Simple Electronic Combination Lock

The following diagram is a very easy and simple electronic combination lock based on IC LS7220.

electronic combination lock LS7220

Component Part List:

  • C1 = 1uF 25V
  • C2 = 220uF 25V
  • R1 = 2.2K Ohm
  • Q1 = 2N3904 / 2N2222
  • D1 = 1N4148 / 1N4001-1N4007
  • K1 = 12V SPDT Relay / Any appropriate relay with 12V coil
  • U1 = LS7220 Digital Lock IC
  • S1-S12 = SPST Momentary Pushbutton  Keypad (see notes)
  • HD1 = 12 Position Header

Circuit Notes:

This could be the circuit diagram of a easy electronic combination lock by using IC LS 7220.This circuit may be applied to activate a relay for controlling (on & off) any device each time a preset combination of 4 digits are pressed.The circuit may be operated from 5V to 12V.

To set the mixture 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 is going to 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, then the output ( relay) will be activated for a preset time driven by the capacitor C1.Here, it is set to be 6S.Increase the value of C1 to increase the preset time.

For the keypad, arrange the switches in a 3X4 matrix on a PCB (Printed Circuit Board).Write the digits on the keys by using a marker.Instead of applying numbers I wrote some symbols!.The bad guys is going to be more confused with this.

LS7220 Pin Assigment

Download LS7220 datasheet from the following link:


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