This circuit was found in the September 1992 issue of Radio Electronics magazine. The circuit does not use a transistor to drive the relay and has no adjustments.
This circuit operates with frequencies from 0.5 to 400 MHz. We find it in a 90’s NASA...
This circuit was found in a 1968 EEE and can be implemented with any current general-purpose transistor....
This circuit was found in an old Poptronics but is still current as transistors can be replaced...