rmd6502

Agreed - that also exposes the connection for the earth ground, which might be a good idea, though everything is already pretty well isolated so may not make a difference.

I like the idea of recycling in this way - one thing to be careful of is what you might be breathing using this technique.

The ATTiny85 has a timer that can be clocked using a PLL, so you can get a PWM frequency of 256KHz, as opposed to 62.5KHz with an Arduino. That having been said, the Leonardo has a PLL-drivable timer too.See Great Scott's Buck converter tutorial for why the higher frequency is better (spoiler alert: greater efficiency and lower values needed)