23 which is connected to receive power from battery 17 and has access to the power stored thereby invertor 23 may also be of conventional design which generally includes an oscillator operating at a predetermined frequency and driven circuitry for switching battery power through a transformer the output of which is the substitute mains voltage on conductors 22 during normal operation when mains voltage is present at input 13 power switching unit 21 is controlled to connect the voltage from input 13 to the barrier movement controller 12 alternatively when line active detector 19 senses an interruption of mains voltage at terminal 13 a signal is sent to an enable circuit 25 and power switch 21 is controlled to connect substitute mains power when present from invertor 23 and conductors 22 to barrier movement control 12 by operation discussed below no such substitute power is present until additional signals are provided to the enable circuit 25 in the present embodiment such additional signals are generated by the barrier movement control 12 barrier movement controller 12 responds to user requests by opening and closing a barrier such as a garage door in the embodiment of fig1 user requests are directed to the barrier movement control 12 by a portable rf transmitter 27 and a wired wall control unit 29 as is described in detail in the aforementioned u s pat no 6 025 785 a user can press a button 31 on transmitter 27 to cause a security code to be rf transmitted the barrier movement control 12 which is shown in block diagram form in fig4 includes an antenna 35 which is connected to an rf receiver 37.