"When the input sequence is all 0's (your opto circuit places a 0 on the input when active) for 8 samples (80mS total) then you can assume the input is actually active."

And if you have the Number_of_Samples as a variable, it gives you even more flexibilty. I have the procedure as described above inside an interrupt that happens at 5 millisecond intervals. Depending on the source of input - operator key-press, pressure switch, level switch etc the variable is set at anything from 4 to 20. ( Yes some industrial limit switches are really bad in this respect)

Raghu