Did you use OSPi 1.2 or OpenSprinkler DIY 1.2? OpenSprinkler DIY 1.2 already has rain sensor port integrated. OSPi 1.2 does not, and yes you can add a rain sensor by following the OSPi 1.4 circuit diagram to figure out the GPIO pin. Add a 10K pullup, and a 4.7K current limiting resistor (in series) should suffice. You don’t even need 4.7K resistor — it was added because we’ve occasionally had users who plugged in 24VAC to the rain sensor port. The 4.7K resistor will make sure it won’t damaged the processor even if 24VAC is plugged into a GPIO.