Electronic Combination Lock using IC LS 7220

Electronic Combination Lock Circuit


This is the circuit diagram of a simple electronic combination lock 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.

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 entire key code has to be re entered.

When the correct key combination is pressed the out put ( 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 key pad, 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.

Electronic Lock Circuit Diagram .

Electronic lock circuit

Pin Assignment of LS7220.

LS 7220 Pin Configuration and Pin Diagram
Parts List

C1              1   1uF 25V Electrolytic Capacitor
C2              1    220uF 25V Electrolytic Capacitor
R1              1    2.2K 1/4W Resistor
Q1              1    2N3904 NPN Transistor    2N2222
D1              1    1N4148 Rectifier Diode    1N4001-1N4007
K1              1    12V SPDT Relay    Any appropriate relay with 12V coil
U1             1     LS7220 Digital Lock IC
S1-S12    12    SPST Momentary Pushbutton    Keypad (see notes)
HD1          1    12 Position Header

We have more Electronic Keys & Locks circuits of your interest, Take a look below:

1. One Transistor Code Lock

2. Digital Code Lock

Show Comments

43 thoughts on “Electronic Combination Lock using IC LS 7220

  1. priyanka

    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

  2. ben

    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

  3. Bivin


    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

  4. kartik

    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????????????

  5. Akash

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

  6. be zie

    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

  7. Dale

    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.

  8. 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.


    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?

  10. pjb students

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

  11. veenu

    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

    • admin

      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

  12. milad

    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)?

  13. Aman Ram

    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..

  14. 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.


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