Getting Power to the Board
The very first Arduino I/O Boards had a very simple power-supply circuit. Despite its simplicity, it was quite versatile, allowing you to provide either regulated or unregulated power, as well as routing power to the expansion connectors to power any shields that might be attached. The power-supply section of the original Arduino schematic is shown in Figure 4-2.
The Barrel Connector
The unlabeled block on the left side of the drawing is the barrel connector for unregulated direct current (DC) power input. The part outline vaguely resembles ...