I agree that all he cares about is that it sees something or not and as long as a pulse(s) manage a trigger that could be latched for a moment using maybe a 555 as a one shot it could likely work. I just didn't know for sure it would do as he wants so I didn't suggest it or attempt a circuit for it.
Ron
As it so often happens, the OP stopped posting, leaving us to debate what he wanted amongst ourselves. :^) I realize the method of just detecting pusles might be crude, but it would allow one to easily repurpose a common household object, as I'm sure we all have a drawer full of them.