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 switchs are pressed the circuit will reset and you will need to start over.
Depending on how you wire the switchs, you can use any 4 switch combination
Reference : ?