@icedquinn@sun@disabled
the tl;dr is that their are a lot of viable versions, that work on completely different paradigms. Many patents have been issued over the last 100 years where some are illustrated. And, Patrick Kelly has the best compendium of these devices. His website has a simplified document that covers many.
The rules for not getting shut down are, stay away from anti-gravity, don't file for a patent, and don't try to build and distribute them at scale.
The mechanical ones are given more leeway than the solid state electrical versions.
And, have fun, you can build a nice over unity joule thief circuit probably within a day.