| 摘要 |
1528172 Detecting edges INTERNATIONAL COMPUTERS Ltd 26 April 1976 [3 June 1975] 23999/75 Heading G1A [Also in Divison G4] In an improvement of an apparatus for detecting the passage of an edge of a hole in a punched card passing between a LED 1 and a photo-transistor 2, the photo-transistors output is initially compared by an operational amplifier 8 (before the card is inserted) with a reference voltage. The reference voltage is derived from a Zener diode 10. The output of the operational amplifier 8 is used to control the voltage across the LED 1 by way of a resistance chain 12 and 13 as shown. A capacitor 11 is in a feedback loop of the operational amplifier 8 and has the effect of maintaining the voltage level across the LED when a card is obstructing the light path. Thus the LED's output is maintained substantially constant. The photo-transistor 2's output is also compared by a difference amplifier 15 with a further reference voltage derived from a resistance chain 16 associated with the Zener diode 10. When the signal level drops to below a threshold voltage determined by the further reference voltage, the difference amplifier output at 17 changes to indicate that an edge is being detected. |
