An encoding circuit is provided permitting energization of an electromagnetic unlocking solenoid if a series of push-button switches is operated in accordance with a predetermined code. The circuit includes a memory which stores the sequence of operation of the push-buttons in accordance with the code...http://www.google.ca/patents/US4149212?utm_source=gb-gplus-sharePatent US4149212 - Electrically encoded, electrically controlled push-button combination lock
