This solution does not use few parts (I didn’t try to optimise it at all) but it has the interceptor at the bottom, and it uses only 5 balls : each ball reverses a bit, the last ball reverses the middle bit and then is intercepted. Hope you like it !
I love it! 
It’s good to see a solution that prioritizes minimizing the number of balls used, because that’s basically the same as minimizing computation time. Also, I spent a lot of time tinkering with gear bits on the bottom row, but I feel that you have unlocked the true potential of this technique. This was such eureka moment for me that I was inspired to make my own version:
I basically replaced the top logic with something similar to the book solution. This reduces the part count to 32, which is 18 fewer than the original, 3 fewer than the book solution, and only 4 more than @menderbug’s solution. It also allows either the blue or red lever to be used to start, whereas red is required for the original. There is one drawback, though: there are three parts that need to be reset between each run; in the original there were only two (there are also two in the book solution and in Menderbug and @Samuel’s solutions).
Awesome !! This is indeed a huge improvement on my original version. Now it minimizes both computation time and chip size. This is efficient computing !
