This is the AC light dimmer on the picture. I wonder how to use interrupts to detect cross-zero voltage and manage radio communication.

Hello, Theese relays is already with triacs. You could look at themes in here: https://forum.mysensors.org/topic/4020/controlling-an-inductive-ac-load/2 ...and here: https://forum.mysensors.org/topic/3238/ac-light-dimmer-with-2x-triac There is discusion about it, and also shematics. Check it :)