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When I bought my aruino uno and connected it to my computer, I tried to make the simple blink circuit(before programming arduino) and suddenly the led started blinking.
Is the blink example is the defualt arduino sketch ?
I am sure that I didn't uploaded any thing to the board.

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Some may have the "Blink" sketch pre-installed, but many don't. However, when there is no sketch installed the board sits in bootloader mode waiting for you to upload a sketch. While it's in bootloader mode the LED pin pin D13 blinks.

If your LED is blinking about once a second then it's the Blink sketch. If it's rapidly blinking three or four times per second or so then it's in the bootloader and you have no sketch loaded.

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For me it was blinking 1 time at a second. By the way, what makes the bootloader blink the pin 13 if there is no sketch loaded, is it a normal sketch ?, and why that don't happen when I upload the "BareMinimum" example ?. – Asad Marmash Nov 29 '15 at 15:19
    
@AsadMarmash The bootloader is just software. It runs before the sketch runs if there is a sketch installed. If there isn't it just runs and runs and runs. It blinks the LED because it blinks the LED. That is what the programmer that wrote the bootloader decided to program it to do. It doesn't blink with BareMinimum because BareMinimum doesn't blink the LED, and it is a sketch, so the bootloader runs it. – Majenko Nov 29 '15 at 15:21

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