Stupid little board to allow a Sunon MF40100V1-1000U-F99 fan, which has a good/fail output, to replace a -G99 type, which has a tachometer output. The ATTiny looks for the fan’s pin to be low, and if it’s low it sends pulses back to the Einsy. Why? Because I have some -F99 fans but want to put a good Sunon fan in my Prusa I3 Mk3. The Prusa will think the fan failed without some tachometer pulses. Plus, this gives it that one more thing to go wrong. U1 is an ATtiny85 but a 25 would work just as well. I happen to have 85s here.

Software: Easy as pie and not included… if pin D0 is low, send out pulses on pin D1, otherwise don’t do that.