I am fond of those slave clocks you could see in city-halls, railways stations, schools, banks, ... Their distinctive (and sometimes loud) ticking and more importantly the perfect alignment between the minute-hand and the dots make them attractive, They are usually quite expansive (considering they are totally unusable) on second-hand market, but their display is really unique.
I decided to build a driver (master clock) with following constraints:
- as small as possible (so it could be installed behind any model)
- that can runs on 2 AA batteries as the minimum power requirement
- as universal as possible (whatever are the voltage, polarity and the 'frequency' required)
- precise enough to be usable in real life as any other clock (ok, not in your room)
- simple enough for grand-mother to use it

I ended up with a quite small PCB - through hole - , an ATTINYx5 and a RTC DS3231 module, and many hardware and software options. Particular efforts were done to save energy.