7.4 external hardware devices

input devices

devices that send data to a computer allowing it to react to user interaction or environmental changes

output devices

devices that convert data from a computer into a perceptible form such as light or sound

barcodes

printed diagrams of light and dark portions that code for information

the light and dark portions corresponding to binary 1s and 0s respectively

there are 1D and 2D barcodes(qr codes)

2D barcodes can code more information than 1D barcodes but they require more processing to extract the information so more complex readers are needed

barcode readers

barcode readers consist of a laser light source, a lens, photodiodes and a mirror

the mirror directs light from the laser onto the barcode and the light portions of the barcode reflect more light while dark sections reflect less

the light reflected by the barcode passes through the lens and onto the photodiode which turns light into electrical charge(more light=more charge), which can then be measured and processed to form a digital signal corresponding to the content coded onto the barcode

barcode error detection and prevention methods

barcode readers use methods such as parity bits and check digits to detect whether a barcode has been read erroneously, and will continue to scan the barcode until it is read successfully

digital camera

digital cameras consist of a lens that focuses light onto a sensor with a shutter that regulates the path of the light between the lens and the sensor

a CMOS(complementary metal oxide semiconductor) or CCD(charged coupled device) sensor converts incoming light into electrical charge, and each cell of the sensor represents one pixel of the image generated by the camera

the charge in each of the cells is measured and converted to a digital value corresponding to the colour of the pixel and the charges are processed into a digital image

in colour cameras each cell has a sensor for red, green and blue light, and images are created for the intensity of each colour. the images are then combined to form a full colour image

laser printer

laser printers consist of a laser light source, a mirror, a drum, a toner roller and fusers

the drum is positively charged before the laser is directed at its surface by the mirror

areas the laser hits are discharged leaving behind an impression in electrical charge on the drum, and a toner roller dispenses negatively charged toner onto the drum which is attracted to the positively charged portions of the drum

the drum applies the toner to the paper, then the paper is heated by fusers, fixing the toner to the paper

colour printers use CMYK inks (inc diagram)

RFID

aka radio frequency identification- a method of transferring information wirelessly between a tag and a reader

an RFID tag contains a chip with a small amount of memory, attached to a coil of wire which acts as an antenna

most RFID tags are passive meaning they are activated by a reader when it is in range as a charge is induced in the coil. however active tags contain a small power supply and can be used further away from readers

RFID readers emit radio waves which induce a charge in the antenna that power the chip. the antenna then emits its own radio wave containing the information held on the chip which the reader detects and decodes, and the decoded message can be relayed to a computer

secondary storage

persistent, usually high-capacity, storage for data that does not need to be accessed as frequently as data in primary storage. includes HDDs, SSDs and optical disks

hard disk drives

disks consisting of circular platters made of magnetic material

each plate has a read-write head hovering over it on an actuating arm which allows it to move over the surface of the plate and read all of the sections

data is written in cocentric tracks divided into sectors

the plate rotates thousands of times per minute and the read-write head changes the magnetic polarity of magnetic dots on the disk surface to change the binary value they represent and measures the polarity to read the binary values

dots with the north pole facing up represent 1s and dots with the south pole facing up represent 0s

characteristics of hard disk drives

-cheap

-fast read/write speeds but slower than SSDs

-high storage capacity

-breakable due to moving parts

-high power consumption

-large

-relatively noisy

solid state drives

disks consisting of non-volatile NAND flash memory cells and a controller that manages the structure of data

the memory cells are formed of floating gate transistors which store informatuion by trapping electrical charge. the presence or absence of charge indicate a binary 1 and 0 respectively

data is stored in pages which are combined to form blocks. SSDs cannot overwrite data so pages are erased and rewritten with the changes applied

characteristics of solid state drives

-expensive

-very fast read/write speeds

-high storage capacity but less so than HDDs

-durable as there are no moving parts

-very low energy consumption

-can be made very small

-operate almost silently

optical disks

disks that store data on a track that spirals from the center to the edge of the disk, in a pattern of pits burned into the surface by a high-power laser, and lands, corresponding to 0s and 1s respectively (excluding rewriteable optical disks)

optical disks are read by a low-power laser beam which passes over the disk, reading along the track. the laser beam reflects back onto a photodiode when not passing over a pit, while pits scatter the light

this variation in recieved light is used to create an electrical signal corresponding to the coded for binary value

rewriteable optical disks

0s and 1s are stored as the absence or presence of opaque dye on the surface of the disk. the disk is covered in photosensitive dye which can change from opaque to transparent under the high-power laser

the laser beam reflects back onto a photodiode when not passing over a section with opaque dye, while dots of opaque dye absorb the light. this variation in recieved light is used to create an electrical signal corresponding to the coded for binary value

types of optical disk

-ROM(read only)

-R(recordable- writeable to once)

-RW(rewriteable)

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-CD(compact disk)

cheapest, lowest storage capacity(~700MB)

-DVD(digital versatile disc)

cheap, low storage capacity(4.7GB per layer)

-Blu-Ray

expensive and high storage capacity(relative to other disks- 25GB per layer)

characteristics of optical disks

-very cheap

-very low storage capacity

-low read/write speeds

-easily portable, though easy to damage

-high power consumption

-large, but conveniently flat shaped

-noisy