Circuit for establishing a reference voltage in bar code readers

Claim 1

A circuit for establishing a reference signal useful in a bar code character reader, said reference signal being established from a reader scan signal having positive and negative peaks and an instantaneous amplitude which is proportional to the amount of light reflected from bar code characters scanned by said reader, said circuit comprising: first circuit means responsive to said reader scan signal for generating a first varying amplitude signal, including means for increasing said first varying amplitude signal proportionally in amplitude with said reader scan signal as long as said reader scan signal rises positively with respect to and is more positive in amplitude than said first varying amplitude signal, and further including means for decreasing said first varying amplitude signal at a first predetermined rate as long as said reader scan signal is less positive in amplitude than said first varying amplitude signal, said first circuit means further including first storing means for storing said first varying amplitude signal as said first varying amplitude signal increases positively in amplitude, thereby providing a first stored voltage, means for discharging said first storing means at said first predetermined rate as long as said reader scan signal is less positive in amplitude than said first varying amplitude signal, said first circuit means further including a first amplifier and a first diode having an anode and a cathode, said first amplifier having in operation an output applied to the anode of said first diode, said cathode being connected to said first storing means, said amplifier operative to produce at said cathode of said diode a first diode output voltage, thereby charging said first storing means, said first diode operative to block discharge of said first storing means once said first diode output voltage is less positive in amplitude than said first stored voltage; second circuit means responsive to said reader scan signal for generating a second varying amplitude signal, including means for increasing said second varying amplitude signal in amplitude at a second predetermined rate as long as said reader scan signal is more positive in amplitude than said second varying amplitude signal, and further includes means for decreasing said second varying amplitude signal to conform to said reader scan signal as long as said reader scan signal is less positive in amplitude than said varying amplitude signal, said second circuit means further including second storing means for storing said second varying amplitude signal as it increases positively in amplitude, thereby providing a second stored voltage, said first and second predetermined rates being sufficiently slow such that relatively little change occurs in the amplitudes of said first and second stored voltages between successive positive peaks and successive negative peaks of said reader scan signal, said second circuit means further including means for discharging said second storing means to substantially conform said second varying amplitude signal to said reader scan signal as long as said reader scan signal is less positive in amplitude than said second varying amplitude signal, said second circuit means further including a second amplifier and a second diode having an anode and cathode, said second amplifier having in operation an output applied to the cathode of said second diode, the anode of said second diode being connected to said second storing means, said second diode operative to block said second amplifier output from said second storing means and permit said second storing means to charge from a fixed voltage source, said second diode further operative to permit said second storing means to discharge through said second diode and said second amplifier when said second stored voltage is more positive in amplitude than said second amplifier output; means coupled to said first and second circuit means for establishing a third varying amplitude signal having a predetermined relationship with said first and second varying amplitude signals, said third varying amplitude signal being said reference signal, said third circuit means including a resistance means having two ends and a midpoint connection therebetween, means applying said first varying amplitude signal to one end of said resistance means and means applying said second varying amplitude signal to the other end of said resistance means, said reference signal being provided at said midpoint connection of said resistance means, said reference signal being substantially instantaneous intermediate in amplitude of said first and second varying amplitude signals; and a comparator, responsive to said reference signal and said reader scan signal, said comparator providing a first output signal when said reference signal is more positive in amplitude than said reader scan signal, and a second output signal when said reference signal is less positive in amplitude than said reader scan signal.