Welcome to EnviroDIY, a community for do-it-yourself environmental science and monitoring. EnviroDIY is part of 
We’ve encountered a few SDI12 sensors that are more picky about the data line voltage being actually 5v instead of 3.3v, and when sensor excitation line needs 9v or 12v, then we definitely need something between the Mayfly and the sensor. I’ve hand-built a bunch of little adapter boards (see attached photo) for those sensors, using a BSS138, some resistors, and a really cheap 9v or 12v step-up module from Pololu. This seems to work for every sensor we’ve tried it on, but it’s not ideal. I’m working on an even better SDI12 level shifting module using a different technique, but I’ll write more about that in the next week or two.
