// 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 = LED_BUILTIN;  // the number of the LED pin
  // LED_BUILTIN is set to the correct LED pin independent of which board is used

volatile int state = LOW; // To make sure variables shared between an ISR

void buttonPinISR()
{

  state = !state; //toggle the state when the interrupt occurs
  
}

void setup() {
  // initialize the LED pin as an output:
  pinMode(ledPin, OUTPUT);
  // initialize the pushbutton pin as an input:
  pinMode(buttonPin, INPUT);

  attachInterrupt(digitalPinToInterrupt(buttonPin), buttonPinISR, FALLING);

}

void loop() {

  digitalWrite(ledPin, state); //pin 13 equal the state value

}