Based on Radio Shack 276-137, this circuit was found in the December 1995 issue of Electronics Now magazine. The circuit drives an LED but could use a relay instead.
We found this circuit in the 1983 Radio Electronics Special projects magazine. The components are...
With the presented circuit it is possible to detect signals from transmitters in a frequency range...
This circuit was found in a British documentation from 1969. It can be implemented with modern...