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I had one project recently that required more than 30 Mayfly boards where each would interface with one SDI-12 soil moisture sensor array that requires 9v excitation. So I drew up this little custom PCB (not in Eagle), had it printed, and assembled the boards myself. It’s essentially one channel of the Sparkfun level converter board mentioned above, using a BSS138 MOSFET and two resistors. I also added a spot for a 9v boost converter board (https://www.pololu.com/product/2116) which also comes in 5v and 12v versions. It requires cutting the boards apart on a band saw, and then the hardest part is hand-assembling the 3 SMD parts, but if you’ve got a steady hand and a hot-air rework station or oven, it’s easy. I used a 3-position spring-latch terminal block, but you could use screw terminals or Molex pins as shown in the other photos.