Arduino/build/shared/examples/10.StarterKit/p15_HackingButtons/p15_HackingButtons.ino

/*
  Arduino Starter Kit example
 Project 15  - Hacking Buttons
 
 This sketch is written to accompany Project 15 in the
 Arduino Starter Kit
 
 Parts required:
 batery powered component
 220 ohm resistor
 4N35 optocoupler
 
 Created 18 September 2012
 by Scott Fitzgerald
 
 http://arduino.cc/starterKit
 
 This example code is part of the public domain 
 */

const int optoPin = 2; // the pin the optocoupler is connected to

void setup(){
  // make the pin with the optocoupler an output 
  pinMode(optoPin, OUTPUT);
}

void loop(){
 digitalWrite(optoPin, HIGH);  // pull pin 2 HIGH, activating the optocoupler
                              
 delay(15); // give the optocoupler a moment to activate 
 
 digitalWrite(optoPin, LOW);  // pull pin 2 low until you're ready to activate again
 delay(21000);                // wait for 21 seconds 
}