I’ll take one stepper over eight motors every time.
That’s the Meridian Orrery by illusionmanager, as Hackaday reported on May 1. All eight planets, the Sun and the Moon are on it. Pluto isn’t, and I’ll let that one go.
It then drives a small stepper motor via a TMC2209 driver, turning the mechanism back and forth until all the pieces are positioned correctly
To me that sentence is the whole machine. The ESP32 is the right amount of brain, since the entire trick is software deciding where one motor stops. A reed switch and a magnet are the cheap, honest way to find zero. The small tabs that interlock the rings are the clever part, and my guess is the stepper swings one way, the tabs pick up the next ring and carry it along, and the swing back leaves each ring parked where the simulation wants it. The Moon gets its own geared mechanism. The maker says that if his version updates in the middle of the night, you never see it move.
It’s 3D-printed parts plus that ESP32, the TMC2209 driver, a stepper, the reed switch and the magnet, and the maker says this version went for simplicity and added the Moon. The Instructables page is a 10-step tutorial with pictures, so you can build one.
I want to know how much backlash those tabs pick up over a few hundred reversals, since the reed switch only fixes zero.