The Arduino IDE has a feature that can be a great help in debugging sketches or controlling Arduino from your computer's keyboard.

The Serial Monitor is a separate pop-up window that acts as a separate terminal that communicates by receiving and sending Serial Data. See the icon on the far right of the image above.

Serial Data is sent over a single wire (but usually travels over USB in our case) and consists of a series of 1's and 0's sent over the wire. Data can be sent in both directions (In our case on two wires).

Step 1:

You will use the Serial Monitor to debug Arduino Software Sketches or to view data sent by a working Sketch. You must have an Arduino connected by USB to your computer to be able to activate the Serial Monitor.

To get familiar with using the Serial Monitor, Copy and Paste the following example Sketch into a blank Arduino IE window. Then Verify it and if it's OK, Upload it. The next step will show you what to expect.


/* YourDuinoStarter_SerialMonitor_SEND_RCVE<br> - WHAT IT DOES: 
   - Receives characters from Serial Monitor
   - Displays received character as Decimal, Hexadecimal and Character
   - Controls pin 13 LED from Keyboard
 - SEE the comments after "//" on each line below
   - None: Pin 13 built-in LED
 - V1.00 02/11/13
   Questions: */

/*-----( Import needed libraries )-----*/
/*-----( Declare Constants and Pin Numbers )-----*/
#define led 13  // built-in LED
/*-----( Declare objects )-----*/
/*-----( Declare Variables )-----*/
int ByteReceived;

void setup()   /****** SETUP: RUNS ONCE ******/
  Serial.println("--- Start Serial Monitor SEND_RCVE ---");
    Serial.println(" Type in Box above, . ");
//--(end setup )---

void loop()   /****** LOOP: RUNS CONSTANTLY ******/
  if (Serial.available() > 0)
    ByteReceived =;
    Serial.print("        ");      
    Serial.print(ByteReceived, HEX);
    Serial.print("       ");     
    if(ByteReceived == '1') // Single Quote! This is a character.
      Serial.print(" LED ON ");
    if(ByteReceived == '0')
      Serial.print(" LED OFF");
    Serial.println();    // End the line

  // END Serial Available

//--(end main loop )---

/*-----( Declare User-written Functions )-----*/

/*********( THE END )***********/

Step 2: What the Serial Monitor Looks Like

When you click on it, the Serial Monitor will pop up in a new window. Above is what our example Serial Monitor Sketch looks like with the Monitor opened.

Look at the Serial Monitor window.

  • - The small upper box is where you can type in characters (hit or click "Send")
  • - The larger area (Corner can be dragged to enlarge) is where characters sent From Arduino will be displayed.- At the bottom are two pulldowns:
  • - One sets the "line ending" that will be sent to Arduino when you or click Send
  • - The other sets the Baud Rate for communications. (If this does not match the value set up in your sketch in Setup, characters will be unreadable). Example: Serial.begin(9600); Some sketches or other applications may use a different Baud Rate.


If you are testing a new sketch you may need to know what's happening when you try to run it. But "Software Is Invisible ! ". So you need the tell the software to tell you what it's doing, and sometimes the value of changing variables. You do this my using the Serial Monitor and adding code to your sketch to send characters that you can see.

In Setup you need to begin Serial Communications and set the Baud Rate (speed) that data will be transferred at. That looks like this:

Serial.begin(9600); // Other baud rates can be used...
Serial.println("My Sketch has started");

The second line is optional...

Here you can print helpful info to the Serial Monitor. Examples:

Serial.println("Top of loop");
Serial.println("Reading Temperature Sensor");
Serial.print("LoopCounter value = ");

For details of how to show different data, see "Serial_Print.html" in the Reference section of your Arduino IDE:

Top menu: Help>reference/Serial_Print.html

Experiment with changing the Sample Software Sketch.

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