Electronic Combination Lock using IC LS 7220


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Electronic Combination Lock Circuit

Description
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.