Was anyone else hoping it would detect the ball by simply completing a circuit (since the rails are metal and so is the ball)? Then it could give a magnetic tug after an appropriate time. Maybe this would be flaky because of the poor contact between rails and a rolling ball, but an inductive proximity sensor feels like overkill.
It looks like the rails are already a complete circuit - I think they're a single piece of wire that is bent at the end to make a loop, and there are a couple lateral braces soldered on to keep them properly spaced.
You'd have to split the rails into two pieces and replace the lateral braces with something nonconductive (plastic) which wouldn't look nearly as nice; and the device would be a lot more prone to going out of calibration if the spacing drifted.
Its probably possible but there's a lot of downsides to save $2 on a sensor, some of which will be eaten up elsewhere as it will add some additional parts.
Edit: It would also somewhat spoil the magic, as that's the first thing most of us would think of (I certainly did).
I was thinking a pair of tiny contacts could touch the ball and complete the circuit, but you're right, this isn't an engineering opportunity, but a performance issue. You shouldn't expose electricical components in a perpetual motion simulator.
