A transmitter (20) is provided for transmitting a command signal, the transmitter including a rolling code generator (26) operable in response to a control logic (22) to provide a code forming at least part of said transmitted signal. The code is automatically changeable at intervals. A receiver (30) is provided which receives the transmitted signal for operating the lock (37). The receiver also includes a rolling code generator (34) operable in response to a control logic (35) to provide a code associated with or related to the transmitted code to facilitate operation of the security device (37). The code in the receiver (30) is also automatically changeable at intervals substantially in synchronism with the timing of code changes in the transmitter (20). The receiver is arranged to operate the security device even if the codes in the transmitter and receiver are up to a certain number of steps out of synchronism, that number increasing with time.