12C508 Smart Lock: The base sends a data sequence (the password) which is recognized by the key. The key checks the sequence and -if it recognizes it -sends to the base another data sequence, as an answer to the password. At the Time when the base gets a correct answer (which means that the key has been introduced), the relay is put into action, and it can command a number of consumers.

Combination Lock (PIC16F84):

Combination Lock (Simple Version):

Combination Lock: This circuit is very basic to build. To open a the lock which is connected to the K1 Load. you must press each momentary switch in the correct sequence. The sequence used in this circuit is S1,S2,S3,S4. If any of the other switches are pressed the circuit will reset and you will need to start over. Depending on how you wire the switches, you can use any4 switch combination

Digital Combination Lock:

Digital Entry Lock:

Electronic Card Lock System:

Electronic Door Codelock:

Electronic Lock:

Enhanced Alarm Keypad:

Simple Code Lock:

Simple Electronic Lock Uses Single Transistor Circuit: 05/26/03 Electronic Design - Ideas for Design /  Building on the recent Design Brief theme of one-pin keypads, this design uses only one analog microcontroller (MCU) input to scan numerous keys, plus one resistor per key and another resistor and capacitor. The circuit is simply a crude...

Simple Low Cost Digital Code Lock:

Single Button Lock Provides High Security: 03/04/99 EDN-Design Ideas / PDF contains multiple circuits - scroll to find this circuit.