24. 4x4 Matrix Keypad

#include <Keypad.h>

const byte ROWS = 4;
const byte COLS = 4;

char keys[ROWS][COLS] = {
  {'1', '2', '3', 'A'},
  {'4', '5', '6', 'B'},
  {'7', '8', '9', 'C'},
  {'*', '0', '#', 'D'}
};

byte rowPins[ROWS] = {9, 8, 7, 6};
byte colPins[COLS] = {5, 4, 3, 2};

Keypad myKeypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);

void setup() {
  Serial.begin(9600);
  Serial.println("Keypad ready. Press any key...");
}

void loop() {
  char pressedKey = myKeypad.getKey();

  if (pressedKey) {
    Serial.print("Key pressed: ");
    Serial.println(pressedKey);
  }
}

#include <Keypad.h>

// ---- Keypad setup ----
const byte ROWS = 4;  // 4 rows on the keypad
const byte COLS = 4;  // 4 columns on the keypad

// What each button on the keypad represents
char keys[ROWS][COLS] = {
  {'1', '2', '3', 'A'},
  {'4', '5', '6', 'B'},
  {'7', '8', '9', 'C'},
  {'*', '0', '#', 'D'}
};

// Which Arduino pins are connected to the keypad's rows and columns
byte rowPins[ROWS] = {9, 8, 7, 6};
byte colPins[COLS] = {5, 4, 3, 2};

Keypad myKeypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);

// ---- Code / password setup ----
const String secretCode = "1234";  // the correct code
String inputCode = "";             // what the user has typed so far

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  digitalWrite(LED_BUILTIN, LOW);
  Serial.begin(9600);
  Serial.println("Enter code, then press #");
}

void loop() {
  char key = myKeypad.getKey();

  // If no button was pressed, do nothing this round
  if (key == NO_KEY) {
    return;
  }

  // '#' means "check the code now"
  if (key == '#') {
    Serial.println();  // move to a new line in Serial Monitor

    if (inputCode == secretCode) {
      Serial.println("Access Granted!");
      digitalWrite(LED_BUILTIN, HIGH);
      delay(3000);
      digitalWrite(LED_BUILTIN, LOW);
    } else {
      Serial.println("Access Denied!");
      flashErrorLED();
    }

    inputCode = "";  // reset for next attempt
  }
  // Only accept digits 0-9, and only if there's room left
  else if (key >= '0' && key <= '9' && inputCode.length() < 4) {
    inputCode += key;   // add the digit to the code
    Serial.print("*");  // print * to hide the code (use key instead to show it)
  }
}

// Blinks the LED quickly 3 times to show a wrong code
void flashErrorLED() {
  for (byte i = 0; i < 3; i++) {
    digitalWrite(LED_BUILTIN, HIGH);
    delay(100);
    digitalWrite(LED_BUILTIN, LOW);
    delay(100);
  }
}