I wanted to learn how to interface with I²C devices using my Arduino, so I bought a DS1307 clock and crystal kit from Earthshine Electronics. I’ve no association to them, but I’d recommend having a look at their shop. Their prices are very reasonable, unlike most Arduino component retailers.
I²C is an addressable serial interface. That means that you can communicate with many I²C devices using the same 4 pins all the time.
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