The board can operate on an external supply of 6 to 20 volts. If supplied with less than 7V, however, the 5V pin maysupply less than five volts and the board may be unstable. If using more than 12V, the voltage regulator may overheatand damage the board. The recommended range is 7 to 12 volts.
What I don't see, is what voltage the 2.1mm jack takes.