mArduino is a simple library for Arduino. mArduino uses multiple Arduinos and I2C Protocol to expand Arduino's IO capacity. By using mArduino, master arduino can read and write slave's I/O very straightforwardly. Master can make slave's LED in a nearly identical manner to writing an arduino sketch, for example, the following sketch will blink slave's LED: #include "Wire.h" // You need to include both Wire.h and mArduino.h #include "mArduino.h" const int SLAVE_ADDRESS = 1; const int ledPin = 13; void setup() { mArduino(); // join I2C bus as a Master pinMode(SLAVE_ADDRESS, ledPin, OUTPUT); } void loop() { digitalWrite(SLAVE_ADDRESS, ledPin, HIGH); delay(100); digitalWrite(SLAVE_ADDRESS, ledPin, LOW); delay(100); } For more information, see: http://coopermaa2nd.blogspot.com/2011/12/marduino-arduino-i2c.html