PCF8574T I2C I/O Expander Module | ATMOSFÆR ENGINEERING

I2C-based GPIO expansion module using the PCF8574T chip to add 8 additional digital I/O pins using only 2 wires (SDA/SCL). Features onboard DIP switches for configurable I2C addressing, allowing up to 8 modules (64 I/O pins total) on a single I2C bus. Ideal for expanding limited microcontroller pins for LED control, button inputs, relay driving, and LCD interfacing.

Technical Specifications:

• IC: PCF8574T (or PCF8574AT variant)
• Interface: I2C (TWI)
• I/O Pins: 8 bidirectional quasi-bidirectional I/O (P0-P7)
• Operating Voltage: 2.5V - 6V (typically 5V or 3.3V compatible)
• I2C Clock Frequency: Up to 100kHz (standard mode) or 400kHz (fast mode)
• I2C Address Range: 0x20-0x27 (PCF8574T) or 0x38-0x3F (PCF8574AT)
• Address Selection: 3-position DIP switch (A0, A1, A2)
• Output Current: 25mA per pin (max), 100mA total
• Pull-up: Internal weak pull-ups (~100μA)
• PCB Dimensions: 37.5mm x 16.7mm
• Operating Temperature: -40°C to +85°C

Pin Configuration:

• VCC: Power supply (3.3V or 5V)
• GND: Ground
• SDA: I2C data line
• SCL: I2C clock line
• P0-P7: 8 I/O pins
• INT: Interrupt output (optional, alerts on input change)

Address Configuration (DIP Switch):

• All switches ON: Address 0x27 (111 binary)
• All switches OFF: Address 0x20 (000 binary)
• Mix positions for addresses 0x21-0x26

Features:

• Simple 2-wire I2C control
• Bidirectional I/O (input or output per pin)
• Cascadable up to 8 modules (64 I/O total)
• Interrupt-on-change capability
• 5V and 3.3V compatible
• Onboard address selection switches
• Low power consumption

Applications:

• Expanding Arduino/ESP32/Raspberry Pi GPIO pins
• LED matrix control
• Button/switch input expansion
• LCD character display interface (I2C backpacks)
• Relay module control (multiple relays)
• Keypad matrix scanning
• Multi-sensor polling
• Industrial control panels

Usage Example:

• Connect VCC, GND, SDA, SCL to microcontroller
• Set DIP switches for desired I2C address
• Use I2C library to write/read P0-P7 states
• Default address with all switches ON: 0x27

Important Notes:

• Outputs are open-drain with weak pull-ups (use external pull-ups for driving high current)
• For output use, write HIGH first, then can read inputs
• Interrupt pin alerts when any input changes state
• Multiple modules require unique addresses via DIP switches

Compatible with Arduino (Wire library), ESP32, Raspberry Pi, STM32, and any microcontroller with I2C support. Widely used in I2C LCD adapters and multi-relay boards.