push-buttin changing the state of an LED --> https://arduino.stackexchange.com/questions/63343/changing-state-of-an-led-using-a-pushbutton-leads-to-unstable-result

test.ino

const int buttonPin = 2;     
const int ledPin =  7;      

int buttonState = 0;         
int lastButtonState = buttonState;
bool flag = true;
bool flag_ = flag;

volatile byte state = LOW;
volatile byte state_ = state;

void setup() {
  pinMode(ledPin, OUTPUT);
  pinMode(buttonPin, INPUT); digitalWrite(buttonPin, HIGH);
  attachInterrupt(digitalPinToInterrupt(buttonPin), changeState, FALLING);
}

void loop() {
  buttonState = digitalRead(buttonPin);

  if (buttonState == HIGH && lastButtonState != buttonState) {
    flag = !flag;
  }

  if (state != state_ || flag != flag_) {
    digitalWrite(ledPin, flag ? LOW : HIGH);
  }

  flag_ = flag;
  state_ = state;
  lastButtonState = buttonState;
//  delay(5);
}


void changeState() {
  state = !state;
}