This simple circuit is the electronic version of the combination lock. Using the special purpose LS7220 digital lock IC, the circuit allows a 4 digit combination of your choice to activate a relay for a set period of time. This relay can then be used to trigger a lock solenoid, enable a starter button, open a motorized door, or many other tasks that require a momentary signal.
C1 1uF 25V Electrolytic
C2 220uF 25V Electroytic
R1 2.2K 1/4W
Q1 2N3904 NPN
K1 12V SPDT Relay
U1 LS7220 Digital Lock IC
S1 to S12 SPST Momentary Pushbutton
HD1 12 Position Header
- To set the combination, wire the appropriate switches to U1 pins 3, 4, 5 and 6 using the header. For example if S1 was connected to pin 3, S2 to pin 4, S3 to pin 5 and S4 to pin 6, the combination would be 1,2,3,4. Now wire all other unused switches across the header to pin 2 of U1. In this way you can create any 4 digit combination you want. Pin 2 is the reset pin, so connecting all unused keys to it assures that the entire combination must be reentered if an incorrect key is pressed.
- When the appropriate combination is entered, the relay is activated for a period of time determined by C1. The 1uF capacitor specified in the parts list will result in an on-time of roughly 5 seconds. Increase the value of C1 to increase this time.
- An easy way to make a keypad is to buy 12 PC board mount pushbuttons and then etch a PC board so that the buttons are in 4 rows of 3, similar to a telephone keypad. Place this in a case and then use a label maker or transfer letters to add your numbers to the tops of the pushbuttons. You can also use a pre made keypad but keep in mind that you need a pad which provides an output for each key. Most pads available have the keys connected to provide a row and column signal when they are pressed.