Debounced Scmitt Trigger Switch

/* Debounced Switch Input
*/

const int buttonPin = 2;          // Button pin
const int ledPin =  7;            // LED pin

int ledState = LOW;

// variables will change:
volatile int buttonState = 0;     // Pushbutton Status Variable

unsigned long previousMillis = 0; // Store last time LED was updated

const long interval = 1000;

void setup() {
  // initialize the LED pin as an output:
  pinMode(ledPin, OUTPUT);
  // initialize the pushbutton pin as an input:
  pinMode(buttonPin, INPUT);
  // Attach an interrupt to the ISR vector
  attachInterrupt(0, pin_ISR, CHANGE);
}

void loop() {
  // Nothing here!
}

void pin_ISR() {
  unsigned long currentMillis = millis();

  if (currentMillis - previousMillis >= interval) {
    // save the last time you blinked the LED
    previousMillis = currentMillis;

    // if the LED is off turn it on and vice-versa:
    if (ledState == LOW) {
      ledState = HIGH;
    } else {
      ledState = LOW;
    }

    // Set LED with the ledState of the variable
    digitalWrite(ledPin, ledState);
  }
}