const uint8_t interruptPin = 2;

void interruptFn() {
  if ( digitalRead(interruptPin) == HIGH ) {
    Serial.println(F("PUMP: ON ⇒ OFF"));  // not a great idea to print in an ISR but for demo purpose
  } else {
    Serial.println(F("PUMP: OFF ⇒ ON"));  // not a great idea to print in an ISR but for demo purpose
  }
}

void setup() {
  pinMode(interruptPin, INPUT_PULLUP);
  Serial.begin(115200); Serial.println();
  attachInterrupt(digitalPinToInterrupt(interruptPin), interruptFn, CHANGE);
}

void loop() {}