Arduino Password Door Lock System – DIY Project

Building a password-protected door lock system with Arduino is a great beginner-to-intermediate electronics project. You will learn how to create a secure digital door lock using an Arduino, a 4×4 keypad, an I2C LCD, a buzzer, and a solenoid door lock.

The system allows a user to enter a password through the keypad. If the password is correct, the door lock opens for a few seconds. If the password is incorrect multiple times, the system temporarily blocks access and activates a buzzer alarm.

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📦 Components Required

To build this project, you will need the following components:

• Arduino Uno (or compatible board)
• LCD Display 16×2   (with I2C adapter – highly recommended to save pins)
• 4×4 Keypad Matrix
• Solenoid Door Lock
• Relay Module or NPN Transistor (to drive the lock)
• Buzzer (Optional)
• Jumper wires
• Breadboard
• Power supply

I highly recommend getting a complete starter kit that includes everything you need to create a wide range of projects.

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🔌 Circuit Connections

Here’s how to connect all the components:

Keypad Connections:

• Row pins (9,8,7,6) → Arduino pins 9,8,7,6
• Column pins (5,4,3,2) → Arduino pins 5,4,3,2

I2C LCD Connections:

• SDA → Arduino A4
• SCL → Arduino A5
• VCC → 5V
• GND → GND

Output Devices:

Component	Arduino Pin
Solenoid Lock (via relay/transistor)	11
Buzzer	12

⚠️ Important:
The solenoid lock must NOT be powered directly from the Arduino pin. Always use a relay module or transistor with external power.

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💻 Arduino Code

Here is the complete Arduino code for the password door lock system.

// Include LCD display library for I2C
#include <LiquidCrystal_I2C.h>
// Include Keypad library
#include <Keypad.h>

// Password Length
const int Password_Length = 7;
// String to hold password input
String Data;
// Put The Right Password
String Master = "1234567";

// Pin connected to lock (relay OR Transistor)
const int lockOutput = 11;

// Pin connected to Buzzer
const int buzzerPin = 12;

// Counter for character entries
byte data_count = 0;

// Counter for number of tries
byte tries_count = 0;

// Character to hold key input
char customKey;

// Constants for row and column sizes
const byte ROWS = 4;
const byte COLS = 4;

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

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

// Create keypad object
Keypad customKeypad = Keypad(makeKeymap(hexaKeys), rowPins, colPins, ROWS, COLS);

// Create LCD object : Use 0x27 If 0x3F Doesn't work
LiquidCrystal_I2C lcd(0x3F, 16, 2);

void setup() {
  // Setup LCD with backlight and initialize
  lcd.init();
  lcd.backlight();

  // Set lockOutput, buzzer as an OUTPUT pin
  pinMode(lockOutput, OUTPUT);
  pinMode(buzzerPin, OUTPUT);
}

void loop() {

  // Initialize LCD and print
  lcd.setCursor(0, 0);
  lcd.print("Enter Password:");

  // Look for keypress
  customKey = customKeypad.getKey();
  if (customKey) {
    // Enter keypress into array and increment counter
    Data += customKey;
    lcd.setCursor(data_count, 1);
    lcd.print(Data[data_count]);
    data_count++;

    // Key Press Sound Effect
    digitalWrite(buzzerPin, HIGH);
    delay(50);
    digitalWrite(buzzerPin, LOW);
  }

  // See if we have reached the password length
  if (data_count == Password_Length) {
    tries_count++;
    lcd.clear();

    if (Data == Master) {
      // Correct Password
      lcd.print("Correct Password");
      lcd.setCursor(0, 1);
      lcd.print("Please Enter Sir");
      delay(500);
      // Open Door Lock for 5 seconds
      digitalWrite(lockOutput, HIGH);
      delay(5000);
      digitalWrite(lockOutput, LOW);

      //Reset tries_count
      tries_count = 0;
    } else {
      // Incorrect Password
      if (tries_count >= 3) {
        for (int i = 0; i < 20; i++) {
          lcd.clear();
          lcd.print("Try in: " + String(20 - i));
          lcd.setCursor(0, 1);
          lcd.print("Seconds");

          digitalWrite(buzzerPin, HIGH);
          delay(500);
          digitalWrite(buzzerPin, LOW);
          delay(500);
        }
        //Reset tries_count
        tries_count = 0;

      } else {
        //Wrong Password Sound Effect
        digitalWrite(buzzerPin, HIGH);
        delay(500);
        digitalWrite(buzzerPin, LOW);

        lcd.print("Wrong Password");
        delay(1500);
      }
    }

    // Clear data and LCD display
    lcd.clear();
    clearData();
  }
}

void clearData() {
  //Reset data_count
  data_count = 0;
  //Reset Data
  Data = "";
}

🎯 Testing and Troubleshooting

Libraries Used:

Two libraries are used in this project.

• Keypad Library → reads keypad button presses.
• LiquidCrystal_I2C Library → controls the LCD through the I2C interface.

Before uploading, make sure to install these libraries from the Arduino Library Manager:

Uploading the Code:

1. Connect your Arduino board to your computer.
2. Open Arduino IDE.
3. Install the required libraries.
4. Copy and paste the code.
5. Select your board and port.
6. Click Upload.

Once uploaded, you can attach the solenoid lock to your door and start testing.

Common Issues and Solutions:

1. LCD not displaying anything?
   • Check the I2C address (try 0x27 instead of 0x3F)
   • Verify SDA and SCL connections
   • Adjust the potentiometer on the I2C module for contrast
2. Keypad not responding?
   • Double-check row and column pin assignments
   • Ensure no loose connections
   • Test each key with a simple sketch first
3. Solenoid not locking?
   • Verify external power supply (12V for solenoid)
   • Check relay connections
   • Ensure the transistor/relay can handle the current

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🧠 Code Explanation

1️⃣ Password Storage:

The right password is stored in the variable:

String Master = "1234567";

You can change this password to anything you want.

2️⃣ Reading Keypad Input:

The keypad reads user input using:

  // Look for keypress
  customKey = customKeypad.getKey();

Every pressed key is added to the Data string.

3️⃣ Password Verification:

Once the entered characters reach the password length:

if (Data == Master)

The system compares the entered password with the stored one.

If the password is correct ✅:

digitalWrite(lockOutput, HIGH);
delay(5000);
digitalWrite(lockOutput, LOW);

The solenoid lock activates for 5 seconds, allowing the door to open.

If the Password is wrong 🔄:

• Failed Attempt Tracking: The system counts incorrect attempts using tries_count
• Lockout Mechanism: After 3 failed attempts, the system enters a 20-second cooldown period
• Audible Feedback: The buzzer provides different sounds for keypresses, correct/incorrect passwords, and lockout warnings

4️⃣ User Interface:

The LCD provides clear feedback:

• “Enter Password:” prompt on the first line
• Asterisks (*) show as you type (though in this version, it shows the actual characters)
• Success or failure messages
• Countdown timer during lockout

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🚀 Possible Improvements

Once you have the basic system working, try these interesting modifications:

1. Add an EEPROM to store multiple user passwords
2. Include an RFID reader as a secondary authentication method
3. Add a real-time clock for time-based access
4. Create a logging system to track access attempts
5. Add a Bluetooth module for remote control and password changes

Final Thoughts

This Arduino password door lock system is an excellent project that combines multiple concepts: user input, display output, password authentication, and physical actuation. It’s practical, expandable, and teaches valuable lessons about security system design.

The code is well-structured and includes important security features like the lockout mechanism after failed attempts. Whether you’re building this for a school project, to secure a cabinet, or just for learning, it’s a rewarding build that results in a functional security device.

Happy building, and stay secure!

Projects like this are also a great introduction to smart home systems.