Reply To: Hydros21 Reporting -9999 on Mayfly 0.5b

All Mayfly logger board from version 0.3 to the current v1.1 (and including your v0.5v) have a separate 3.3v regulator that powers external sensors, and also the 5v boost circuit.  This secondary 3.3v regulator is turned on or off by controlling pin D22.  When the regulator is turned on, a red LED in the lower left corner of the Mayfly should turn on.  There’s a possibility that perhaps the sensor died and damaged the 3v (or 5v) output circuitry on your Mayfly.  Do you see the red LED (labeled LED3 on the Mayfly v0.5b) turning on during samples?  Have you tried putting a voltmeter across the ground (GND) and V pins of the SDI12 grove jack during readings to see what voltage you’re sending to the sensor?  You can set the voltage selection jumpers to either 3.3v or 5v for the Hydros21 sensor, it should work with either.

How old is the sketch that you’re using on the v0.5b board?  If your old board was programmed before May 2022, then the old code won’t work with the newer Gen2 Hydros21 sensors because the SDI12 library for communication with the library had to be adjusted for some slight timing changes in how the Mayfly communicates with the sensors.  Your v1.1 board is newer and probably has the updated SDI12 library and therefore is able to successfully talk to the sensor.