Dimming LEDs

In the real world we want to do more than just turn things on and off. For example, what if we wanted the LED to be 50% bright? However, if you remember from the Blink exercise, our GPIO pins can only output HIGH and LOW. Given this, how can we output anything else?

The solution is pulse-width modulation (PWM). Basically, if you turn the LED on and off really fast it will appear dim. If you wanted it to be 50% bright, you would turn it on half the time and off the other half. If you wanted it to be 75% bright, you would keep it on for three times as long as you keep it off. This ratio of on to off is known as a duty cycle.

Coding PWM

Given what we know, we could do this ourselves with digitalWrite and delay. However, Arduino has the handy function analogWrite built-in to do it for us! Let’s open up the Arduino IDE to a new sketch and set up our LED pin just like last time.

void setup() {
  // put your setup code here, to run once:
  pinMode(11, OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:

}

Notice that this time we are using pin 11 instead of pin 13. analogWrite only works on pins that support PWM, which are marked with a tilde ~ in front of the number. Pin 11 is one of these.

Next, let’s add some analogWrites in the loop.

void setup() {
  // put your setup code here, to run once:
  pinMode(11, OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  analogWrite(11, 255);
  delay(1000);
  analogWrite(11, 0);
  delay(1000);
}

What do these numbers in the second argument of the function mean?! analogWrite is like digitalWrite, but it outputs analog values rather than digital. This second argument is the duty cycle, which can be any value between 0 and 255 (2^8 - 1). Choose any two numbers you want for your two analogWrite commands.

With this, you can go ahead and re-upload your code! Play around with different brightness values and see how you perceive them.

Bonus exercise: Can you make the LED fade in and out continuously?

Next, if we have time we’ll play with a fun RGB LED :)

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