Arduino - Keypad Door Lock

In this tutorial, we learn how to make door lock system with password using keypad and solenoid lock.

When the door is unlocked by a correct password, It keeps the door unlocked for 20 seconds, and then automatically lock the door again. The Arduino code supports multiple passwords.

Hardware Required

1×Arduino UNO or Genuino UNO
1×USB 2.0 cable type A/B
1×Keypad 3x4 and 4x4 Kit
1×(Alternative) Keypad 3x4
1×(Alternative) Keypad 4x4
1×Solenoid Lock
1×12V Power Adapter
1×DC Power Jack
1×Jumper Wires
1×(Optional) 9V Power Adapter for Arduino
1×(Recommended) Screw Terminal Block Shield for Arduino Uno
1×(Optional) Transparent Acrylic Enclosure For Arduino Uno

Or you can buy the following sensor kits:

1×DIYables Sensor Kit (30 sensors/displays)
1×DIYables Sensor Kit (18 sensors/displays)
Please note: These are Amazon affiliate links. If you buy the components through these links, We will get a commission at no extra cost to you. We appreciate it.

About Keypad and Solenoid Lock

If you do not know about solenoid lock and button (pinout, how it works, how to program ...), learn about them in the following tutorials:

Wiring Diagram

  • Arduino - Door lock with keypad, solenoid lock
Arduino, keypad, solenoid lock wiring diagram

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Arduino Code - Door lock system with password using keypad, solenoid lock

/* * Created by * * This example code is in the public domain * * Tutorial page: */ #include <Keypad.h> const int RELAY_PIN = A5; // the Arduino pin, which connects to the IN pin of relay const int ROW_NUM = 4; // four rows const int COLUMN_NUM = 4; // four columns char keys[ROW_NUM][COLUMN_NUM] = { {'1','2','3', 'A'}, {'4','5','6', 'B'}, {'7','8','9', 'C'}, {'*','0','#', 'D'} }; byte pin_rows[ROW_NUM] = {9, 8, 7, 6}; //connect to the row pinouts of the keypad byte pin_column[COLUMN_NUM] = {5, 4, 3, 2}; //connect to the column pinouts of the keypad Keypad keypad = Keypad( makeKeymap(keys), pin_rows, pin_column, ROW_NUM, COLUMN_NUM ); const String password_1 = "1234ABC"; // change your password here const String password_2 = "5642CD"; // change your password here const String password_3 = "4545B"; // change your password here String input_password; void setup() { Serial.begin(9600); input_password.reserve(32); // maximum input characters is 33, change if needed pinMode(RELAY_PIN, OUTPUT); // initialize pin as an output. digitalWrite(RELAY_PIN, LOW); // lock the door } void loop() { char key = keypad.getKey(); if (key){ Serial.println(key); if(key == '*') { input_password = ""; // reset the input password } else if(key == '#') { if(input_password == password_1 || input_password == password_2 || input_password == password_3) { Serial.println("The password is correct, unlocking the door in 20 seconds"); digitalWrite(RELAY_PIN, HIGH); // unlock the door for 20 seconds delay(20000); digitalWrite(RELAY_PIN, LOW); // lock the door } else { Serial.println("The password is incorrect, try again"); } input_password = ""; // reset the input password } else { input_password += key; // append new character to input password string } } }


The I2C address of LCD can vary according to the manufacturers. In the code, we used 0x27 that is specified by DIYables manufacturer

Quick Steps

  • Connect Arduino to PC via USB cable
  • Open Arduino IDE, select the right board and port
  • Navigate to the Libraries icon on the left bar of the Arduino IDE.
  • Search “keypad”, then find the keypad library by Mark Stanley, Alexander Brevig
  • Click Install button to install keypad library.
Arduino keypad library
  • Copy the above code and open with Arduino IDE
  • Click Upload button on Arduino IDE to upload code to Arduino
  • Open Serial Monitor
  • Press 12345 keys and press #
  • Press 1234ABC keys and press #
  • See the the lock tongue's state during 20 seconds.
  • See the result on Serial Monitor
The password is incorrect, try again The password is correct, unlocking the door in 20 seconds
Autoscroll Show timestamp
Clear output
9600 baud  

Code Explanation

Valid passwords are pre-defined in the Arduino code.

A string is used to store the input password from users, called input string. In keypad, two keys (* and #) are used for special purposes: clear password and terminate password. The system works as follows:

  • Except for two special keys, if another key is pressed, it is appended to the input string
  • If * is pressed, input string is clear. You can use it to start or re-start inputing the password
  • If # is pressed:
    • The input string is compared with the pre-defined passwords. If it matched with one of the pre-defined passwords, control the relay to unlock the door.
    • No matter the password is correct or not, the input string is clear for the next input

    ※ NOTE THAT:

    • In the above code, To make it simple, we used the delay function. It is better to use millis() instead of delay(). See How to use millis() instead of delay()
    • You can add a piezo buzzer to make the beep sound each time keypad is pressed.
    • In the above codes, the door is locked again after 20 seconds. You can replace it by a door sensor. The door is locked when the door sensor detect the door is closed by user.
    • In the above code, the passwords are hard-coded. In the practice, it should be able to add/delete/change passwords dynamically via a special mode. If so, the passwords should be saved on EEPROM memory. The number of passwords can be saved depends on the EEPROM's size. The code will become complicated. If you want to build such system, we provide a coding service. Please feel free to contact us.

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