user28276: Can anyone please help me figure out/ point the reason of this arduino problem? I uploaded a simple arduino sketch (blink an LED ) just to test if my real program was the reason of the "abnormality" of this board. It works fine when I connected it to PC via arduino cable, but doesn't run the program when I use an external power supply(9v) and connected it to the Vin pin of arduino.
Need help and hoping for fast replies. I will be having a competition tomorrow, this is very urgent. please guys.
for the schematic diagram, there are no other parts inserted to the arduino board, aside from the 9v battery. I was able to say that the program work on "LED blink" by observing the lights of "TX" LED on the board. please help me. :( 

user28033: TX LED is blinking because of communication (indicating data transmit via serial). It will not blink if there are no communication from Arduino through USB cable.      
For simple observation, use LED_BUILTIN pin. Its a build in-LED indicator on Arduino.    
pinMode(LED_BUILTIN, OUTPUT);

then
digitalWrite(LED_BUILTIN, HIGH);
delay(500)
digitalWrite(LED_BUILTIN, LOW);
delay(500)

Arduino fail on external power supply is usually because of voltage drop or your system draw too much current.

28276: <p>Can anyone please help me figure out/ point the reason of this arduino problem? I uploaded a simple arduino sketch (blink an LED ) just to test if my real program was the reason of the "abnormality" of this board. It works fine when I connected it to PC via arduino cable, but doesn't run the program when I use an external power supply(9v) and connected it to the Vin pin of arduino.</p>

<p>Need help and hoping for fast replies. I will be having a competition tomorrow, this is very urgent. please guys.</p>

<p>for the schematic diagram, there are no other parts inserted to the arduino board, aside from the 9v battery. I was able to say that the program work on "LED blink" by observing the lights of "TX" LED on the board. please help me. :( </p>
 28033: <p>TX LED is blinking because of communication (indicating data transmit via serial). It will not blink if there are <strong>no communication from Arduino through USB cable</strong>.      </p>

<p>For simple observation, use LED_BUILTIN pin. Its a build in-LED indicator on Arduino.    </p>

<pre><code>pinMode(LED_BUILTIN, OUTPUT);
</code></pre>

<p>then</p>

<pre><code>digitalWrite(LED_BUILTIN, HIGH);
delay(500)
digitalWrite(LED_BUILTIN, LOW);
delay(500)
</code></pre>

<p>Arduino fail on external power supply is usually because of voltage drop or your system draw too much current.</p>
