LED Blink

// Blink an LED
int ledPin = 13; // Digital output pin num 13
int dTime = 500; // this is a delay of one second

int counter = 0; // Keep a counter to change the speed every step
int speedInc = 100; // Store the increase of delay in a variable

void setup()
{
  // Set the digital pin as output
  pinMode(ledPin, OUTPUT);
}

void loop()
{
   digitalWrite(ledPin, HIGH);
  delay(dTime); // Let the light burn for a while
  digitalWrite(ledPin, LOW);
  delay(dTime); // Let the light be turned off for a while

  // Keep the counter for every step in the loop
  counter = counter + 1;
 
  // Every 10 steps switch the speed increment to positive or negative
  if(counter % 10 == 0) {
    speedInc = speedInc * -1;
  }

  // Increase (or decrease) the delay
  // Led will flash slower and longer or faster and shorter
  dTime = dTime + speedInc;
}