There is one other consideration when replacing the switch with the opto-sensor. Might the obstruction move very slowly? If it moves slowly enough it won't trigger the '555.

No, I dont think it will be moving slow... Any way, I'll model it and tes with slow obsticle on my breadboard tomorrow.

 

Sorry for the delay, I was on a wedding of my friend. I ended up just using pic microcontroller. Tested it with slow moving and fast moving obstruction and it works perfectly. Current setup uses one IR diod-phototransistor pair to fire LED flash, and second pair to allow to fire another flash from the first pair. Thank you all very much for all your help!!!