Circuitry for bar code scanners

Abstract

In order to decode from electrical signals derived by a photodetector which responds to the light returned from the relatively less reflective (black) bars of the code and relatively more reflective (white) bars (also known as spaces), these signals must be digitized into a digital data signal where the black and white bars are represented by opposite logic levels, (for example logic high voltage levels representing the black bars, and logic low voltage levels representing the white bars). The high logic levels are pulses which are separated in time by the low logic levels. A cascade of amplifiers is used to amplify the bar code signal from the photo detector. An amplifier having a logarithmic signal compression characteristic is included in the cascade, preferably after an amplifier which serves to differentiate the signal. The amplified signal is applied to a digitizer including a comparator which is sensitive only to the leading edge. Additional features are networks which enable the maximum gain of amplifiers in the cascade to be varied in accordance with a digital code, amplifier circuitry which provides linear amplification until the signal level reaches a threshold at which time amplification follows the logarithmic compression characteristic, hysteresis which prevents the comparator from being reset until a transition between a signal corresponding to black and white bars is detected, which hysteresis is a fixed voltage level, and circuits for setting the comparator to a state corresponding to a white bar (logic low).