OpenSprinkler 3.0 uses I2C to communicate with expander; previous versions (2.3 and before, also including OSPi) use 74HC595 shift register to daisy chain expanders. You can choose either — if you are building your own custom circuit, 74HC595 is probably easier since it’s cheap and you can simply follow the pin connections of OSPi circuit (it uses just 4 GPIO pins to control up to 56 zones).