There are lots of examples on the internet on how to program an Arduino as an I²C master to communicate with I²C slave devices.  There are, however, very few examples out there on how to program your Arduino as a slave device.  One of the best sites I’ve seen for documentation on I²C slave programming is over at Nick Gammon’s blog. 

Well I thought I would take it a step further and put together a step by step guide showing you a real life example of how to make an I²C slave device using an Arduino. I did a lot of research while making my I²C GPS Shield and thought I’d pass along some of what I learned.