Seventy years ago this month, the August 1954 issue of Popular Mechanics showed how to put together this CONELRAD monitor for the ham.  This one was more extravagant than most.  It was hooked into the AVC circuit of a broadcast radio, and would trigger when the station being monitored left the air.  In addition to setting an alarm, the author had it wired to automatically shut down the transmitter and switch to receive.