I2C keypad door lock with an LCD on an Arduino Uno

// I2C keypad door lock: a 4x4 keypad, a 16x2 LCD and a servo bolt, and the
// keypad and the screen need only two wires between them.
//
// A 4x4 keypad usually takes eight pins. Here a PCF8574 I2C expander (at
// 0x20) scans it, and the LCD's I2C backpack (at 0x27) sits on the same two
// wires, so the whole lock uses A4, A5 and pin 9.
//
//   PCF8574   SDA -> A4  SCL -> A5   P0..P3 -> keypad R1..R4
//                                    P4..P7 -> keypad C1..C4
//   LCD I2C   SDA -> A4  SCL -> A5   (address 0x27)
//   servo     PWM -> 9   VCC -> 5V   GND -> GND
//
// Type the code and press # to open: the servo pulls the bolt back for five
// seconds, then locks again. * clears what you typed. Three wrong codes in
// a row lock the keypad for 30 seconds, with a countdown on the screen.
//
// Things to try. Change CODE to your own, up to eight digits. Change
// OPEN_MS to keep the door open longer.

#include <Wire.h>
#include <I2CKeyPad.h>
#include <LiquidCrystal_I2C.h>
#include <Servo.h>

const char CODE[] = "1234";
const unsigned long OPEN_MS = 5000;
const unsigned long LOCKOUT_MS = 30000;
const uint8_t MAX_TRIES = 3;
const uint8_t LOCKED_ANGLE = 0;
const uint8_t OPEN_ANGLE = 90;
const uint8_t SERVO_PIN = 9;

I2CKeyPad keyPad(0x20);
LiquidCrystal_I2C lcd(0x27, 16, 2);
Servo bolt;

// The keypad's layout, then N for no key and F for a failed read.
char keymap[19] = "123A456B789C*0#DNF";

char entered[9];
uint8_t length = 0;
uint8_t wrong = 0;
bool keyWasDown = false;
enum State { LOCKED, OPEN, LOCKOUT } state = LOCKED;
unsigned long since = 0;
unsigned long lastSecond = 0;

void prompt() {
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print(F("Enter code:"));
  lcd.setCursor(0, 1);
  for (uint8_t i = 0; i < length; i++) lcd.print('*');
}

void lockDoor() {
  bolt.write(LOCKED_ANGLE);
  state = LOCKED;
  length = 0;
  prompt();
  Serial.println(F("locked"));
}

void check() {
  entered[length] = '\0';
  if (strcmp(entered, CODE) == 0) {
    wrong = 0;
    bolt.write(OPEN_ANGLE);
    state = OPEN;
    since = millis();
    lcd.clear();
    lcd.print(F("Access granted"));
    lcd.setCursor(0, 1);
    lcd.print(F("Door open"));
    Serial.println(F("open"));
    return;
  }
  wrong++;
  length = 0;
  Serial.print(F("wrong code "));
  Serial.println(wrong);
  if (wrong >= MAX_TRIES) {
    wrong = 0;
    state = LOCKOUT;
    since = millis();
    lastSecond = 0;
    lcd.clear();
    lcd.print(F("Too many tries"));
    Serial.println(F("keypad locked out"));
    return;
  }
  lcd.clear();
  lcd.print(F("Wrong code"));
  lcd.setCursor(0, 1);
  lcd.print(MAX_TRIES - wrong);
  lcd.print(F(" tries left"));
  delay(1500);
  prompt();
}

void onKey(char key) {
  Serial.print(F("key "));
  Serial.println(key);
  if (key == '*') {
    length = 0;
    prompt();
  } else if (key == '#') {
    if (length > 0) check();
  } else if (key >= '0' && key <= '9' && length < sizeof(entered) - 1) {
    entered[length++] = key;
    lcd.setCursor(length - 1, 1);
    lcd.print('*');
  }
}

void setup() {
  Serial.begin(9600);
  Wire.begin();
  lcd.init();
  lcd.backlight();
  bolt.attach(SERVO_PIN);
  if (!keyPad.begin()) {
    lcd.print(F("No keypad found"));
    Serial.println(F("No PCF8574 at 0x20: check SDA, SCL and its power"));
    while (true);
  }
  keyPad.loadKeyMap(keymap);
  lockDoor();
  Serial.println(F("keypad lock ready"));
}

void loop() {
  // One press is one key: act when a key goes down, then wait for it to come up.
  bool down = keyPad.isPressed();
  if (down && !keyWasDown) {
    char key = keyPad.getChar();
    if (key != 'N' && key != 'F' && state == LOCKED) onKey(key);
  }
  keyWasDown = down;

  if (state == OPEN && millis() - since >= OPEN_MS) {
    lockDoor();
  }
  if (state == LOCKOUT) {
    unsigned long left = (LOCKOUT_MS - (millis() - since) + 999) / 1000;
    if (millis() - since >= LOCKOUT_MS) {
      Serial.println(F("keypad unlocked"));
      lockDoor();
    } else if (left != lastSecond) {
      lastSecond = left;
      lcd.setCursor(0, 1);
      lcd.print(F("Wait "));
      lcd.print(left);
      lcd.print(F(" s  "));
    }
  }
  delay(20);
}