Keypad Combination Lock

Article :  Andy Collinson
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A digital combination lock using a push button keypad. The combination can be changed and the lock has an entry delay. keypad combination lock circuit 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:
LS7220 pinout 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:
keypad 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

1 See under US Component Suppliers on my links page.

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