What you'll learn

Sending text to the board over serial
Checking how many characters have arrived
Timing with millis instead of delay
Doing two jobs at the same time

Difficulty: Beginner
Time: About 10 minutes
Board: Arduino Uno R3
Parts: 1

How it works

There is nothing to wire. The serial port runs over the USB cable. When the board starts, it prints UNO is ready and waits for you to type.

Each character you send comes back as UNO received followed by that character. At the same time, once a second, the board prints a counter that goes up by one. It uses the clock instead of a long pause, so it can count and listen together.

Parts list

1 part, plus the jumper wires. They retail for about $27; on Mokxi they're free.

1 × Arduino Uno R3In Starter Kit, Claw Machine Kit, Vending Machine Kit.

Kits that can build it

These 13 starter kits have every part in the circuit. Each kit's page lists what else it can build.

The code

The firmware exactly as the editor opens it. Change a line there and press Run: it compiles in the browser.

sketch.ino · Arduino Uno R3

// Talk to the board: it counts out loud and repeats what you type.
//
// Nothing to wire: the serial port runs over the USB cable.
//
// Serial.available() says how many characters have arrived; Serial.read()
// takes the next one. The counter is printed once a second with millis(), so
// the board can listen and count at the same time.

unsigned long lastCount = 0;
int counter = 0;

void setup() {
  Serial.begin(9600);
  Serial.println("UNO is ready! Type something.");
}

void loop() {
  while (Serial.available() > 0) {
    char c = Serial.read();
    if (c == '\n' || c == '\r') continue;
    Serial.print("UNO received: ");
    Serial.println(c);
  }
  if (millis() - lastCount >= 1000) {
    lastCount = millis();
    Serial.print("counter: ");
    Serial.println(counter++);
  }
}

Code walkthrough

The key functions of the sketch, and what each one does.

`setup` sketch.ino

Starts the serial port at 9600 baud and prints a greeting, so you know the board is listening.

void setup() {
  Serial.begin(9600);
  Serial.println("UNO is ready! Type something.");
}

`loop` sketch.ino

First, Serial.available() tells it if characters arrived, and Serial.read() takes them one at a time and echoes each, skipping newlines. Then millis() is compared with lastCount, so once a second it prints the counter and adds one.

void loop() {
  while (Serial.available() > 0) {
    char c = Serial.read();
    if (c == '\n' || c == '\r') continue;
    Serial.print("UNO received: ");
    Serial.println(c);
  }
  if (millis() - lastCount >= 1000) {
    lastCount = millis();
    Serial.print("counter: ");
    Serial.println(counter++);
  }
}

Troubleshooting

The usual mistakes with this circuit, and the fix for each.

Nothing comes back when you type. This project
Check the serial monitor baud rate is 9600, matching Serial.begin.
Your typing seems to get cut up. This project
The board echoes one character at a time, so a word comes back as separate lines.
Upload fails or nothing runs after wiring pins 0 and 1 to a module. Arduino Uno R3
Pins 0 and 1 are the USB serial port. Move the module to other pins, or unplug it while uploading. Also check the right board is selected.
Pins misbehave or the board resets when you power motors or servos from it. Arduino Uno R3
Each I/O pin gives about 20 mA, 40 mA max. Power motors and servos from their own supply and join its ground to the Uno's GND.