int swButton = 12;
int relayPin = 16;

bool relayOn = false;   // Present on/off state of the relay.

int debounceTime = 200;   // Debounce time in ms.

volatile bool switchToggle = false;   // True when button press is valid (and not bounce noise).
volatile unsigned long nowPush = 0;   // The millis() time of the present button press.
volatile unsigned long lastPush = 0;  // The millis() time of the previous button press.
volatile unsigned long timeGap = 0;  // The difference of the nowPush and lastPush times.


// Interrupt Service Routine on button press (falling edge).
ICACHE_RAM_ATTR void swButtonISR() {
  nowPush = millis();
  timeGap = nowPush - lastPush;
  
  // Debouncing stuff, recognizing this button-press as a valid one.
  if (timeGap > debounceTime) {
    switchToggle = true;
    lastPush = nowPush;
  }
}

// Function that toggles the relay when called upon.
void toggleRelay () {
    if (relayOn) {
      digitalWrite(relayPin, LOW);   // Turn off relay.
      relayOn = false;
      Serial.println("Relay ON");
      return;
    }
    else {
      digitalWrite(relayPin, HIGH);    // Turn on relay.
      relayOn = true;
      Serial.println("Relay OFF");
      return;
    }
    return;
}


void setup() {  
  Serial.begin(115200);
  pinMode(swButton, INPUT);
  pinMode(relayPin, OUTPUT);
  //pinMode(greenLED, OUTPUT);
  attachInterrupt(digitalPinToInterrupt(swButton), swButtonISR, FALLING);
}

void loop() {
  if (switchToggle == true) {
    toggleRelay();
    switchToggle = false;
  }
}