Concept: 

This is the prototype for the final auto bike light. There is much more info in that post including the parts list and files.

 

 

This one used no resistors on the LEDs and relied on the Attiny85 to limit current.

The layout of the later version is also much nicer reducing the number of jumpers required.

I used a smaller battery in this one but I haven’t run out of power yet and it’s been on and off for at least a week. It will be good to see how long this will last to extrapolate to the final version.