I wanted to build a binary clock that didn't display BCD time. So I used an obsolete even then AT90S1200 (which has to be programmed in assembler), some perfboard and some dim green LEDs to make one. This build is more than ten years old.

From left to right, minutes, seconds, and hours (24 hour time) on the bottom.

1, 2
4, 8,
16, 32 minutes or seconds

1, 2, 4, 8, 16 hours.

So the first photo displays 13 hours, 19 minutes and 11 seconds.

The worst thing about the clock is that the 4.096 MHz crystal does not keep very accurate time. It would be fun to rebuild with the built-in microbit display or a nice STM32 with lfxo and integrated RTC.