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.
Schematic
Parts
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| 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 |
Notes
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.
