I always loved these X-Mas lights which look like a raindrop.
I just wondered, how they are working. First I thought of using a decade counter. Long time ago I build a night rider light with such a component. This time I wanted to use the ATTiny85, inspired by HAZE TECH's led chaser.
BTW: this is a different but neat way to approach the challenge:
https://tronicspro.com/rain-drop-led-chaser-electronics-project/