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Are you referring to the two Grove ports in the lower left corner of the Mayfly? All of the Grove ports (and the Sw3 and Sw5 pins in the headers) are designed to only be powered when you set pin D22 high. This is so that you can turn any external sensors on or off independently from other things on the Mayfly. Setting D22 high turns on the external 3v and 5v regulators, and anytime they are on you’ll see a red LED in the corner of the board light up (LED3).
Also be aware that the maximum incoming voltage that the ADS1115 can read is 3.3v, as determined by the Vcc of the Mayfly. You can buy the OBS3+ in different voltage output options, so if you bought the one with 5v output at full scale, you’ll need to use a voltage divider to protect the Mayfly from seeing anything greater than 3.3v. We always buy the OBS3+ with the 2.5v output option so that the max voltage will never exceed Vcc. And if you do use a voltage divider, make sure to account for that in the formula you use for the ADS1115 to convert bits to volts which and translate into sensor output to match up with the sensor calibration equations provided with each sensor.