I do love gadgets, linkages, clockworks, all that 19th-century cogs-and-wheels technology. So much easier to see than our 21st-century nanotubes and zillion transistors on a chip.

Sándor Kabai has written many Demonstrations illustrating such mechanisms.

What makes these interesting is that playing with the various parameters lets you vary both the geometric setup and the controls that make the mechanism go through its motion.