A heart cam type locking mechanism is provided by which in response to a first push button depressing operation, a moving member is locked at a first position in a casing, the moving member is locked at a second position in response to a second push button depressing operation and the moving member is...http://www.google.ca/patents/US4661667?utm_source=gb-gplus-sharePatent US4661667 - Two-stage locking push switch
