Digital Keypad Combination Lock

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This simple circuit is the electronic version of the combination lock. Using the special purpose LS7220 digital lock IC, the circuit allows a 4 digit combination of your choice to activate a relay for a set period of time. This relay can then be used to trigger a lock solenoid, enable a starter button, open a motorized door, or many other tasks that require a momentary signal.


Schematic for combination lock


Total Qty.
C111uF 25V Electrolytic Capacitor
C21220uF 25V Electrolytic Capacitor
R112.2K 1/4W Resistor
Q112N3904 NPN Transistor2N2222
D111N4148 Rectifier Diode1N4001-1N4007
K1112V SPDT RelayAny appropriate relay with 12V coil
U11LS7220 Digital Lock IC
S1-S1212SPST Momentary PushbuttonKeypad (see notes)
HD1112 Position Header


  1. To set the combination, wire the appropriate switches to U1 pins 3, 4, 5 and 6 using the header. For example if S1 was connected to pin 3, S2 to pin 4, S3 to pin 5 and S4 to pin 6, the combination would be 1,2,3,4. Now wire all other unused switches across the header to pin 2 of U1. In this way you can create any 4 digit combination you want. Pin 2 is the reset pin, so connecting all unused keys to it assures that the entire combination must be reentered if an incorrect key is pressed.
  2. When the appropriate combination is entered, the relay is activated for a period of time determined by C1. The 1uF capacitor specified in the parts list will result in an on-time of roughly 5 seconds. Increase the value of C1 to increase this time.
  3. An easy way to make a keypad is to buy 12 PC board mount pushbuttons and then etch a PC board so that the buttons are in 4 rows of 3, similar to a telephone keypad. Place this in a case and then use a label maker or transfer letters to add your numbers to the tops of the pushbuttons. You can also use a pre made keypad but keep in mind that you need a pad which provides an output for each key. Most pads available have the keys connected to provide a row and column signal when they are pressed.

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Digital Keypad Combination Lock
Sunday, September 29, 2013 12:02:01 PM
If i was to put a switch and jumper cable before C1, will this allow me to keep the relay active permanently when the switch is set to bypass (valet mode)? I basically want to have a set up where i punch the code in as normal to activate the relay then after X number of seconds the relay will deactivate Then at certain times like for car servicing i want to punch the code in and keep the relay active permanently so i dont have to give the code to the service manager? hope you can under stand where i am coming from? C|heers, Mike.
Digital Keypad Combination Lock
Tuesday, March 13, 2012 8:50:08 PM
I am new and 1st year 2nd semester. My course teacher give a project of electronic lock. Please somebody help me giving a helpful link. or help me via Facebook. search me at Please help me . I have got only 20 days to complete this project.
rituraj chouhan
Digital Keypad Combination Lock
Sunday, September 06, 2009 2:09:10 AM
its nice for major project
Digital Keypad Combination Lock
Tuesday, June 23, 2009 6:03:47 AM
you can do something similar by connecting the 1st button to the s pin of a SR flip flop. Take the Q output and use an AND gate to combine it with the next button and run the output to another S pin. then using the Q bar outputs use not or nor gates to get a number of high outputs which you can AND together
Digital Keypad Combination Lock
Friday, April 10, 2009 1:19:09 PM
They can also be found on ebay for a reasonable price. There is a kit there that does something similar to this
Digital Keypad Combination Lock
Friday, March 27, 2009 8:05:25 AM
I cant get the LS7220 in any of the shops in kolkata .can you plz suggest an alternate chip for the circuit?
Digital Keypad Combination Lock
Sunday, February 15, 2009 12:04:39 AM
how do i connect my keypad together with an lcd to display the code
john ng'ang'a ngugi
Digital Keypad Combination Lock
Thursday, January 22, 2009 3:20:29 AM
can i get less expensive alternative components to enable a student present the project to the market?bearing in mind the student is not earning but al for expertise.
Digital Keypad Combination Lock
Thursday, January 15, 2009 12:26:47 PM The parts is sold there..... but expensive..... > $19
Digital Keypad Combination Lock
Tuesday, November 04, 2008 8:24:32 AM
how do i connect my keypad together with an lcd to display the code
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