From what I'm finding the Arduino can't simply add 1 to a 7seg display based on seeing a random voltage change from an optical interupter.
Psuedocode:
Optical interrupter is set around glass vial with 5V supply
When object passes thru the vial interrupter voltage drops to 3.5V
Interrupter is activated as a small particle pass the IR therefore the interrupt only last .1mS
Vout from interrupter goes to Arduino and reads voltage change
processor counts +1 for each voltage drop (drop >1V)
count shown on 2 seperate 7segment displays
after idle for 3 minutes the count should return to 0