I am in need of help to complete a home made laser beam photo cell detector timer, which I will use to time my kids horses in an arena for barrel racing. I have the major components but need to figure out how to change (very simply) from the photo cell being always on (while sending no signal) to sending a simple ground to the timer when the beam is broken. I have thought of a hold open switch, that when the beam is broken closes, but the response time has to be in the nano second range (my timer measures out to the .000) so I was wondering if anyone knew of a simple electronic chip that would do the same thing. (Commercial timers of this type retail for 800.00 bucks) any help is appreciated.