clockwork wind-up. It would be bulky but maybe you could put a permanent magnet on a sled or wheel that would impart the necessary force on the metal ball as it

clockwork wind-up. It would be bulky but maybe you could put a permanent magnet on a sled or wheel that would impart the necessary force on the metal ball as it approached but then and flipped repelled it. Would be difficult to make work smoothly or at all...