Electronic Combination Lock using IC LS 7220

jacky April 12, 2011 43 Comments

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

  • giri
    November 30, 2014

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

  • naane
    October 9, 2013

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

  • priyanka
    July 5, 2013

    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

  • akanksha
    January 20, 2013

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

  • ben
    November 11, 2012

    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

    October 20, 2012

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

  • October 5, 2012


    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

  • kartik
    March 27, 2012

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

  • Akash
    February 19, 2012

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

  • Salu
    January 15, 2012

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

  • S,A
    November 8, 2011

    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.

  • be zie
    November 6, 2011

    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

  • archana
    October 21, 2011

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

    August 18, 2011

    what is the price of LS 7220…????

  • asichi
    August 12, 2011

    what is HD1..?

  • August 9, 2011

    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

  • Dale
    July 24, 2011

    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.

  • May 24, 2010


  • seetharaman
    May 24, 2010

    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.

    • banu
      August 4, 2013

      Please what is the equivalent of IC 7220

  • sandeep b
    May 23, 2010

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

    May 6, 2010

    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?

  • Harsha
    May 5, 2010

    Hi there,

    Could you please explain what is header (HD1)?

    Thanks in advance.

  • April 12, 2010

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

  • marc
    March 2, 2010

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

  • pjb students
    February 2, 2010

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

  • kashif
    January 20, 2010

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

  • December 30, 2009

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

  • pavan
    December 22, 2009

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

  • October 20, 2009

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

  • veenu
    August 16, 2009

    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
      August 18, 2009

      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

  • sivakumar
    July 31, 2009

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

    • admin
      August 4, 2009

      i will add one soon

  • hadi rizvi
    May 30, 2009

    please send price of IC LS7220 thanks

  • May 27, 2009

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

  • May 12, 2009


  • Patrick Mitchell
    April 12, 2009

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

  • milad
    March 15, 2009

    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)?

    • admin
      April 5, 2009

      i will add them soon

  • Aman Ram
    February 24, 2009

    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..

  • sam mendez
    February 23, 2008

    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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