Digital Code Lock using Arduino

Digital code lock systems are most common on security systems. An electronic lock or digital lock is a device which has an electronic control assembly attached to it. They are provided with an access control system. This system allows the user to unlock the device with a password. The password is entered by making use of a keypad. The user can also set his password to ensure better protection. The major components include a keypad, LCD and the controller Arduino. This article describes the making of an electronic code lock using arduino.

What you will learn?
  1. How to connect keypad and LCD to arduino?
  2. How to take keypad input?
  3. Comparing keypad input?
  4. Limiting the input?
  5. How to make complete digital code lock application?
Components Required
  1. Arduino Uno
  2. 16×2 LCD Display
  3. 4×4 keypad
  4. Relay
  5. 1K Resistors Qty. 3
  6. BC548
  7. LEDs

Digital Code Lock Circuit

Code lock circuit is constructed around arduino uno, using LCD and keypad. LCD and keypad forms the user interface for entering the password and displaying related messages such as “Invalid password”, “Door open”, etc. Two LEDs are provided to indicate the status of door whether it is locked or open.

Arduino Code Lock Circuit
Arduino Code Lock Circuit

Digital Code Lock Arduino Code

                Program is constructed using two libraries “LiquidCrystal” and “Keypad”. Program have different modules, Setup, Loop, Lock. In setup we initialize all the IO connections and LCD, Keypad. In main loop we are taking pressed keys in array “code[]”, Once the four digits are entered we stop accepting keys. We are using numeric keys and ‘C’ , “=” key. ‘C’ key is used to lock or clear the display in case wrong password is entered. We can hide the entered password by putting  Star character ‘*’.

                After entering password ‘=’ key acts as ok. If password is correct door is kept unlocked for few seconds. If it is incorrect message will be displayed.

   Digital Code Lock Demo
#include <Keypad.h>
#include <LiquidCrystal.h>

// initialize the library with the numbers of the interface pins
LiquidCrystal lcd(9, 8, 7, 6, 5, 4);

const byte ROWS = 4; //four rows
const byte COLS = 4; //four columns
//define the cymbols on the buttons of the keypads
char hexaKeys[ROWS][COLS] = {
byte rowPins[ROWS] = {3, 2, 19, 18}; //connect to the row pinouts of the keypad
byte colPins[COLS] = {17, 16, 15, 14}; //connect to the column pinouts of the keypad

//initialize an instance of class NewKeypad
Keypad customKeypad = Keypad( makeKeymap(hexaKeys), rowPins, colPins, ROWS, COLS); 

const int LED_RED=10; //Red LED
const int LED_GREEN=11; //Green LED
const int RELAY=12; //Lock Relay or motor

char keycount=0;
char code[4]; //Hold pressed keys
//                  SETUP
void setup(){
   // set up the LCD's number of columns and rows: 
  lcd.begin(16, 2);
  // Print a message to the LCD.
  lcd.print("Password Access:");
  lcd.setCursor(0,1); //Move coursor to second Line
 // Turn on the cursor
  digitalWrite(LED_GREEN,HIGH);  //Green LED Off
  digitalWrite(LED_RED,LOW);     //Red LED On
  digitalWrite(RELAY,LOW);       //Turn off Relay (Locked)
//                  LOOP
void loop(){
  char customKey = customKeypad.getKey();

  if (customKey && (keycount<4) && (customKey !='=') && (customKey !='C')){      
      //lcd.print(customKey); //To display entered keys
      lcd.print('*');    //Do not display entered keys
  if(customKey == 'C')      //Cancel/Lock Key is pressed clear display and lock
    Lock();    //Lock and clear display
  if(customKey == '=')   //Check Password and Unlock
    if((code[0]=='1') && (code[1]=='2') && (code[2]=='3') && (code[3]=='4'))  //Match the password
      digitalWrite(LED_GREEN,LOW);  //Green LED Off
      digitalWrite(LED_RED,HIGH);     //Red LED On
      digitalWrite(RELAY,HIGH);       //Turn on Relay (Unlocked)    
      lcd.print("Door Open       ");
      delay(4000);      //Keep Door open for 4 Seconds
      lcd.print("Invalid Password");  //Display Error Message
      delay(1500); //Message delay

//                  LOCK and Update Display
void Lock()
    lcd.print("Door Locked     ");
    lcd.print("                "); //Clear Password
    digitalWrite(LED_GREEN,HIGH);  //Green LED Off
    digitalWrite(LED_RED,LOW);     //Red LED On
    digitalWrite(RELAY,LOW);       //Turn off Relay (Locked)


This code demonstrates how to construct digital code lock and its application using arduino.

We have used almost all the IO lines of arduino, now you know that analog lines have digital numbers from 14 to 19.

6 thoughts on “Digital Code Lock using Arduino

  1. I have successfully compiled and uploaded your code to an Uno and a Mega2650 with no problem. I am using the latest library releases. However, when I try to compile the code for a Nano, which is my choice to use in this situation, I get the following errors:

    C:\Users\STEVE_~1\AppData\Local\Temp\ccZH18FF.ltrans0.ltrans.o:(.rodata+0x10): undefined reference to `Print::write(unsigned char const*, unsigned int)’

    c:/users/steve_000/appdata/local/arduino15/packages/arduino/tools/avr-gcc/4.9.2-atmel3.5.4-arduino2/bin/../lib/gcc/avr/4.9.2/../../../../avr/lib/avr5/crtatmega328p.o:(.init9+0x0): undefined reference to `main’

    collect2.exe: error: ld returned 1 exit status

    Any ideas for me?

    Thanks. I’ve learned a lot.

    Steve Fowler

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