Simple Planetary Gears
by jonducrou, published
Set of planetary gears and arms to hold the planet gears in place.
Based off OpenSCAD code from thing:7390 (after cutting out significant portions of it - because the original thing is much more than a toy).
The gear ratios are very interesting, and (at least at first) non-intuitive, depending on the part that is held stationary.
With a fixed Annulus or Sun, this gives 1:6.
With a fixed Planet carrier, this gives 1:5.