The Adafruit PCF8574 GPIO Expander Breakout is an affordable 8-channel I2C GPIO expander that adds extra digital I/O to any microcontroller project. Simply connect over I2C and control up to 8 additional pins for buttons, LEDs, or other digital peripherals — no extra GPIO-rich board required.

The PCF8574 uses an unusual open-drain architecture — there is no pin direction register. Each pin is either a lightly pulled-up input (100K pull-up, reads high by default) or a strong 20mA ground-sinking output. Arduino and CircuitPython libraries abstract this away, letting you use familiar input/output modes. With three I2C address jumpers, you can chain up to 8 expanders on a single bus for 64 total GPIO.

Key Features

• 8 I/O Pins – Each pin can act as a pulled-up input or a ground-sinking output
• 3 Address Jumpers – Up to 8 expanders on one I2C bus for 64 total GPIO
• IRQ Output – Automatic interrupt alert when any input pin changes value
• Open-Drain Architecture – 100K pull-up inputs and 20mA sink outputs (no direction register)
• STEMMA QT / Qwiic – Solderless STEMMA QT connectors for easy daisy-chaining
• Breadboard Friendly – Standard 0.1″ header pinout with mounting holes

How the Pins Work

• Buttons/Switches – Connect one side to the PCF8574 pin and the other to ground. Pin reads high when open, low when pressed
• LEDs – Connect the LED anode to positive voltage through a resistor. The PCF8574 sinks current to ground to turn the LED on
• Digital I/O – Light pull-up acts as logic high output; strong ground acts as logic low output

Ideal For

• Adding extra buttons, switches, or keypads to I2C-equipped boards
• Expanding LED or relay control beyond available GPIO
• Multi-expander setups requiring up to 64 digital I/O pins
• Projects needing interrupt-driven input change detection

Also Consider

• MCP23017 I2C GPIO Expander – 16 channels with full input/output direction control

Resources

• Arduino PCF8574 Library
• CircuitPython/Python PCF8574 Library
• STEMMA QT Introduction