AutocadHow-ToBeginner · 4 min read

Arduino Project for Password Door Lock: Simple Code & Guide

An Arduino password door lock uses a Keypad to enter a code and a Servo motor to control the lock. The Arduino checks the entered password and opens the door by turning the servo if the password is correct.

📐

Syntax

This project uses the Keypad library to read password input and the Servo library to control the lock mechanism. The main parts are:

Keypad: Reads button presses from a matrix keypad.
Servo: Controls the lock by rotating to open or close positions.
password[]: Stores the correct password sequence.
inputPassword[]: Stores the user's entered password.
checkPassword(): Compares entered password with the correct one.

arduino

#include <Keypad.h>
#include <Servo.h>

const byte ROWS = 4; // Four rows
const byte COLS = 4; // Four columns

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}; // Connect to the row pinouts of the keypad
byte colPins[COLS] = {5, 4, 3, 2}; // Connect to the column pinouts of the keypad

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

Servo lockServo;

char password[5] = {'1','2','3','4', '\0'}; // Correct password
char inputPassword[5]; // Stores user input
byte inputIndex = 0;

bool checkPassword() {
  for (byte i = 0; i < 4; i++) {
    if (inputPassword[i] != password[i]) {
      return false;
    }
  }
  return true;
}

💻

Example

This example shows a complete Arduino sketch for a password door lock. It reads keypad input, checks the password, and moves a servo to unlock or lock the door.

arduino

#include <Keypad.h>
#include <Servo.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 keypad = Keypad( makeKeymap(keys), rowPins, colPins, ROWS, COLS );

Servo lockServo;

char password[5] = {'1','2','3','4', '\0'};
char inputPassword[5];
byte inputIndex = 0;

void setup() {
  Serial.begin(9600);
  lockServo.attach(10); // Servo connected to pin 10
  lockServo.write(0); // Lock closed position
  Serial.println("Enter 4-digit password:");
}

bool checkPassword() {
  for (byte i = 0; i < 4; i++) {
    if (inputPassword[i] != password[i]) {
      return false;
    }
  }
  return true;
}

void loop() {
  char key = keypad.getKey();
  if (key) {
    if (key == '#') { // # to submit password
      if (inputIndex == 4) {
        if (checkPassword()) {
          Serial.println("Password correct. Door unlocked.");
          lockServo.write(90); // Open door
          delay(5000); // Keep door open for 5 seconds
          lockServo.write(0); // Lock door again
          Serial.println("Door locked.");
        } else {
          Serial.println("Wrong password. Try again.");
        }
      } else {
        Serial.println("Enter 4 digits before submitting.");
      }
      inputIndex = 0; // Reset input
    } else if (key == '*') { // * to clear input
      inputIndex = 0;
      Serial.println("Input cleared.");
    } else if (inputIndex < 4) {
      inputPassword[inputIndex] = key;
      inputIndex++;
      Serial.print("Digit entered: ");
      Serial.println(key);
    }
  }
}

Output

Enter 4-digit password: Digit entered: 1 Digit entered: 2 Digit entered: 3 Digit entered: 4 Password correct. Door unlocked. Door locked.

⚠️

Common Pitfalls

Common mistakes include:

Not resetting the input index after a wrong or correct password attempt, causing input errors.
Forgetting to attach the servo to the correct pin or not initializing it properly.
Not debouncing keypad inputs, which can cause multiple readings of one key press.
Using a password length different from the input buffer size, leading to incorrect checks.

Always clear the input after each attempt and verify wiring carefully.

arduino

/* Wrong way: Not resetting inputIndex after wrong password */
if (checkPassword()) {
  // unlock
} else {
  Serial.println("Wrong password.");
  // inputIndex not reset here causes errors
}

/* Right way: Reset inputIndex after every attempt */
if (checkPassword()) {
  // unlock
}
Serial.println("Attempt done.");
inputIndex = 0; // Reset for next input

📊

Quick Reference

Tips for building an Arduino password door lock:

Use a 4x4 matrix keypad for easy input.
Store the password in a fixed-size array.
Use # to submit and * to clear input.
Control a servo motor to lock/unlock the door.
Print messages to Serial Monitor for feedback.

✅

Key Takeaways

Use a keypad and servo with Arduino to create a password door lock.

Check the entered password against a stored code before unlocking.

Reset input after each attempt to avoid errors.

Use '#' to submit and '*' to clear password input.

Provide user feedback via Serial Monitor for easier debugging.