This is a standard Arduino example sketch, copied here for reference.
Full Source Code
The full code is all in one file Blink.ino.
/* Blink Turns on an LED on for one second, then off for one second, repeatedly. Most Arduinos have an on-board LED you can control. On the Uno and Leonardo, it is attached to digital pin 13. If you're unsure what pin the on-board LED is connected to on your Arduino model, check the documentation at http://www.arduino.cc This example code is in the public domain. modified 8 May 2014 by Scott Fitzgerald */
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