https://github.com/speedcontrols/ac_sc_grinder

One more rework:

• Ported to STM32F0 family (with limited  hardware multiplication and no division).
• Significantly reduced RAM demands.
• Added HALs for new boards.
• Open triac with short pulses.
• Fixed max possible speed.
• Other minor changes.

Now firmware can fit into any MCU with 4K RAM & 32K flash. Our choice is STM32F042F6P6 - because it can be flashed by USB without ST-Llink.

Also, final schematic was simplified again, and routed for actual grinder models. Here is temporary link: https://easyeda.com/speed/ac-speed-control-v2. It was not tested yet, but i already ordered PCB and components, to check how i is. As soon as i works, i will update project docs. 

What is changed (except MCU), if compare with dev board?

• Control triac via capacitor, instead of make negative MCU power. More accessible and cheap components.
• Changed step-down chip to LNK3204, because LCSC discontinued MP157.

So, optocouple with snubber network was removed in more elegant way. The only limitation - have to use 10ma triacs. But that's ok for expected kinds of use.