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The OBS sensor comes a with bare-wire cable with 6 leads. Three of them need to be connected to the Mayfly ground (sensor power ground, sensor signal ground, and sensor shield). Then the 3 other sensor wires get connected to the remaining 3 pins of a Grove terminal (note, you have to use one of the two Aux A/D Grove inputs only). Seeedstudio make a 4-pin screw terminal Grove board, but the hard part is getting all 3 ground wires into one tiny screw terminal, so I designed the custom 6-pin screw terminal board because it has two additional ground terminals. If you didn’t want to use a screw terminal, you could just take a Grove cable, cut it in half, strip the ends, and solder them together, making sure you get the power and two analog signal wires connected to the right wires, and solder all 3 sensor ground wires to the single Grove cable ground wire. But the Grove cables have a very small gage wire in them which is hard to solder properly, and will likely break under frequent flexing unless you properly protect it. And if you do all of this cable soldering outside of the Pelican case, it’s impossible to cram the Grove connector through the cable gland in the bottom of the case.
So the easiest thing is to just run the 6 bare wires into the bottom of the Pelican case and use the 6-pin screw terminal. If you can wait about a week, they’ll be available in the Amazon store for purchase.