arhi posted an update on his HAKKO soldering pencil driver:
The driver actually works like a charm – if you take the “numbers” on display as some “arbitrary value” and not temperature in C :D. The tip temperature is held pretty good and it all works great, it’s just that “values” are meaningless. The 500C heater temp is actually around 220C on the tip … I’ll see if there’s anything to be done wrt that problem but most probably some “custom calibration” is the way to go.
Attached all files required to reproduce the controller. PCB/Schematic are made in PROTEUS (pdf files also available as is gerber), firmware in CSC PICC.