Ho Gompka,
it’s in the file OpenSprinkler.cpp, just search for void OpenSprinkler::apply_all_station_bits() {
You can find a fully working variant for the ESP8266 WIFI 4 Channels Relay Module on my Github account (with more information about the hardware setup):
https://github.com/ThFischer/OpenSprinkler-for-ESP12F_Relay_X4#readme
salute & happy engineering
Thomas