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

Parts

Part	Total Qty.	Description	Substitutions
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

Notes

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.

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.

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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Comments

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rituraj chouhan Digital Keypad Combination Lock Sunday, September 06, 2009 2:09:10 AM

its nice for major project

catprog 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

Patrick 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

SAN 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?

anonymous 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.

HappyMan Digital Keypad Combination Lock Thursday, January 15, 2009 12:26:47 PM

http://www.abra-electronics.com/products/product.php?productid=1395&cat=0&page=1 The parts is sold there..... but expensive..... > $19

anonymous 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

ielle Digital Keypad Combination Lock Tuesday, August 12, 2008 5:52:57 AM

i just want to ask, where do you buy a sample of the LS7220 Digital Lock IC chip.

RJL Digital Keypad Combination Lock Tuesday, May 06, 2008 11:48:34 AM

is there any substitute for LS7220 ?

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rituraj chouhan	Digital Keypad Combination Lock	Sunday, September 06, 2009 2:09:10 AM
its nice for major project
catprog	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
Patrick	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
SAN	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?
anonymous	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.
HappyMan	Digital Keypad Combination Lock	Thursday, January 15, 2009 12:26:47 PM
http://www.abra-electronics.com/products/product.php?productid=1395&cat=0&page=1 The parts is sold there..... but expensive..... > $19
anonymous	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
ielle	Digital Keypad Combination Lock	Tuesday, August 12, 2008 5:52:57 AM
i just want to ask, where do you buy a sample of the LS7220 Digital Lock IC chip.
RJL	Digital Keypad Combination Lock	Tuesday, May 06, 2008 11:48:34 AM
is there any substitute for LS7220 ?