Serial

Your Arduino has no screen. Serial is how it talks to your computer — for debugging, for logging sensor data, for everything. Learn this early; it makes every later chapter easier.

The basics

void setup() {
  Serial.begin(9600);        // open the serial port at 9600 baud
  Serial.println("Hello!");  // print a line
}

void loop() {
  Serial.print("Reading: ");
  Serial.println(analogRead(A0));
  delay(500);
}

Upload, then open Tools → Serial Monitor. Set the baud rate dropdown to 9600 to match Serial.begin(9600) — mismatched baud rates produce gibberish, the classic serial symptom.

Check: The monitor shows Hello! once, then Reading: 512 (or whatever A0 sees) every half second.

print() vs println()

• Serial.print(x) — prints with no line ending.
• Serial.println(x) — prints plus a newline.

Combine them to build readable lines: print("Temp: ") then println(temp) gives Temp: 23.5 on one line.

Baud rate

Baud is symbols per second — roughly bits per second here. 9600 is the traditional default; 115200 is common for faster data. Both ends must agree. Higher is faster but more error-prone over long wires; for USB debugging it barely matters.

Reading from the computer

Serial goes both ways — your computer can send commands to the Arduino:

void setup() {
  Serial.begin(9600);
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.println("Type 1 for ON, 0 for OFF");
}

void loop() {
  if (Serial.available() > 0) {       // any incoming data?
    char cmd = Serial.read();         // read one character
    if (cmd == '1') {
      digitalWrite(LED_BUILTIN, HIGH);
      Serial.println("LED on");
    } else if (cmd == '0') {
      digitalWrite(LED_BUILTIN, LOW);
      Serial.println("LED off");
    }
  }
}

Type 1 in the Serial Monitor's input bar, press Send — the LED lights. You've built a remote control.

Careful: The Serial Monitor's line-ending dropdown (bottom right) appends invisible characters. With "Newline" selected, typing 1 sends '1' plus '\n' — your sketch reads the 1, then the \n hits the else branch harmlessly here, but in stricter parsing it causes phantom inputs. Set it to "No line ending" when it matters.

The plotter

Tools → Serial Plotter graphs numbers in real time — one value per println. Point a sensor at things and watch the waveform. It's the fastest way to "see" what a sensor sees.

Next: Sensors