so i want to build a hoop that small pellets can fly through.

>>916674
lasers and laser detectors around the ring

make the loop a coil (lots of windings) and connect it to a lc oscillator. then connect that to a freq detector that has a go-high-when-detects-freq output. then drive a counter, buzzer, bell, etc.
the lc osc acts like a tuned osc that changes freq when something enters the coil.

>>916675
Sonic transducer. Or for a metal pellet, hall effect sensor

>>916675
at the minute the budget is pretty low and there will be multiple of these set ups being developed so i think lasers might be out of the picture. unless you know of any cheap, lightweight lasers on the marketplace?

>>916677
someone recommend something like this but would it be completely accurate? if a pellet flew by the outside of the coil, would it not still set it off?