| 摘要 |
An adaptive threshold optical reader detects optically encoded information, such as the presence or absence of opaque dots 12 in a translucent card 10. Light is projected from an LED 18 through a dot position to a photodiode 22. In order to provide a reliable threshold for discriminating between the dot absent level and the dot present level (preferably at the geometric mean of these two levels), the photodiode signal produces a reference voltage by way of an operational amplifier 24 for storage on a capacitor 32 when there is no card in a holder 14. A card detector 40 closes switches 36 and 38 when there is no card in the holder. The amplifier then has a low feedback resistance (R26 and R34 in parallel). When a card is present, the switches are open, the feedback resistance is R26 alone and a comparator compares the amplifier output voltage with the reference voltage on C32. The ratio of R26 to the parallel combination of R26 and R34 is chosen to position the threshold correctly. The adaptive threshold compensates for variations with age of the LED characteristics, the photodiode characteristics and the transmissivity of the holder 14. The reference voltage may be stored digitally and a microprocessor can perform digital comparison. Instead of a switched feed back resistor a transistor may be employed which automatically switches between effective resistances in accordance with operational amplifier current. <IMAGE> |
