PCF8574T I2C IO Expansion Board

A microcontroller only has so many GPIO pins, and a project driving several LEDs, buttons or relays can run out fast. The PCF8574T solves that by adding 8 parallel I/O pins accessible over the same two I2C wires used for any other I2C device - each pin can be individually set as an input or output, controlled entirely through I2C commands rather than a direct wire per pin.

Up to eight of these boards can share one I2C bus at once, each set to a different address, which scales to 64 additional I/O pins off a single two-wire bus - a project genuinely short on pins gains far more headroom than adding a few extra GPIO breakouts would provide.

Common uses are driving multiple LEDs or relays from a pin-constrained board, reading several buttons or switches without dedicating a pin to each, and any project design that has simply run out of direct GPIO pins.