Adafruit
Adafruit PCF8574 I2C GPIO Expander Breakout
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 c...
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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
Jargon buster
Plain-language definitions for the technical terms used above.
- Address jumpers
- Address jumpers are small solder pads, links or switches used to change a device's address on a shared bus such as I2C. They matter when you want to connect several identical devices to the same controller, since each one needs a unique address to avoid conflicts.
- breakout
- A breakout board carries a small or fine-pitched component and brings its connections out to standard, breadboard- and header-friendly pins. Describing a part as a breakout means it can be wired into a project without soldering directly to the component's tiny contacts.
- CircuitPython
- A beginner-friendly version of Python designed to run directly on microcontroller boards. If a product supports CircuitPython, you can often program it by copying code files onto the board rather than setting up a more complex toolchain.
- GPIO
- General-purpose input/output pins are microcontroller pins you can set in software to read signals, switch devices on and off, or connect to peripherals. The number of GPIO pins matters because it limits how many buttons, LEDs, sensors, and other parts you can wire directly to the board.
- I2C
- I2C is a two-wire communication bus used by many sensors and small modules. It matters because several I2C devices can share the same two wires, but each device needs a compatible address and your controller must support I2C.
- IRQ
- IRQ (interrupt request) is a signal line a device uses to alert a microcontroller that something needs attention, so the microcontroller does not have to poll continuously. Wiring an IRQ pin to a free input lets code respond promptly to events such as new data being ready.
- LED
- A light-emitting diode (LED) is a small electronic component that emits light when current flows through it in the correct direction. Because it only conducts one way, its polarity matters, and a through-hole LED must be soldered the correct way around to light up.
- microcontroller
- A microcontroller is a small computer on a single chip that runs a stored program and controls connected inputs and outputs such as buttons, sensors, displays and communication interfaces. In a device built around one, it is the part that executes the code and coordinates the device's behaviour.
- Qwiic
- Qwiic is a plug-in connector system for I2C devices that uses small 4-pin cables, so you can connect compatible sensors without soldering. It matters because your controller or adapter also needs Qwiic, or you will need a cable or breakout to wire it up.
- STEMMA QT
- A small plug-in connector system for I2C boards that lets you connect compatible sensors and controllers without soldering. It matters because it can make wiring faster and less error-prone, especially when adding several small modules to a project.
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