How long will it take for this bullet to pass through this square frame?How long could the bullet obstruct the IR between the emitter and photodiode?In other words, what's the maximum pulse width that could result?[Very short time, very short pulse width - probably too short. Could an Arduino interrupt even notice it?]
So, your objective is simply to annunciate1 a projectile's presence in the "window", ideally from as close to its breaking the plane as possible?To me, that would be an Arduino misapplication. I would look at using:1) an RS Latch (a/k/a SR Latch)http://www.jameco.com/Jameco/Products/ProdDS/13004.pdfpossibly conditioned with a2) a "one-shot" multivibrator [trigger]http://www.jameco.com/Jameco/Products/ProdDS/252259TI.pdfStill, you need to supply more information.The projectile has some length and travels at some speed, within tolerances, so you should (need to) come up with a figure for how long the detection event will last, even if ideally so.Can the IRED emitter/detector operate at that speed?1 - trip an LED, trigger a flash, etc.