A simple electronic combination lock is designed using IC LS 7220. This circuit can be used to activate a relay for controlling (ON & OFF) any device when a preset combination of 4 digits are pressed. The circuit can be operated from 5V to 12V.

The LS7220 IC is a MOS digital lock IC that can recognize up to 5040 different combinations of 4 digit combinations. The 4 digital password combination has to be hard-wire programmed.

Let’s build the Combination Lock Circuit

To set the combination, connect the appropriate switches to pin 3,4,5 and 6 of the IC through the header. As an example, if S1 is connected to pin 3, S2 to pin 4, S3 to pin 5, S4 to pin 6 of the IC, the combination will be 1234. This way we can create any 4 digit combinations. Then connect the rest of the switches to pin 2 of IC. This will cause the IC to reset if any invalid key is pressed, and the entire key code has to be re-entered.

When the correct key combination is pressed the output (relay) will be activated for a preset time determined by the capacitor C1. Here it is set to be 6S. Increase C1 to increase on time. For the keypad, arrange switches in a 3X4 matrix on a PCB. Write the digits on the keys using a marker. Instead of using numbers, I wrote some symbols! The bad guys will be more confused by this.

This electronic combination lock circuit can be further designed with the LS7220 IC to make an Automotive Ignition Anti-Theft Digital Lock.  To build this, additional features that are integrated into the IC must be used. Some of the features include the Sense Input feature which enables the IC operation, the save memory feature, the static or momentary lock output, and the convenience delay feature (controlled by an external capacitor).

The IC output condition is provided to activate and de-activate a relay. The relay can be used to turn ON and OFF both AC and DC devices. The relay driving power can be fully utilized to control an alarm, a small display, a lamp or even an electromagnet. A door can be symbolized as OPEN or CLOSE using a DC motor that turns the door according to condition.

If you are interested in building other related applications, try our digital code lock circuit. You can also try our one transistor code lock application.

Electronic Lock Circuit Diagram

Electronic lock circuit

Pin Assignment of LS7220.

LS 7220 Pin Configuration and Pin Diagram

Electronic Combination Lock Circuit – Components and

C11uF 25V Electrolytic Capacitor1
C2220uF 25V Electrolytic Capacitor1
R12.2K 1/4W Resistor1
Q1 2N3904 NPN Transistor  or 2N22221
D11N4148 Rectifier Diode (1N4001-1N4007)1
K112V SPDT Relay (Any appropriate relay with 12V coil)1
U1LS7220 Digital Lock IC1
S1-S12SPST Momentary Pushbutton  Keypad (see notes)12
HD112 Position Header1

An electronic combination lock comes in safety applications category. This circuit helps you to learn the basics of digital electronics and understand how to design digital circuits. Having a strong hand in digital electronics is very important for all electronics engineers.



  1. hi can u tell me the alternate of ls 7220
    or can u tell me the place

  2. hi can u tell me the alternate of ls 7220 or can u tell me the place

  3. i has not found ic ls 7220 can u tell me another ic in place of this and what chances make on using it? please reply i have to make this

  4. can i just connect an LED at pin 13 to show that the code entered is right ?? will that work ???

  5. hey guys a quick question how i design a electronic lock circuit which rejects an incorect combination then allows 8 second delay before entering a new combination

  6. i m making digital lock bt i m feeling difficulty in finding ls7220 so plz hlp me ?????????????????? thanks

  7. Hi

    I had gone thru ur website and quiet interesting, U r proving the theory part of the electronics content where in we are into giving virtual lab. I am sure the synergy between will help the students/hobby users to learn your theory content and practice as well. Please let me know your good time to talk.

    Bivin Jacob – 9900211303

  8. simply i want to say that the hd1 (header 1 ) is a connector port for the combination lock.

    in a case if you are not sure of perfect lock. you can change the pin combination or pair of pin to make ti difficult to unlock for thief

    that<s my point of view . whats your????????????

  9. what is functionality of position header in “Electronic Combination Lock using IC LS 7220”. Pls give me reply on my E-mail.

  10. Hello, please I want toSimple electronic combination lock circuit
    and circuit pcb
    And please send

  11. hi can u tell me the alternate of ls 7220 or can u tell me the place where i can buy it in karachi pakistan.

  12. I want to make the simple project for digital lock. I am in 3rd semester, webmaster, would you help. I need the help of people

  13. kindly send me the pcb layout for digital lock code using IC LS7220.thank you

  14. www.the-g-man.wen.ru

    is the IC need to programed at fisrt, for entering the right code? and, how to make the analog lock, which it turned by relay? thanks

  15. I too would like a PCB Layout of this project. I have been trying to make one myself but with very little success. Please help.



  17. seetharaman

    Hi Sundeep as per proelectron standards first digit stands for material. B stands for silicon and if it is A it is germanium. The second digit indicates the application C stands low power small signal low frequency transistor. 107 stands for the type number (may be voltage current etc)for details refer manufacturers catalog. the suffic A, B, C stands for small signal current gain of the transistor. A 100 t0 250, B 200 to 450, C 400 to 900.
    The following do not fall on any standards.
    SL 100 & CL100 S stands for the manufacturer Semiconductor India Ltd., C stands for Continental devices India Ltd. L100 stand for medium power NPN transistor. if it is K100 stands for medium power PNP transistor. BEL100N and BEL100P are also similar trasistors.

  18. sandeep b

    what does BC stand for in BC107 SL stands for in SL 100 CL stands for in CL 100

  19. Ilove the circuit. What I need to know is the Header (HD1). What is it? Is it part of the keypad or do I need to buy it?

  20. Hi there,

    Could you please explain what is header (HD1)?

    Thanks in advance.

  21. Bhadrappa.H.R

    please! i have project based on sequence timers.
    could u display a circuit about sequence timers using 555 0r 8051 micro controllers?

  22. Hello admin your site is really interesting and helpful I’m impressed.. : )

  23. pjb students

    please! please! please!
    i have project based on solenoid.
    could u display a circuit about solenoid??
    please again!!

  24. i still have problem with this will you please make it more simpler that would be very much kind of you

  25. moye Alex

    Hello, please I want to know how smart card digital lock system can be realized with possible circuit diagram.

  26. really a valuable circuit, but what is a 12 position header. sorry to ask i am new to electronics.

  27. I would like to have a PCB layout for the digital lock

  28. I am student of 4th year.let me know that i should make project on digital lock code or not???kindly send me the pcb layout for digital lock code.
    thank you

    • Currently I dont have a PCB lay out of this circuit.I tested this circuit on the bread board only.
      You can easily make the PCB by yourself using software like PCB123.Just have a try.
      If you cant make one properly, mail me. I will make one for you.

      regards CT

  29. sivakumar

    good website…….please send me the circuit of instrumentation amplifier………

  30. Алексей

    Самый интересный web сайт о новинках гаджетах, а также тесты мобильных устройств

  31. Patrick Mitchell

    Hi there.
    Just wanted to let everyone know that LS7220 ICs can be purchased over ebay.

  32. Hi hello
    could you please post the circuite diagram of timer with 555(for power equipment)
    and second
    would you please post the circuite (and information ) about motor control( with pc)?

  33. The ckt. used for making an electronic combinational lock is very easy to understand.
    I must congratulate the concerned people for making site user friendly..

  34. sam mendez

    Hi Hello webmaster ! thanks i had been looking for this for a long time.nice work.would you please post the circuit diagram of a fridge door alarm.thanks in advance.