PWM: Dimming an LED

Overview

The Uno cannot output an in-between voltage. Pulse-width modulation (PWM) switches a pin fully on and off hundreds of times per second; the fraction of time spent on, the duty cycle, sets the average. An LED flickers too fast to see, so it simply looks dimmer.

You will need

• Arduino Uno R3
• LED and 330 Ω resistor
• 10 kΩ potentiometer
• Breadboard and jumper wires

Wiring

Diagram

The sketch

const int LED_PIN = 9;     // must be a PWM pin: 3, 5, 6, 9, 10 or 11
const int POT_PIN = A0;

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

void loop() {
  int reading = analogRead(POT_PIN);              // 0..1023
  int brightness = map(reading, 0, 1023, 0, 255); // 0..255
  analogWrite(LED_PIN, brightness);
  delay(10);
}

How the code works

analogWrite() takes 0–255, where 0 is always off and 255 always on. Pins 5 and 6 run at about 980 Hz, the others at about 490 Hz. The Servo library disables PWM on pins 9 and 10, and tone() interferes with pins 3 and 11, so plan pins when combining features.

Perceived brightness is not linear: the change from 0 to 30 looks bigger than from 200 to 255. For an even-looking fade, square the value before writing it.

What you should observe

• Turning the knob dims the LED smoothly from off to full.
• Waving the LED quickly in a dark room at mid brightness shows a dotted trail: the pulses.

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