Arduino Serial Monitor explained — Serial.print, baud rate and input

#The short answer

The Serial Monitor is a window that shows text sent by your Arduino over the USB cable. Three lines are all you need:

void setup() {
  Serial.begin(9600);              // open the connection at 9600 baud
  Serial.println("Hello from the UNO!");
}

void loop() {
}

Serial.begin(9600) opens the connection; Serial.println() sends a line of text. Open the Serial Monitor (in the Arduino IDE: the magnifying-glass button at the top right, or Tools → Serial Monitor), set it to 9600 baud, and the message appears.

#What "serial" and "baud" mean

Serial means the bits travel one after another down a single wire, like people walking through a door in a line. On the UNO, that wire is pins D0 (RX, receive) and D1 (TX, transmit), which a small chip on the board turns into USB.

The baud rate is the speed: 9600 baud is 9600 bits per second — roughly 960 letters a second. That is plenty for messages and sensor readings.

#print vs println

• Serial.print(x) sends x and stays on the same line.
• Serial.println(x) sends x and then starts a new line.

Mix them to build one tidy line from several pieces. This is from our Knob project:

Serial.print("knob ");
Serial.print(reading);
Serial.print("  ->  LED ");
Serial.println(brightness);

It prints lines like knob 512 -> LED 127. You can print text in quotes, whole numbers and decimals. Serial.print(celsius, 1) prints a decimal number with one digit after the point — the Thermostat project uses it to show 24.3 C fan off.

Play in 3DRun the code · press the parts · turn it around

#Using it to find bugs

The Serial Monitor is the best debugging tool a beginner has. When something does not work, print what the board thinks is happening:

• Is the code even running? Print "setup done" at the end of setup(). If you never see it, the board is not running your sketch.
• What is the sensor reading? Print the raw number. If a light sensor always shows 0 or 1023, the wiring is wrong, not the code.
• Which way did the if go? Print inside each branch: Serial.println("dark"); / Serial.println("bright");.
• Did the button change? Print only when the value changes, like our Push-a-button project:

int now = digitalRead(BUTTON);
digitalWrite(LED, now);
if (now != was) {         // only report a change
  Serial.println(now == HIGH ? "pressed" : "released");
  was = now;
}

#Reading what you type

The Serial Monitor works both ways: type in the box at the top, press Send (or Enter), and the text goes to the board. Serial.available() tells you how many characters have arrived; Serial.read() takes one of them.

Here is a small sketch that switches the board's LED with the letters 1 and 0:

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Send 1 for on, 0 for off");
}

void loop() {
  if (Serial.available() > 0) {
    char c = Serial.read();
    if (c == '1') {
      digitalWrite(LED_BUILTIN, HIGH);
      Serial.println("LED on");
    }
    if (c == '0') {
      digitalWrite(LED_BUILTIN, LOW);
      Serial.println("LED off");
    }
  }
}

Every character you type arrives separately, including the invisible newline at the end of the line — that is why we check for '1' and '0' and simply ignore everything else. To read a whole number like 120, use Serial.parseInt().

#The Serial Plotter

The Arduino IDE also has a Serial Plotter (Tools → Serial Plotter). It draws a live graph of the numbers you print, one per line. Print a sensor reading with Serial.println(reading); and watch it as a wave — great for knobs, light sensors and distance sensors.

#Common mistakes

• Nothing appears. You forgot Serial.begin(9600); in setup(), or the monitor is closed, or the wrong port is selected.
• Gibberish. The baud rates don't match. Set both to 9600.
• Upload fails while the circuit uses pins 0 and 1. Those pins are the serial line. Move your parts to pins 2–13.
• Everything prints on one long line. Use println, not print, for the last piece.

#Check yourself

#Questions people ask

#Why is my Arduino Serial Monitor not showing anything?

Check four things: Serial.begin(9600) is in setup(), the monitor's baud rate is 9600, the right COM port is selected under Tools → Port, and your code actually reaches a Serial.print line.

#What baud rate should I use for Arduino?

9600 is the classic choice and works for almost every beginner project. Use 115200 when you print a lot of data quickly. Just make sure the sketch and the monitor use the same value.

#Can I use pins 0 and 1 while using the Serial Monitor?

Better not. On the UNO, pins 0 (RX) and 1 (TX) carry the USB serial data, so parts on them can block uploads and garble messages.

#Does Serial.print slow down my Arduino?

A little. At 9600 baud each character takes about 1 ms to send, and if the send buffer fills up, Serial.print waits. Print less often, or use a faster baud rate.

Last updated 2026-10-29