Data-logging made simple with Arduino
One of the best capabilities provided by Arduino regards its very high modularity, which helps users to quickly translate ideas into physical artifact, as practically demonstrated by Mauro, which shows on his blog how to build a simple data-logger by properly combining different shields.
By using few additional components (mainly resistors and buttons) a fully-functional data logger can be easily implemented.
For more detail: Data-logging made simple with Arduino
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