MCP23017 16-Bit I2C I/O Expander IC (DIP-28)

A microcontroller can run out of free GPIO pins fast once a project grows past a handful of buttons and LEDs, and the MCP23017 solves that by adding sixteen fresh input/output pins that are all controlled over the same two-wire I2C bus already used for other sensors. Several of these chips can even share the same bus at once, each addressed separately, which lets a single microcontroller control dozens of extra pins without ever running short. That makes it a practical building block for large keypad matrices, bar-graph LED displays, panels full of indicator lights, and any control panel project where the number of switches and indicators needed outstrips a typical microcontroller's own pin count. Its DIP-28 package also breadboards easily for prototyping before being moved to a permanent board.