Automatic threshold control circuit for optical card readers and sorters

Claim 2

A punched card optical reading device comprising coupled light source means and photodetecting means for converting optical stimuli into electrical high and low voltage signals, a threshold circuit comparator coupled to said photodetecting means and responsive to said voltage signals and a threshold voltage to convert said voltage signals into electrical binary level signals, and automatic threshold control means for compensating for changes in the performance of said reading device, said threshold control means comprising storage means for storing a reference threshold voltage and circuit means to enter into said storage means said reference threshold voltage derived from and proportional to the voltage signal produced by said photodetecting means during the interval between passage of each card and the next through said reading device, and means to apply said reference threshold voltage to said threshold circuit comparator.

Claim 3

The punched card reader of claim 1, wherein said automatic threshold control means comprises a first amplifier stage having input and output leads, a voltage divider for providing an output voltage, and an electronic switch, said input lead being coupled to said phOtodetecting means, said output lead being connected to said voltage divider, the output voltage of said voltage divider being coupled to said storage means of said automatic threshold control means through said electronic switch, said switch being closed by an external timing signal during the time interval between the passage of two consecutive cards.

Claim 4

The punched card reader of claim 2, wherein said first amplifier stage is a linear amplifier and said storage means is a capacitor having a capacitance sufficient to maintain a substantially constant threshold voltage during the time for reading a card.

Claim 5

The card reader of claim 3, wherein said linear amplifier consists of a differential amplifier having direct and inverting input leads, the signals delivered by said photodetecting means are applied to said direct input lead, and a negative feedback network is connected to said differential amplifier to provide negative feedback for increasing the linearity and stability of said amplifier.

Claim 6

The card reader of claim 3, wherein said electronic switch consists of a field effect transistor whose source electrode is connected to said voltage divider, whose drain electrode is connected to said storage means, and whose gate electrode is connected to the output lead of an external timing signal generator for controlling said field effect transistor to conduct during the time interval between the passage of two consecutive cards.

Claim 7

The optical card reader of claim 3, wherein said threshold circuit comparator comprises a differential amplifier, and a positive feedback means is coupled to said differential amplifier to provide positive feedback for enhancing the amplitude discriminating characteristics for the voltage signals applied to said threshold circuit comparator.

Claim 8

In a punched card reader wherein the presence of a hole is determined by the quantity of light transmitted to a photosensitive detector, and wherein a comparator compares an input signal representing the quantity of light received by said detector with a threshold signal and delivers an output signal having a first value when the level of said input signal is greater than the level of said threshold signal and having a second value when the level of said input signal is less than the level of said threshold signal, an improved threshold signal generating means comprising: threshold signal storage means for storing a signal level for the time required for a punched card to pass said photosensitive detector, and means for inserting into said storage means a new signal level in the interval between the passage of each two successive cards past said photosensitive detector, said new signal level being a predetermined proportion of the level of said input signal during said interval to provide that said new signal level is intermediate to the level of said input signal that represents the presence of a hole in a punched card and the level of said input signal that represents the presence of an unpunched portion of a card.

Claim 9

In a punched card reader wherein the presence of a hole is determined by the quantity of light transmitted to a photosensitive detector, and wherein a comparator compares an input signal representing the quantity of light received by said detector with a threshold signal and delivers an output signal having a first value when the level of said input signal is greater than the level of said threshold signal and having a second value when the level of said input signal is less than the level of said threshold signal, an improved threshold signal generating means comprising: threshold signal storage means for storing a signal level for the time required for a punched card to pass said photosensitive detector, and means for inserting into said storage means a new signal level in the interval between the passage of each two successive cards past said photosensitive detector, said new signal level being a predetermined proportion of the level of said input signal during said interval to provide that said new signal level is intermediate to the level of said input signal that represents the presence of a hole in a punched card and the level of said input signal that represents the presence of an unpunched portion of a card.