For those who are interested in hooking up the 3-wire flow sensors to OpenSprinkler, I’ve just explained how to do so in this post:
This is the same on both OpenSprinkler and OSPi. Basically, red wire -> 5V (VIN pin, also the 5V pin on the USB port); yellow wire -> rain sensor’s left port; black wire -> rain sensor’s right port.
Because there is a 4.7k series (protective) resistor between the sensor (which outputs up to 5V) and the digital input pin (which is only 3.3V tolerant), you don’t need a level shifter (most digital pins have internal zener diodes, which will clamp the voltage coupled with the series resistor).
The high pulse/liter count could be a concern as it may cause the number of clicks to exceed the software limit (but this can be lifted by changing a number in software). In general, I would advise against these type of cheap sensors as they are not very accurate or reliable.