrated into management information systems so that, for example, managers can tell which books are never taken out of a library and should be removed, which members never attend an event or which products in a supermarket sell best on Sundays. In supermarkets, bar codes are an integral part of the stock control system.
- One problem with any system that is completely reliant on computerised systems, however, is what to do when the system breaks down.
Magnetic Stripe Cards and Smart Cards:
- Data can be entered into computer systems by using cards that have a magnetic stripe on them. The magnetic stripe holds coded information. This can be retrieved by 'swiping' the card through a magnetic card reader. The information can then be used directly, or used to retrieve more information from a central computer.
- This type of data input is typically used for credit cards, debit cards, loyalty cards, membership cards and security access cards. They are quick to use, can be used many times, are cheap to produce and the important information held on them cannot be read unless you have the right equipment.
- Magnetic stripe cards can be read but not written to. The data is placed on the card when it is made. For this reason, smart cards have become more widely used. These are cards that have an electronic chip on them. This can be used to record each time a transaction occurs, for example. In other words, smart cards can be written to as well as read from!
Magnetic Character Recognition:
- When banks produce cheque books, they print on the bottom of the cheque the sort code of the bank, the account number and the cheque number.
- These numbers are printed in magnetic ink because the cheques can then be read automatically once a cheque has been written.
- It doesn't matter if the cheque gets creased or a little dirty because the data on them can still be read by the special magnetic ink readers.
Data Logging:
- A classic example of an automatic data entry method is data logging.
- Consider an experiment where a pupil wants to know how the temperature of a computer room varies over 24 hours. They would need to select a transducer to read the temperature.
- In this case, they select a thermistor. This is an electronic device with a special property. The output voltage from the thermistor constantly varies, depending upon the temperature.
- It is known as an ‘analogue transducer’ because there are an infinite number of outputs from it.
Analogue to Digital Converter:
- We have said that an interface is often required in control and data logging applications. You certainly need one if you want to attach any analogue transducers to a computer. An interface may perform other functions:
- An interface may convert voltage signals from analogue into digital (ADC) or digital into analogue (DAC).
- An interface may be used like a switch, so that e.g. a small dc voltage from the computer can be used to switch on and off a large motor that uses 240Vac.
- An interface may provide compatible physical connections for a computer and the devices that need to be connected to it. Devices may use plugs and sockets that are physically different to a computer’s and so they cannot be directly attached to it. They have to go via an interface.
Sample Times:
- The pupil wants to know how the temperature varies in a 24-hour period. A suitable output for this would be a graph.
- If the pupil set up the computer to take a reading every 10 minutes, then the computer would take 6 readings an hour, or 144 readings in 24 hours. This sample would be more than sufficient to produce a good graph.
- Of course, the pupil could have set up the computer to take hundreds of readings every second! This would have been unnecessary in this case because it wouldn't have told the pupil any extra information.
Data Logging:
- Suppose we have a data logging system set up on the top of a mountain, to record the temperature.
- The data logger might take a reading every hour and store the value. How can the recorded values be sent back without having to send a person up to collect the readings? It can be achieved using satellite technology.
- A transmitter attached to the data logger sends a microwave signal to one of a small number of geostationary satellites that cover the planet. The message is amplified and retransmitted back to Earth.
- The signal is possibly bounced back up to another satellite and down again, so that the data is passed around the planet. The data is then collected and analysed by computer and possibly converted into graphs so trends can be examined.
- We can log data in any place where people couldn’t realistically go for long periods, perhaps because they are dangerous, such as in a nuclear reactor, in space or at the bottom of an ocean. Readings can potentially be taken 24/7 and are likely to be far more accurate than readings people take.