Pulse Width Modulation (en)

German Version

There is no way to generate an analogue value on the Arduino directly, so to make an LED fade we need to use pulse-width modulation. A digital signal from the Arduino switches the corresponding PIN (PWM pins are marked with PWM or ~) ON and OFF very quickly. The longer the ON times of the pin, the brighter the LED appears to us - in contrast, the LED appears darker to us when the OFF times are longer. The PWM function of the microcontroller enables us to set simple values at the corresponding PIN. 0 corresponds to 0V or OFF and 255 corresponds to 5V or ON. Any intermediate value (e.g. 127) can be seen as a combination of 0V and 5V and then corresponds to e.g. 2.5V.

Visualization

This visualization shows how a PWM value is generated by the Arduino.

Function
 

To use a PWM-enabled pin on the Arduino e.g. To dim an LED, we call the following function:

analogWrite(PIN, value);
Value (0-255) at the specified PIN

Here is a short example that fades an LED

#define LED_PIN 11 // PWM enabled pin
 
void setup()
{
  pinMode(LED_PIN, OUTPUT); 
}
 
void loop()
{
  for(int i=0; i<255; i++)
  {
    analogWrite(LED_PIN, i); // Set the value i on LED_PIN 
    delay(10);
  }
   
  for(int i= 255; i>0; i--)
  {
    analogWrite(LED_PIN, i); // Set the value i on LED_PIN 
    delay(10);
  }
}

Further Information:

PWM - on Arduino.cc
The Secrets of Arduino PWM - on Arduino.cc
analogWrite() - on Arduino.cc
Pulse-Width-Modulation - on Wikipedia.org