At first i tried to connect the IR sensors directly to the Arduino inputs. Like this it may happen that a disc is detected twice as the signal may oscillate nearby the thresold level. The signal oscillates because the disc may move sideways while dropping. To solve this problem I added a smith trigger for each sensor that have an isteresys of 1/3 of the supply voltage. I choose to use Opamps because i can set the isteresys i want while if i used a cmos IC with 6 inverting Schmitt triggers the isteresys would be fixed at 0.9 Volts.