I have hooked up a secondary (12 volt, low-power) monitor to an occasional RF output. Currently, the monitor is always on, however, I'd like to be able to automatically switch the monitor off after a period of RF inactivity. I'm looking for a circuit design that will monitor the RF signal, and if present, switch (12v) power on; if not present, switch power off. Any simple circuit that I can construct, or is there anything out there pre-made? Thanks for any info.