Deciphering a DC jack schematic
It's a visual representation of how the socket works.


Pin 3 represents the pin that mates with the center of the plug (typically V+)
Pin 1 is the sprung contact that connects to the outside of the plug (typically Gnd)
Pin 2 contacts to Pin 1 (Gnd) but breaks contact when the plug is inserted (as the Pin 1 contact springs away)