Bar Code Reader

Registration and dispensing of goods with immediate information of the properties by bar code reading, like the price for super markets, destination for postal purposes etc. has become an indispensable tool for industry and warehouses.
This set-up, therefore, is concerned with the method of operation of regularly used bar code scanners and combines the use of laser light with optical and mechanical assemblies to read the bar code under nearly all directions of observation which is written on an object. The bar code itself is an arrangement of vertical dark lines printed on a light reflecting or scattering background where the dark lines absorbe the light. The thickness of the individual line represents a logical low or high. When a laser beam is scanned across such a structure, a series of light / dark events will occur due to the alternating absorption and scattering of the laser light. A dark event is detected when the beam hits a dark, absorbing line. The duration of a dark event corresponds to the value of the bit; a long one indicates the presence of a logical high and vice versa. In principle it is not a must to use a laser for this process. Light emitting diodes (LED) are also used to read bar codes as known from hand held scanner. However automated reading especially from curved surfaces requires a certain distance from the object and can only reliably be performed using a scanning laser beam. Once the scanned information is recorded, it has to be decoded, stored and displayed. For certainty reasons each code is generated in such a way that the reading device is able to validate the correct recognition. All this and a lot of more background information is given in the comprehensive manual. Within this workshop a regular bar code reader as known from supermarkets is prepared in such a way that all components, like rotating polygon mirror, detection and other important subassemblies can be visually and electronically accessed. The connection to a PC provided with special software allows the display of the recorded data as well as the printing of own bar codes.

Principle of operation

The heart of the scanner is a polygon wheel (PG) with mirrors at the vertical sides. The wheel is turned by a motor at a constant speed. The beam of the laser (LS) passes a beam splitting plate and hits one of the mirrors of the polygon wheel. Depending on the momentary position of the mirror the beam is deflected. Due to the spinning wheel the laser beam is fanned out and its projection onto a surface forms a line. The scanning range (SR) is defined by the design of the polygon. Once the laser hits an object, light is scattered back which is imaged by means of the beam splitter plate (BSP) and the lens (L1) onto the photo detector. The intensity of the back scattered light de-pends on the surface properties of the object. In case the object is a printed bar code the detected intensity is low when the beam hits a bar and is high when it hits the space between the bars. The sequence of the dark / light events is passed to an analogue digital converter and subsequently to a logic circuitry where the validity of the recorded binary word is checked.