Barcode Reader. Have you ever noticed that you can see a barcode on almost everything—food packaging, books, magazines, letters, household goods, the circuit boards inside your computer. It's a kind of high-tech graffiti. There are different types of barcode readers: a light pencil, a laser scanner that resembles a gun, a CCD scanner. But all of them contain a device that emits red light with a wavelength of 645-690 nm (an LED or a laser), and a receiver (a photodiode or a CCD chip—a charge-coupled semiconductor device). When such a device is moved across the pattern of lines, the receiver receives a reflected light signal, which changes in intensity in accordance with the width and sequence of the lines. The receiver converts the light signal into an electrical signal, the amplitude of which is proportional to the light intensity. The resulting output is an electrical signal in the form of a square wave. This signal is then converted into a digital signal - a sequence of logical "0" and "1" - and sent to a computer or cash register. Light pens are relatively inexpensive and are commonly used in libraries and hospitals. However, using them without skill is not possible: when reading, it is necessary to maintain a certain angle of inclination and a certain speed of movement. Laser scanners are more reliable and are used in stores, but they are also more expensive. The scanner can be held at a distance of 10 to 40 cm from the product, and special models can read a barcode at a distance of up to 10 m. CCD scanners - with LEDs, - They are cheaper than laser scanners, but their reading range is limited to 20 cm. They are typically used in daylight or with low-contrast barcodes.

In a laser scanner (center), an oscillating mirror or rotating prism sends a beam of visible red light to the barcode. A photodiode reads the reflected beam and produces an electrical output signal corresponding to the barcode pattern. Inside a LIGHT PENCIL (bottom) are LEDs and photodiodes: to read a barcode, you just pass the tip of the pencil over it. In CCD scanners (top), there is not just one LED, but a whole row, and the reading is not done by a single photodiode, but also a whole row of CCD detectors.

CODES. Of the twenty barcodes developed at various times, the twelve-digit UPC (Universal Product Code) is the most popular in retail. Code 39 allows for the transmission of ASCII text, numbers, and letters. US Postal Service uses PostNet Code to quickly read envelope addresses using sorting machines. Parcel Service's MaxiCode allows for the transmission of even more information and its correct reading even if 25% of the content is lost.

FROM BARS TO PULSES. The size of the reading beam is smaller than the width of the narrowest bar and the space between bars, but still not so small as to detect accidentally adhering dust particles or hairs. The typical bar thickness is 0.3 mm, i.e., it occupies exactly four adjacent dots of an image printed at a resolution of 300 dpi. The photodiode receives the reflected light signal and converts it into an analog electrical signal, and the decoder, in turn, reads it as Morse code dots and dashes and converts it into a digital signal.

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