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The Mayfly logger processor is just like any other Arduino board, and loading sketches onto it is just like any other Arduino board. The uploaded sketch is “burned” into the microprocessor chip in the center of the board. The microSD card slot is just an optional device to allow users to store recorded variables or other data during the subsequent operation of the logger. The Mayfly is not like a Raspberry Pi that has an actual operating system and executable files on the memory card.
Regarding SDI-12 addressing, you can connect a Hydros 21 sensor to any pins you want, you just need one data line, one power excitation line, and one ground line. Are you using a sensor with bare wires or did you purchase one with the 3-pin stereo plug (3.5mm)? If you’re using the stereoplug-to-grove adapter board that we sell, then just use a grove cable to connect the adapter board to the Mayfly’s D6-D7 grove jack, and then you would use data pin 7 in the sketch. If you’re using a bare wire sensor, then you’re free to connect them to any free digital pin, but for simplicity you should use D7 when starting out so you don’t have to change the sketch. The reason for the power excitation pin D22 in the sketch is that the onboard switchable voltage regulator is controlled by turning on pin D22. But you need to hook any external sensors you want to operate to the “Switched 3v” line because that’s where the actual switched power comes from, and not D22. Again, this is taken care of if you’re using the grove jack stereo plug adapter board, but you have to hook it up right if you’re using a bare wire sensor. Whenever D22 is on, there’s a red LED in the bottom corner of the Mayfly that comes on to indicate that the sensor (and anything else on the switched 3v or switched 5v line) is being powered.