The most widely used barrel connector is the 2.1 mm one. They are often used for low-voltage power supplies and wall adaptors. You will probably have several of these lying around from old or broken toys and other equipment. They make great power supplies for various projects. This sub-board allows you to easily connect them to the miniTesla system.
This one has a plug insertion detection, a switch that is closed and thus connected to the sleeve if no plug is inserted. Once a plug is inserted this switch opens. This is useful to automatically switch between battery operation, when no plug is inserted, and power supply operation when it has been plugged in.