The SX1509 Breakout is designed with two headers at the top and bottom, serving as the input and control headers. These headers are where you supply power and terminate your I2C signals – SDA and SCL. For convenience, GPIO and power buses are broken out in every direction, and configurable jumpers cover the majority of the board.
One of the unique features of the SX1509 Breakout is its ability to operate as a level-shifter. With I/O banks that can function between 1.2V and 3.6V (5.5V tolerant) independently of both the core and each other, this device can facilitate direct level shifting between I/O banks and your host controller. This makes it an ideal choice if you're looking to add more I/O onto your Arduino or other I/O limited controller. Plus, Sparkfun has developed an Arduino Library to help you get started with ease.
|SX1509 I/O Expander Breakout Hookup Guide - SparkFun Learn