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I2C pcf8574 8bit port expander

I made a test / prototype setup on a breadboard with two pcf8574 ics ; one for output and one for input. Both connected to the same I2C bus.

Adding more digital inputs to an Arduino was my first objective, as I am planning to use this on a project which has buttons that control a menu.

For now this little write-up should suffice for the moment I am going to design a PCB for this.

The breadboard built


The program called Fritzing does a nice job of making an understandable picture of the board. This is not only nice for using in a write-up but in real-live it is faster than pulling everything from your breadboard just to place components a couple of holes further away (because you need the extra space).

Fritzing can also make a schematic, I've included it here but it looks horrible. When I make the final design on a PCB I will use Eagle for this.


I took a photo of the breadboard build, nice to see but difficult to see where what goes:

The code

The Arduino IDE has a library called Wire which is used for I2C stuff.

I wrote code that reads the button input and writes this to the LED output:

// I2C PCF8574 8 bit i/o port expander
// by AEP
// testing / prototyping the Wire library in combination with the ic PCF8574
// Reads data from PCF8574 over I2C and sends data to another PCF8574 over the same I2C bus

// Created 21 oct 2012

#include <Wire.h>

byte iInput=0;
byte iOutput=0;

void setup()

void loop()
  Wire.requestFrom(33,1);// Begin transmission to PCF8574 with the buttons
  if(Wire.available())   // If bytes are available to be recieved
    iInput =;// Read a byte
  if(iInput<255)         //If the value less than 255
    if (iInput==254) // P0
      iOutput = 1; 
    if (iInput==253) // P1
      iOutput = 2; 
    if (iInput==251) // P2
      iOutput = 4; 
    if (iInput==247) // P3
      iOutput = 8; 
  Wire.beginTransmission(32);  //Begin transmission to PCF8574 (with the LEDs)
  Wire.write(iOutput);         //Send data to PCF8574 (with the LEDs)
  Wire.endTransmission();      //End Transmission to PCF8574 (with the LEDs)