Dim an LED With PWM

Part of the Outputs section of Coddy's Arduino journey. Lesson 1 of 24.

digitalWrite() knows two levels: on or off. To make an LED glow dimly, the board switches the pin on and off hundreds of times a second. Your eye cannot follow the flicker and sees the average: a pin that is on a quarter of the time looks a quarter as bright.

This trick is called PWM, pulse width modulation. The share of time the pin is on is its duty cycle.

analogWrite(pin, value) sets it with a number from 0 to 255:

analogWrite(9, 0);    // always off
analogWrite(9, 64);   // on a quarter of the time
analogWrite(9, 127);  // on about half the time
analogWrite(9, 255);  // always on

Once you write a value, the board keeps the pin pulsing on its own, even during delay(), until you write a new one. analogWrite() makes the pin an output by itself; pinMode(9, OUTPUT) in setup() still says what the pin is for.

Challenge

Easy

The red LED is on pin 9. Make it glow at a quarter of its full brightness:

a quarter of 255 is about 64
write it with analogWrite()

Press Run to watch it, then Check.

Try it yourself

const int ledPin = 9;

void setup() {
  pinMode(ledPin, OUTPUT);
  // set the LED to a quarter brightness here
}

void loop() {
  // nothing to repeat: the pin keeps pulsing on its own
}

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Outputs

Practice on your own: Arduino playground