FREE SIMULATOR EXAMPLE · BEGINNER

Serial Hello World

The smallest useful Arduino program that talks back: print two lines in setup(), then report uptime in seconds every two seconds, forever. Ten lines of code — but they prove the entire serial path, USB cable to drivers to baud rate to monitor, before you build anything bigger on top of it.

Board	Arduino Uno (ATmega328P, 16 MHz, 5 V logic)
Difficulty	beginner
Category	communication
Libraries	None — just the Arduino core
Runs in	Sensbench browser simulator — free, no account

The exact circuit you get when you open this example — already wired, ready to run.

The sketch

// Serial Communication Example
// Send messages to Serial Monitor

void setup() {
  Serial.begin(9600);
  Serial.println("Hello, Arduino!");
  Serial.println("System initialized");
}

void loop() {
  Serial.print("Uptime: ");
  Serial.print(millis() / 1000);
  Serial.println(" seconds");
  delay(2000);
}

How it works

Serial.begin(9600) opens the USART at 9600 baud. On the Uno that is the hardware UART on pins 0 and 1, forwarded to USB by the bridge chip (16u2 or CH340 depending on the board). Every Serial.println() appends its own newline, which is why each message lands on its own line in the monitor.

setup() runs exactly once, so the banner prints once; loop() then repeats print-plus-delay(2000) forever. millis() counts milliseconds since boot, so millis() / 1000 is a free running clock — note the uptime keeps counting even while the sketch sits inside delay(), because the timer is hardware, not your code.

The blocking delay() is honest here because there is literally nothing else to do. The moment you add a second task — a sensor read, a button check — this structure falls apart, because nothing else can run during those two seconds. The fix is the millis() scheduling pattern you can see in action in the Serial Echo example.

Before you build it for real

Related examples

FAQ

Why does the Serial Monitor stay completely empty?

Check three things in order: the baud dropdown matches 9600, you selected the right port, and — on a real board — press reset after opening the monitor, because connecting reboots the Uno and the setup banner fires before you attach. Still nothing? Upload a blink sketch to prove the board is alive at all.

Can I print faster than every two seconds?

Sure — shrink the delay(2000). Just remember the serial buffer is 64 bytes and at 9600 baud a line takes ~20 ms to transmit. Print in a tight loop with no delay and Serial.print() starts blocking once the buffer fills, throttling your whole sketch to the baud rate.