My original solution to 38 was a mess; it worked, but it took 33 parts, including two sets of gear bits and five crossovers.
After I finished that, I looked at the back-of-the-book solution, and was chagrined to see how much cleaner and more elegant that was. But while I was trying it out, I streamlined it a little.
I didn’t manage to streamline it as much as menderbug did; nice work on that 25-part solution. But I figured it might still be worth posting this 26-part variation on Paul’s solution.