this is pretty incredible

https://hackaday.io/project/197267-tiny-bit-machine

Solar powered, battery free hardware to run an interpreter on the ATTiny85. Currently have a working game written in 127 byte program. Charge time in full sun: 3-6 minutes. Run Time: 30-45 minutes.

more amazing shit i found on hackaday

https://thetubecomputer.com/