Open the editorEditor
Built-in project · Arduino Uno R3

Breathing LED on pin 5, on an Arduino Uno

An LED on pin 5 fades up from dark to full brightness and back down, like slow breathing, with a function that takes the pin and the speed. The whole build, an Arduino Uno and 2 more parts, runs here in your browser on the firmware below; open it in the editor to change the wiring or the code and run it again.

Open in the simulatorRead the code

Teaching? Assign it to your class in one click.

Beginner About 10 minutes to build and run. Runs in your browser. Free, and no account needed.

The parts retail for about $30. On Mokxi they're free.

The circuit runs here once the page has loaded.

What you'll learn

  • PWM and duty cycle
  • Fading with analogWrite
  • Using a for loop to count up and down
  • Passing a pin and a delay into a function
  • Difficulty: Beginner
  • Time: About 10 minutes
  • Board: Arduino Uno R3
  • Parts: 4

How it works

Pin 5 drives the red LED through a 220 ohm resistor. The sketch uses PWM, which switches the pin on and off very fast. A bigger share of on time looks like a brighter LED.

The sketch steps the brightness from 0 up to 255, then back down to 0, pausing 6 ms at each step. That takes about three seconds per breath. Then it rests half a second and starts again.

Parts list

4 parts, plus the jumper wires. They retail for about $30; on Mokxi they're free.

Kits that can build it

These 12 starter kits have every part in the circuit. Each kit's page lists what else it can build.

Wiring table

Every connection, pin by pin, read from the circuit itself. Each row joins the pin on the left to the pin on the right, by a jumper wire or a breadboard strip.

Part and pinConnects to
LED, red pin CArduino Uno R3 pin GND
Resistor, 220 Ω pin 1Arduino Uno R3 pin 5
LED, red pin AResistor, 220 Ω pin 2

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
// A breathing LED on pin 5.
//
//   pin 5 -> 220 ohm -> LED -> GND
//
// breath() raises the PWM duty cycle from 0 to 255 one step at a time, then
// lowers it again. The delay between steps sets the speed: 6 ms a step is
// about three seconds a breath.

const int LED_PIN = 5;

void breath(int pin, int delayMs) {
  for (int i = 0; i <= 255; i++) {
    analogWrite(pin, i);
    delay(delayMs);
  }
  for (int i = 255; i >= 0; i--) {
    analogWrite(pin, i);
    delay(delayMs);
  }
}

void setup() {
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: a breathing LED on pin 5");
}

void loop() {
  breath(LED_PIN, 6);
  delay(500);
}

Code walkthrough

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

breath sketch.ino

Takes a pin and a delay in milliseconds. The first loop counts i from 0 to 255 and calls analogWrite each time, so the LED gets brighter. The second loop counts back down, so it fades out. The delay sets the speed.

void breath(int pin, int delayMs) {
  for (int i = 0; i <= 255; i++) {
    analogWrite(pin, i);
    delay(delayMs);
  }
  for (int i = 255; i >= 0; i--) {
    analogWrite(pin, i);
    delay(delayMs);
  }
}

setup sketch.ino

Makes LED_PIN (pin 5) an output and starts the serial port at 9600 baud for a short hello message.

void setup() {
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: a breathing LED on pin 5");
}

loop sketch.ino

Calls breath with pin 5 and a 6 ms step, then waits 500 ms before the next breath.

void loop() {
  breath(LED_PIN, 6);
  delay(500);
}

Troubleshooting

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

  • The LED just blinks or does not fade. This project
    Only some Uno pins do PWM. Keep the LED on pin 5, which does.
  • The breathing is too fast or too slow. This project
    Change the 6 passed to breath. A larger number makes each breath slower.
  • The LED is dim, burns out, or the pin reads wrong because the resistor value is off. Resistor
    Read the color bands again. A 220 ohm to 330 ohm resistor suits a typical LED on 5 V. 10k is a common pull-up or pull-down value. Check with a multimeter if unsure.
  • A pull-up or pull-down resistor is missing, so the input floats and reads random values. Resistor
    Tie the input to 5 V or ground through about 10k. Or use INPUT_PULLUP and wire the switch to ground.
  • The LED does not light, or it lights once and then dies. LED
    LEDs have polarity. The longer leg (anode) goes toward the positive side. Always add a series resistor, about 220 ohms to 1 k ohm on a 5 V pin.
  • A blue or white LED stays dark on 3.3 V. LED
    Blue and white LEDs drop about 3 V, so 3.3 V leaves almost nothing for the resistor. Use a 5 V supply, or a smaller resistor and a red or green LED.
  • 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.

Change it and keep it

Open it in the editor, change the circuit or the code, and keep your version in a free account.

Start building free
Open it in the editor