sry been gone a lil while. But anyway it would be detecting Sphere objects. They would be paintballs. They are 68 calliber and filled with a thick milky liquid. they are like plasticy (hard) on the shell. Just curious how to make one. It needs to be able to run off of 1 or 2 9v batteries. And connect to a solenoid. And if it dont sense a ball the solenioid wont shoot. ty