This example demonstrates how to debounce an input, which means checking twice in a short period of time to make sure it's definitely pressed. Without debouncing, pressing the button once can appear to the code as multiple presses. Makes use of the millis() function to keep track of the time when the button is pressed.

Arduino Board momentary button or switch 10K ohm resistor breadboard hook-up wire Circuit

image developed using Fritzing. For more circuit examples, see the Fritzing project page

Schematic

click the image to enlarge

The code below is based on Limor Fried's version of debounce, but the logic is inverted from her example. In her example, the switch returns LOW when closed, and HIGH when open. Here, the switch returns HIGH when pressed and LOW when not pressed.

/* 
 Debounce
 
 Each time the input pin goes from LOW to HIGH (e.g. because of a push-button
 press), the output pin is toggled from LOW to HIGH or HIGH to LOW.  There's
 a minimum delay between toggles to debounce the circuit (i.e. to ignore
 noise).  
 
 The circuit:
 * LED attached from pin 13 to ground
 * pushbutton attached from pin 2 to +5V
 * 10K resistor attached from pin 2 to ground
 
 * Note: On most Arduino boards, there is already an LED on the board
 connected to pin 13, so you don't need any extra components for this example.
 
 
 created 21 November 2006
 by David A. Mellis
 modified 30 Aug 2011
 by Limor Fried
 modified 28 Dec 2012
 by Mike Walters
 
 This example code is in the public domain.
 
 http://www.arduino.cc/en/Tutorial/Debounce
 */
 
// constants won't change. They're used here to 
// set pin numbers:
const int buttonPin = 2;    // the number of the pushbutton pin
const int ledPin = 13;      // the number of the LED pin
 
// Variables will change:
int ledState = HIGH;         // the current state of the output pin
int buttonState;             // the current reading from the input pin
int lastButtonState = LOW;   // the previous reading from the input pin
 
// the following variables are long's because the time, measured in miliseconds,
// will quickly become a bigger number than can be stored in an int.
long lastDebounceTime = 0;  // the last time the output pin was toggled
long debounceDelay = 50;    // the debounce time; increase if the output flickers
 
void setup() {
  pinMode(buttonPin, INPUT);
  pinMode(ledPin, OUTPUT);
 
  // set initial LED state
  digitalWrite(ledPin, ledState);
}
 
void loop() {
  // read the state of the switch into a local variable:
  int reading = digitalRead(buttonPin);
 
  // check to see if you just pressed the button 
  // (i.e. the input went from LOW to HIGH),  and you've waited 
  // long enough since the last press to ignore any noise:  
 
  // If the switch changed, due to noise or pressing:
  if (reading != lastButtonState) {
    // reset the debouncing timer
    lastDebounceTime = millis();
  } 
 
  if ((millis() - lastDebounceTime) > debounceDelay) {
    // whatever the reading is at, it's been there for longer
    // than the debounce delay, so take it as the actual current state:
 
    // if the button state has changed:
    if (reading != buttonState) {
      buttonState = reading;
 
      // only toggle the LED if the new button state is HIGH
      if (buttonState == HIGH) {
        ledState = !ledState;
      }
    }
  }
 
  // set the LED:
  digitalWrite(ledPin, ledState);
 
  // save the reading.  Next time through the loop,
  // it'll be the lastButtonState:
  lastButtonState = reading;
}