How to make a timing gate

Hi, I want to build timing gate with Arduino for race. There will be a gate (width maximum 1.5m), somebody will run through, cross the light beam and this will tell Arduino to stop the time. Because there is already an option to toggle time using button, it will be great if there will be logic 1 when crossing the beam (to make it work on same pin with button - attachInterrupt(0, toggleStopwatch, RISING); ), but it propably shouldn't be problem to reverse it.

I tried to make it using infrared beam detector, these you use to detect if there is a car at automatic garage doors. But they send the beam everywhere, and even if I tried to wrap most ofthe sensor to make the beam tiny, it sometimes didn't switched while I runned through it. And also, it's not so good, when you have wire on the ground between two sticks, because when somebody is running through the gate, he could stumble over the wire (and possibly damage it).

I'd like to use laser beam as it's thin. But I don't know, how to make it. What parts should I buy? I will really apreciate link to eBay or Aliexpress or something like this. And is there any way to put both transmitter and receiver at one side of the gate and just some mirror at the second? I don't want to have wire between the gate, because of reasons described above.

Thanks.

A little red laser-pointer and a photodiode? Yes, a mirror will reflect a beam.

[ and I mean little, nothing dangerously high powered off eBay! ]

MarkT: A little red laser-pointer and a photodiode? Yes, a mirror will reflect a beam.

[ and I mean little, nothing dangerously high powered off eBay! ]

I have these red lasers left from previous projects. Will they work with this photodiode? And as it's just a small place where the beam have to be pointed, is there any lens that I could place in front of the diode, so there will be bigger area where to point the beam to make it easier to set it up?