Bar code readers

Bar code readers are now in common use and can be seen in shops and libraries. A bar code is composed of a series of parallel adjacent vertical lines or bars of various thicknesses. The complete information about the product can be retrieved from these vertical bars. The data coded in the form of small bars are called as bar codes. The electronic device which is used to read the coded information is called bar code reader. A bar code reader is a hand held device which scans the bar code image printed on the product and converts it into ASCII code. A computer can be used to convert ASCII code. Laser beam scanning technology is used in bar code readers. The dark lines reflect less light compared to white spaces. A scanner converts the amount of light reflected into electrical pulses which is decoded into data by a decoder. This data from decoder can be sent to computer by a cable. The advantages of these devices are economical, portable and easy to use. The main limitation is difficulty in scanning images.

Null balance recorder

A recorder is an electronic recording device which is used to record variables such as force, voltage, current, temperature, humidity, flow etc. Possible components of a recorder are graphic recording devices, computer printers, tape recorders, and cathode ray tubes. The two types of recorders are analog recorders and digital recorders. Analog recorders can be further subdivided into three. They are graphic recorders, oscillographic recorders and magnetic tape recorders. In a null balance recorder, a servomotor provides self-balancing action. Here a position type servomechanism which is applied to a variety of potential and bridge type circuits calibrated for different measurements is used. Signal which is to be measured is treated as input signal. Input signal is then compared with a reference voltage. A feedback system provides the reference voltage which is well calibrated with respect to a mechanical position on an output display scale. A servo amplifier compares the input signal and the reference voltage and then actuates a motor which positions an indicator on the output display scale for a zero error (null voltage condition) between the input signal and reference voltage. Based on the error signal producing source, there are three types of null balance recorders. They are potentiometer type, bridge type and linear variable differential transformer (LVDT) type.