MCP23017 I²C GPIO Expander

Turn two MCU pins into sixteen general-purpose pins — perfect when you've run out of GPIO and don't want a bigger board.

Difficulty: Breadboard. Estimated build time: about 40 minutes. Estimated parts cost: about US$5.50. 9-part bill of materials with live supplier pricing. A sixteen-LED chaser that lights every output of an MCP23017 in sequence, driven by an Arduino over a two-wire I²C bus.