It actually consists of a tunable field strength meter. The circuit is of an old documentation and the circuit must be tuned to the frequency to be monitored. The circuit offers the option to use a headset.
With this shield circuit, you can control LEDs from an AND gate or microcontroller. When both...
Found in July 1975, Radio Electronics magazine this circuit can use any operating circuit,...
Found in Popular Electronics from the 90's, this circuit consists of an audio oscillator for Morse...