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Arduino Door Lock with keypad + Solenoid / Relay and changeable code



       Hi, and welcome to this tutorial, today I’m making a door lock project based on a Arduino board, keypad, LCD i²c screen, and I’ll use for the lock a solenoid and a relay, you can check my tutorial about the finger print door lock where I used a lock that uses a dc motor, so it depends on your lock system you’ll have to chose a wiring, codes and adapt them.

      N.B: For a real project, I don’t recommend using a solenoid at all, but rather hack a lock that can be opened by both electronic and mechanic lock and adapt your project for it.



So for this project we will need those components, alongside some jump wires, and a power supply of 12V Sorry I didn’t add it here:


The Push Button is meant to open the lock from inside, you can remove it if you want, the resistor is for debounce.

I used 4×4 Keypad you can use 3×4 but you’ll need to modify somethings in the code like for confirmation I use ‘A’ you can change it to ‘*’ or ‘#’.

The solenoid is powered by 12V external power supply, and driven by the IRF510N MOSFET transistor.

The Transistor is used as a switch and it’s better to use an N-channel, the IRF510N is pretty popular when used with an Arduino, when you apply a 5V voltage across the Gate and the Source, the transistor becomes like a closed switch between the Drain and the Source, and it doesn’t need any resistor like bipolar ones.

And if there’s no voltage applied the transistor acts like an open switch, and this how we control the solenoid.

For the other example I’m using a 1 channel  relay module, it works with 3.3V, and we control its input like controling the transistor, the only difference is that they are inverted (we’ll see in the code).

Algorithm flowchart

To make things easy to understand here’s the flowchart, it’s not completly detailed but it’s an overview of the code.

Untitled Diagram.png
Lock algorithm diagram



Wiring 1: For the solenoid use

Wiring 1
Solenoid keypad lock

Wiring 2: For the relay use

Wiring 2

Relay keypad lock


N.B: You can use the relay to control any electric lock up to 250VAC, you can use it also to control the Solenoid…


The only library I used is the LCD i²c NewLiquidCrystal: Download here
Keypad and EEPROM libraries are already installed.


Here you can download the codes I’ve created for the project:

The codes are exactly the same the only thing is that you switch between (LOW and HIGH) to open the lock.

Just remember first time you must upload the code and change the passcode then uncomment some lines (read the code to find them (they’re in setup)) and reupload the code so it can read the passcode from the EEPROM.


Test 1
Standby mode – waiting for keys or push button
Test 2
If you press the ‘*’ key it asks you to type the code
Test 3
Enter the correct code and press A to confirm
Test 4
A welcome message will show otherwise a wrong one
Test 5
System goes back to standby mode
Test 6
when you press ‘#’ it asks you to type the old code to change it
Test 7
Enter the old code and A to confirm it
Test 8
Old code is correct now you can change it
Test 9
Enter the new code and press A
Test 9_1.jpg
Confirm the new code again and press A
Test 10
If codes are matching it changes and stores in the EEPROM
Test 11
Otherwise the codes doesn’t change

Well that was the test of the project, it’s the same for the solenoid or relay, and the push button opens from the inside if you want or you can remove it.

I hope it’s useful and if you have any problem or question leave a comment or contact my FB page…

Yassine View All

Automation and Electrical Engineer, Electronics amateur trying to share my little projects.

3 thoughts on “Arduino Door Lock with keypad + Solenoid / Relay and changeable code Leave a comment

    • You can make 4 initial passcodes, and before you can change anyone of them you ask the user which one to changes ( you can do it by a switch case and test on the number entred 1-4, other than that is an error), then when getting the passcode you can compare it with the 4 codes and open the correspondant lock…

      It just need some re arrangement, and add few lines.


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