What’s the I2C address configured by your PCF8574 module? Note that it has 3 bits that you can use to set the I2C address (if I understand correctly, these would be the 3 yellow-colored jumpers on your module). It’s not clear to me whether you are following OS 3.0, 3.1 or 3.2 schematic. Only 3.0 uses PCF8574 (whereas 3.1 and 3.2 use PCF9555 16-channel IO expander). So assuming you are following 3.0 schematic, which is available here: https://github.com/OpenSprinkler/OpenSprinkler-Hardware/tree/master/OS/3.0/AC_driver
it assumes that on the AC driver board, the three bits are A0=1 (high), A1=A2=0 (low). You need to set the jumpers correctly to match this address.