Tuesday, August 3, 2010

Electronic Combination Lock

Simple Electronic Combination Lock using IC LS 7220


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.

 















  




Parts List

C1 1 1uF 25V Electrolytic Capacitor

C2 1 220uF 25V Electrolytic Capacitor

R1 1 2.2K 1/4W Resistor

Q1 1 2N3904 NPN Transistor 2N2222

D1 1 1N4148 Rectifier Diode 1N4001-1N4007

K1 1 12V SPDT Relay Any appropriate relay with 12V coil

U1 1 LS7220 Digital Lock IC

S1-S12 12 SPST Momentary Pushbutton Keypad (see notes)

HD1 1 12 Position Header









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