Foul Ball Alert
I thought of this after my brother-in-law nailed my mother-in-law with a foul ball during a church league softball game. He fouled it straight up and over the fence along the first base line. The stands sit just on the other side. She didn't see it in time and didn't hear our shouts of warning. This kind of thing happens two or three times during a game; its common place. Enter The Foul Ball Alert. It is a softball fitted with RFID. It's the spiraled looking device on the right. You'd recognize it as the gadget placed in library books to prevent naughty Brainiacs from stealing Pride and Prejudice and other books.
The RFID tag is small enough and thin enough to fit under the covering of a softball or baseball. Antennas can be placed around the top of the fences of the ball park and an alarm is sounded when a foul ball is popped up in their vicinity. Think of it as hitting that copy of Pride and Prejudice through those annoying stalls at the entrance to your public library.