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.
How to connect the third contact in the DC barrel
dcpcb-design
Related Solutions
Below is a basic schematic of how everything will be connected. V1 is the Arduino power supply, either 5V usb, or 7V to 12V dc barrel jack. V2 is the Lock Strike power supply. It will vary based on your lock strike, probably 12V to 24V. Regardless of the combination, the red wire below connects the grounds of both power supplies together (the DC, barrel jack side, not the AC wall side!). Just connect any ground connection on the Arduino to the emitter of the TIP31/ground of the lock strike power supply.
simulate this circuit – Schematic created using CircuitLab
CN2 is probably the barrel connector you are looking for. It is labelled as a "DC_2.0mm" which would be a 2mm diameter DC-jack, which can be known as barrel connectors. CN2 is connected to the circuit GND and the VCC net.
JP12 looks like a jumber to allow you to set the power source which U2 regulator is fed from to generate 3.3V. If pins 1 and 2 are shorted together U2 is fed from the 5.0V net, where as if pins 2 and 3 are shorted together U2 is fed from the VCC net. The jumper will usually either be a set of 2.54mm posts which you can short together using a special shunt piece that fits on top like this:
Or surface-mount pads close together that you can short with a solder blob or a surface-mount resistor like this:
So in this circuit, the USB connector CN1 always provides 5V when connected, but the current available will be limited by the USB host (usually to 500mA). The designer has provided an optional way of supplying your own supply (which could be greater than 5V as long as U2 does not overheat) if the power drawn by the 3.3V circuit is too great for USB. You can choose where to supply your circuit from by shorting the appropriate pins of jumper JP12.
Best Answer
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.