Keypad Combination Lock


Description
A digital combination lock using a push button keypad. The combination can be changed and the lock has an entry delay.

Keypad Combination Lock

Notes
When wired to a ten-digit keypad, the circuit will recognize one 4 digit combination out of a possible 5040 codes.
When the programmed four-digit combination is entered from the keypad, in proper sequence, LOCK turns on and Lock Status turns off. The lock combination is set by pins 3,4,5 and 6 on the LS7220. These correspond to the digits on
the keypad. So if for example you wanted to set a combination of 8462, wire key 8 to pin3, key 4 to input 4, key 6 to input 5 and key 2 of the keypad to input 6. Any unused keys are wired to pin 2. An input on pin 2 resets the lock combination.

The IC pinout for the LS7220 is shown below:
7220pinout

The keypad must have individual connections for each key and a common return key. The return key is wired to pin 12 of the LS7220. The alternative is to wire a group of switches in parallel.

A suitable keypad is shown below:
12keypad

This keypad is available at AllElectronics.com1 part number KP-12. Once the correct combination has been entered, output 13 goes high, and Q1 will energize the relay for a period of time set by C1. A value of 1uF gives an on-time of roughly 5 seconds. Increase C1’s value for longer time delays. The Red LED serves as a lock indicator and will also light for the duration the relay is energized.

Supplies of the LS7220 IC can be found on EBay.

See AllElectronics.com under US Component Suppliers on my links page.

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