Dekatron Tube Clock
Dekatron clock runs off mains power with no transistors after initial setting.
A Dekatron tube is a Nixie-like device that can sequentially energize (typically) 10 glowing points arranged in a circle on top of its tube. Dekatrons can be used both as visual displays, and counters, but they've been out of use for decades, following the development of transistor-based alternatives. While a bit of a novelty now, such devices make for fun retro displays, sometimes implemented to spin with a power line’s AC frequency.
The Dekatron clock, by grobinson6000, takes things several stages further, and creates an entire working clock out of eight Dekatrons. The clock is first set by a GPS device on top of the main assembly, then it runs off of AC power with no transistors involved.
The way it works, as outlined in the schematics on GitHub, is that the first Dekatron tube spins at 20ms/division. So at 50 Hz–the local mains frequency–it makes a full revolution once every 200ms. This result is then passed on to a full second counter Dekatron, seconds tubes, minutes tubes, and finally hours tubes. Once it reaches 24 hours it then resets itself, starting the day over.
In addition to this impressive chain of counters, the device on top is rather interesting in itself. It monitors how far the system is getting away from GPS time, and can even report this to Home Assistant via an ESP8266 “somewhere in there.” The system can then be manually (semi-automatically?) reset with a flick of the switch on the GPS unit, and there are buttons and switches on the back of the clock further adjustment. Both units also feature nice wooden enclosures, finishing off this brilliant retro-electronics project with a bit of flair.