External hardware devices (topic 7 long explanations)

Barcode reader (how it works)

A barcode is made of black and white lines of different widths that represent digit values. A light source is shone onto the barcode as the reader is moved across it. Black and white areas reflect different amounts of light, with white reflecting more and black reflecting less. A light sensor detects the reflected light. The reflected light is converted into an electrical signal, then into binary, and sent to the computer for processing. Barcodes often include a check digit to help detect errors.

Digital camera (how it works)

Light enters through the lens and is focused onto an array of light sensors. Each sensor generates an electrical current representing a pixel. An analogue to digital converter (ADC) converts measurements of light intensity into binary data. Colour filters are used to generate separate data for red, green and blue colour components. The pixels are recorded as an array.

Laser printer (how it works)

The print drum is coated in a static charge. The printer generates a bitmap of the page. A laser beam is shone at the print drum via rotating mirrors. By modulating the laser beam, the charge on the drum is reversed where the image should be dark. Toner is given a static charge and is picked up by the drum. The toner is transferred to paper and fused by heated rollers.

RFID (how it works)

An RFID tag is a passive device containing circuitry and an antenna. Memory on the tag stores data. The RFID reader, which is an active device, sends a signal to the tag which activates it. The tag sends data by radio to the RFID reader. The RFID reader converts the radio signal back into binary data.

Hard disk (magnetic) (how it works)

The disk is coated in a magnetisable material. The disk is divided into tracks, and the tracks are split into sectors. The disk spins at a high speed. The read/write head moves to the required track. When reading, the head senses magnetic fields and converts them into 0s and 1s. When writing, the head uses a current through a coil to magnetise areas in different directions to represent 0 or 1.

Optical disk (how it works)

Data is stored on a single spiral track on the surface of the disk as pits and lands. The disk spins at a constant linear velocity. When reading, a laser is shone onto a tiny spot on the track. Lands reflect the laser strongly but pits reflect less light. A light sensor measures the reflected light. A change between pit and land is interpreted as a 1, and no change is interpreted as a 0, or vice versa.

Solid-state disk SSD (how it works)

Data is stored electronically in NAND flash memory using floating gate transistors. Presence of trapped electrons represents a binary value. Data is organised into pages and blocks. A controller manages the organisation of data and the complexities of writing. Pages cannot simply be overwritten directly, so blocks must be erased before being rewritten. Because there are no moving parts, access is faster and latency is lower than with a magnetic disk.