// GLOBALS & CONSTANTS
int LED_PIN = 3;
int BTN_PIN = 7;

volatile int btn_val = 1;

void setup() {
  pinMode(LED_PIN, OUTPUT);

  // INPUT_PULLUP necessary for the button, since there’s no external resistor
  pinMode(BTN_PIN, INPUT_PULLUP);

  attachInterrupt(digitalPinToInterrupt(BTN_PIN), toggleLED, FALLING);
}

void loop() {
  
} 

void toggleLED() {
  btn_val ^= 1;
  digitalWrite(LED_PIN, btn_val);
}