// Pins
const int lockPin = 3;
const int readerPin = 2;

// reader state storage vars
int currentReaderState;
int lastReaderState;

int lockState;

/**
 * Init
 */
void setup()
{
  // Setup ping
  pinMode(lockPin, OUTPUT);
  pinMode(readerPin, INPUT);

  // Close the lock on powerup (LOW = LOCKED)
  digitalWrite(lockPin, LOW);

  // Setup current state of reader (HIGH = DENY)
  currentReaderState = HIGH;
  lastReaderState = HIGH;
}

/**
 * Main loop
 */
void loop()
{
  currentReaderState = digitalRead(readerPin);

  // Readers state is changed
  if (currentReaderState != lastReaderState) {

    // If new state is ALLOW (LOW) then toggle the lock
    if (currentReaderState == LOW) {
      lockState = (lockState == LOW) ? HIGH : LOW;
      digitalWrite(lockPin, lockState);
    }

    // Debounce
    delay(50);
  }

  // Save last state
  lastReaderState = currentReaderState;
}