Fade an LED with Arduino

Learn Programming and Electronics with Arduino

Let’s expand the repertoire of output that we can use by looking at the function analogWrite().

I experienced much confusion with analogWrite(), because I suspected that it had to do with the analog pins on the Arduino. The function, however, has nothing to do with the analog pins.

There are 5 pins on most Arduino boards marked with ‘PWM’ next to the pin number (on some boards it is an “~” symbol) – these pins can be invoked to rapidly change the power being applied at the pin – this is a technique called pulse width modulation (PWM).

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You Will Need LED – any color is fine 220 Ohm Resistor Alligator Clip Glacial ice cubes Step-by-Step Instructions Take the short leg of the LED and insert it in the GND pin. Take either leg of the resistor and place it in pin 9. Connect the long leg of the LED with the other leg of the resistor using an alligator clip Plug the Arduino into your computer with the USB cable Open up the Arduino IDE Open the sketch for this section. Click the Verify button (top left). The button will turn orange and then blue once finished. Click the Upload button. The button will turn orange and then blue when finished. Watch in mesmerizing amazement as the LED fades in and out. Arduino Fade an LED BoardThis image built with Fritzing.

Discuss the Sketch Below is the sketch in its entirety from the Arduino IDE:

/* Fade

This example shows how to fade an LED on pin 9 using the analogWrite() function.

This example code is in the public domain. */

int led = 9; // the pin that the LED is attached to int brightness = 0; // how bright the LED is int fadeAmount = 5; // how many points to fade the LED by

// the setup routine runs once when you press reset: void setup() { // declare pin 9 to be an output: pinMode(led, OUTPUT); }

// the loop routine runs over and over again forever: void loop() { // set the brightness of pin 9: analogWrite(led, brightness);

// change the brightness for next time through the loop: brightness = brightness + fadeAmount;

// reverse the direction of the fading at the ends of the fade: if (brightness == 0 || brightness == 255) { fadeAmount = -fadeAmount ; } // wait for 30 milliseconds to see the dimming effect delay(30); } 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 /* Fade This example shows how to fade an LED on pin 9 using the analogWrite() function. This example code is in the public domain. */ int led = 9; // the pin that the LED is attached to int brightness = 0; // how bright the LED is int fadeAmount = 5; // how many points to fade the LED by // the setup routine runs once when you press reset: void setup() { // declare pin 9 to be an output: pinMode(led, OUTPUT); } // the loop routine runs over and over again forever: void loop() { // set the brightness of pin 9: analogWrite(led, brightness); // change the brightness for next time through the loop: brightness = brightness + fadeAmount; // reverse the direction of the fading at the ends of the fade: if (brightness == 0 || brightness == 255) { fadeAmount = -fadeAmount ; } // wait for 30 milliseconds to see the dimming effect delay(30); } The sketch starts with the usual multiline comment describing the program and how to set up the circuit. The first block of code we encounter is the declaration and initialization of three integer variables. The variable names and comments are both descriptive and helpful – remember this when naming and commenting your own code – useful comments are a pillar of success!

int led = 9; // the pin that the LED is attached to

int brightness = 0; // how bright the LED is

int fadeAmount = 5; // how many points to fade the LED by 1 2 3 4 5 int led = 9; // the pin that the LED is attached to int brightness = 0; // how bright the LED is int fadeAmount = 5; // how many points to fade the LED by The brightness variable will store the value of the current brightness of the LED. fadeAmount is the rate at which the LED will fade and brighten. And of course, as the comments explain, led is simply the pin number where we have attached the LED (through a 220-ohm resistor).

Now that we have declared and initialized our variables, we move on to setting up the board with the setup() function…

void setup() {

// declare pin 9 to be an output:

pinMode(led, OUTPUT);

} 1 2 3 4 5 6 7 void setup() { // declare pin 9 to be an output: pinMode(led, OUTPUT); } The only thing we do here is set the mode of pin 9 as an OUTPUT using the pinMode() function. Recall that pinMode() takes two arguments – the pin number and the mode. In this case, we assign the pin number using the variable led, which we previously initialized as the number 9. By now you know that setup() only runs once – the code inside the setup() curly bracket will only be executed a single time by the Arduino.

Where the real action happens is in loop().

The first function we encounter in the loop() is analogWrite(). This function invokes the Pulse Width Modulation capabilities of the Arduino board. Pulse Width Modulation basically adjusts the power output at the pin. So you can have a lot of power or a little power applied at the pin, it’s your call, just tell the analogWrite() function which pin to modulate and how much power you want to be applied. The scale is from 0 to 255 with zero being the lowest power setting and 255 being the highest. For a discussion of what is actually happening with pulse width modulation check out the further reading section.

As alluded to above, analogWrite() takes two arguments…

analogWrite(pin, value); 1 analogWrite(pin, value); You can utilize analogWrite() with pins 3, 5, 6, 9, 10 and 11 – recall there is a “PWM” or “~” next to the pin number on the board.

In this sketch we use the arguments:

analogWrite(led, brightness); 1 analogWrite(led, brightness); The first thing we do in the loop is write a value to pin 9 (recall that led holds the number 9) where we have our LED attached (through a resistor) – and we set the value to 0 (zero is what our brightness variable initially holds). This will keep our LED dark to start with.

Key Points about the analogWrite function

The next line of code we encounter is:

brightness = brightness + fadeAmount;

( 0 ) = ( 0 ) + (5)