Combination lock: keypad, servo bolt and buzzer, on an Arduino Uno

// A combination lock: the right four keys open the bolt.
//
//   keypad rows 1..4 -> pins 11, 10, 9, 8   columns 1..4 -> pins 7, 6, 5, 4
//   servo signal -> pin 12   red -> 5V   brown -> GND
//   pin 13 -> 1k -> an NPN transistor's base; the active buzzer from 5V
//   to its collector; emitter -> GND
//
// The sketch keeps the last four keys. After the fourth it compares them
// with the code: right, and the servo turns to open for two seconds; wrong,
// and the buzzer sounds for half a second.
//
// Type 1, 2, 3, 4 on the keypad.

#include <Keypad.h>

const char KEYS[4][4] = {
  {'1', '2', '3', 'A'},
  {'4', '5', '6', 'B'},
  {'7', '8', '9', 'C'},
  {'*', '0', '#', 'D'},
};
byte rowPins[4] = {11, 10, 9, 8};
byte colPins[4] = {7, 6, 5, 4};
Keypad keypad = Keypad(makeKeymap(KEYS), rowPins, colPins, 4, 4);

#include <Servo.h>

const int SERVO_PIN = 12;
const int BUZZER = 13;
const char CODE[4] = {'1', '2', '3', '4'};

Servo bolt;
char entered[4];
byte count = 0;

void beep(int ms) {
  digitalWrite(BUZZER, HIGH);
  delay(ms);
  digitalWrite(BUZZER, LOW);
}

void setup() {
  bolt.attach(SERVO_PIN);
  bolt.write(45);
  pinMode(BUZZER, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: type the code on the keypad");
}

void loop() {
  char key = keypad.getKey();
  if (!key) return;
  beep(100);
  entered[count++] = key;
  if (count < 4) return;
  bool right = true;
  for (int i = 0; i < 4; i++) {
    if (entered[i] != CODE[i]) right = false;
  }
  if (right) {
    Serial.println("right code: open");
    bolt.write(135);
    delay(2000);
    bolt.write(45);
  } else {
    Serial.println("wrong code");
    beep(500);
  }
  count = 0;
}