This document explains how to connect your Arduino board to the computer and upload your first sketch.
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You also need a standard USB cable (A plug to B plug): the kind you would connect to a USB printer, for example. (For the Arduino Nano, you'll need an A to Mini-B cable instead.)
Get the latest version from the download page. When the download is finished, double click the .zip fle. This will expand the Arduino application.
Copy the Arduino application into the Applications folder (or elsewhere on your computer). If you're using an Arduino Uno or Mega 2560, you don't have any drivers to install.
The Arduino Uno, and Mega automatically draw power from either the USB connection to the computer or an external power supply. The power source is selected with a jumper, a small piece of plastic that fits onto two of the three pins between the USB and power jacks. Check that it's on the two pins closest to the USB port.
Connect the Arduino board to your computer using the USB cable. The green power LED (labelled PWR) should go on.
Double-click the Arduino application. (Note: if the Arduino software loads in the wrong language, you can change it in the preferences dialog. See the environment page for details.
Open the LED blink example sketch: File > Examples > 01.Basics > Blink.
You'll need to select the entry in the Tools > Board menu that corresponds to your Arduino.
Selecting an Arduino Uno
Details of the board menu entries are available on the environment page.)
Select the serial device of the Arduino board from the Tools > Serial Port menu. On the Mac, this should be something with /dev/tty.usbmodem (for the Uno or Mega 2560) or /dev/tty.usbserial (for older boards) in it.
selecting an Uno, Mega2560, or newer board
selecting an older FTDI-based board
Now, simply click the "Upload" button in the environment. Wait a few seconds - you should see the RX and TX LEDs on the board flashing. If the upload is successful, the message "Done uploading." will appear in the status bar. (Note: If you have an Arduino Mini, NG, or other board, you'll need to physically press the reset button on the board immediately before clicking the upload button on the Arduino Software.)
A few seconds after the upload finishes, you should see the pin 13 (L) LED on the board start to blink (in orange). If it does, congratulations! You've gotten Arduino up-and-running. If you have problems, please see the troubleshooting suggestions.
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The text of the Arduino getting started guide is licensed under a Creative Commons Attribution-ShareAlike 3.0 License. Code samples in the guide are released into the public domain.