Fade with PWM

Digital pins can only output full ON (5V) or full OFF (0V). But what if you want something in between — like half brightness? Pulse-Width Modulation (PWM) simulates an analog voltage by rapidly switching a pin on and off.

The key concept is duty cycle: the percentage of time the pin spends HIGH in one cycle. A 50% duty cycle (on half the time) averages out to 2.5V. A 25% duty cycle averages to 1.25V. The switching happens so fast (~490Hz) that an LED appears to dim smoothly.

analogWrite(pin, value) sets the duty cycle on a PWM pin. The value ranges from 0 (always off, 0V) to 255 (always on, 5V). 128 is approximately 50% duty cycle.

analogWrite(9, 0);    // Off
analogWrite(9, 128);  // ~50% brightness (~2.5V)
analogWrite(9, 255);  // Full brightness (5V)

• Connect pin 9 → 220Ω resistor → LED anode (long leg)
• LED cathode (short leg) → GND
• Pin 9 is a PWM pin (~) on the Uno

const int ledPin = 9;

void setup() {
  pinMode(ledPin, OUTPUT);
}

void loop() {
  // Fade in: 0 to 255
  for (int brightness = 0; brightness <= 255; brightness++) {
    analogWrite(ledPin, brightness);
    delay(10);  // 10ms per step = ~2.5 second fade
  }

  // Fade out: 255 to 0
  for (int brightness = 255; brightness >= 0; brightness--) {
    analogWrite(ledPin, brightness);
    delay(10);
  }
}

The for loop increments brightness from 0 to 255, writing each value to the LED. At 10ms per step with 256 steps, each fade takes about 2.5 seconds.