IMPROVED PINS!. While it is possible to get good results with a soldering iron, as suggested here, it's not easy! I've included a new photo to show my progress from wonky to OK on my first practice heart using the soldering iron. It really demands a light touch.
This set of working gears is fitted on a heart background (2"high x 1-3/4" at widest point). It was created in Tinkercad with some help getting started on the Geneva mechanism There are two bases, one without a loop that can be used as a pin (glue on a pin back from a crafts store) or just glued onto a blank card (best bet for giving to a guy). The other base has a loop so that the heart can function as a pendant. The gears are set in motion by turning the large gear on the upper right, which hangs over the base edge just enough for a finger to catch it.
Although I have not done it myself yet - not totally happy with my color choices - my intent is to use a soldering iron to deform the tops of the pegs holding the gears so that nothing can fall off. I'll update as soon as I've done that (successfully).