if you want something available anywhere, you could use a motion detector for a driveway light. available at any home improvement store. if size matters, i know they make motion detectors much, much smaller. we used them to sense basketballs going thru a hoop when i worked at Vera-tech. but that was 20 years ago, and i honestly don't recall the mfgr. or model. i can tell you it was about the size of a car ignition key, and it was adjustable for size, distance, and speed.