Making a Solid-State Relay With I2C interface

As [Mic] often got requests to make high-power switching boards, he recently finally gave in and designed the one shown above based around a solid-state relay. Some of our readers that already play with mains power know that switching should normally occur when the voltage crosses zero volts. The ‘TRIAC BLOC’ is able to do so, which also allows mains frequency measurement. [Mic] then tuned to the internal oscillator of his ATtiny microcontroller with this 50Hz by adjusting its OSCCAL register value, so the switching command can be sent at the ideal moment.

Zero crossing detection is implemented by feeding the mains into an AC optocoupler. [Mic] discovered that the optocoupler diodes are not identical, so he had to adjust his firmware to account for the time differences.
All the resources are available on github, we would be interested to hear your detailed analysis of the circuit implemented with the passives R3/C1/L1/R8/C3.