I was reading this question post What to do with third contact in DC barrel plug with only two input contacts about the third connector of the DC Barrel, and said that you can know if the DC Plug is connected or not. How can you know that? Did the third pin give you ground or voltage? For example if I have an also a USB connector to give some priority to one of the connections.
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If no plug is inserted, the third pin will contact the middle pin. You can put a bias on it from e.g. a battery, and detect presence of an external supply that way.
answered Sep 11, 2015 at 23:14
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\$\begingroup\$ Im not sure what you mean with bias, I was thinking to connect for example in my circuit an USB connector, then the 5v of the USB in that pin and the ground together? but I think I'm going to do a short cicuit. Not sure \$\endgroup\$– Pulse9Commented Sep 11, 2015 at 23:31