I finally gave up and built the solution from the book. It does not work any better than mine, which is to say, when the starting position of the two gear bits are the same, they end up different. The goal is for them to match, regardless of starting position. It does work when they start out not matching. There’s a user submitted version of the puzzle solution on this site which has the same issue.
Something’s clearly going wrong here. I’ve just got my board out, set up the book solution to 33 (rather than saving two ramps by moving the bit lower) and tested it. All 4 starting configurations give the correct results - A stays the same, and B ends up matching it.
Things to check:
- that you’re resetting the bit after runs where A points right (if the bit starts pointing left, the machine will just set B to point left)
- that when a marble passes through A from the top, A ends up back as it was, and the marble comes out of the crossover going in the direction A points
- that when a marble comes into B from either wing, B ends up pointing to the side the marble came from. In particular, if B is already pointing in that direction, the marble should also go through the lower gear bit of B and leave B as it started
So long as A and B work as described, and the bit is set properly at the start of each run, the machine should work.
Thanks, I’ll take another look.
I have a further question about this. Does the solution need to be correct (B matching A, and A being in its starting position) after any number of marbles run through? Mine works great with two marbles (BB or BR), but do I need to make sure it also works after, say, six marbles go through? The book doesn’t say, but it does show 8 balls in each hopper.
To run any setup, you should push the start lever once to release a single ball, let go, and then not touch the board at all. The run will end either with a ball being intercepted, or with a failed attempt to release a ball from an empty hopper (or, occasionally, with a mechanical failure - a ball escaping or getting stuck).
8 balls in each hopper is a fairly standard setup, used when the number of balls is intended to be irrelevant, as a way to avoid giving clues (and to save having to check whether there are otherwise viable solutions that require a couple more balls than the intended solution). It also forces the use of an interceptor - otherwise, you could set up a solution with 2 blue and 1 red in the hoppers which concludes by triggering the empty hopper.
It does also allow you to make multiple runs of your solution without needing to refill the hoppers, though you do need to remember to reset any bits between runs.