Electronic Combination Lock circuit using LS7220


This is the circuit diagram of a simple electronic combination lock using IC LS 7220.This circuit uses a relay for controlling any type of device when the combinations of four digits are pressed.Keypads are used as input here to enter the digits and the correct combination will activate the relay and the door or deice can be operated.


The combination can be set here by connecting the appropriate keys to the pins 3,4,5 and 6 of the IC through Header.Unselected keys are connected to the pin 2.For example if 4568 is your combination then it is to be connected to 3,4,5 and 6th pins and rest of the keys are connected to pin 2.Thus any four key combination can be created using this.

The 2nd pin of this IC was reset so whenever any other keys connected to pin 2 is pressed it will reset the device and the digits have to be re-entered.When the correct combination is pressed the output will be activated for a preset time determined by the capacitor C1.Increase C1 value to increase on time.Using this circuit you can really construct a high efficient lock system.

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Frank Donald

Frank Donald

Junior Network Engineer at Tata Communications
Frank Donald is an Electronics and Communication Engineer who loves building stuff in his free time. Video games, blogging and programming are the things he loves most
Frank Donald

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Cell Buster

Thank you for posting the great content…I was looking for something like this…I found it quiet interesting, hopefully you will keep posting such blogs… cell phone detection



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