Once in a while we find products that have center NEGATIVE DC power jacks and they really make us mad because who keeps around both positive and negative polarity versions of every voltage? now, hopefully USB PD https://www.adafruit.com/product/5807 will solve this long term, but till then this 2.1mm DC flipper flopper adapter will connect in to out and out to in and voila - your standard center-positive is now center-negative. or vice versa! how handy is that.