This circuit was obtained in an Electronics Now from the 90s. The circuit uses operational amplifiers from the time and the indicator is of the Sonalert type with internal oscillator.
This circuit uses a loop antenna to receive and amplify signals from the 60 kHz long wave band....
The frequency of this oscillator depends on R and C (see math section for electronics). The...
This common operational configuration can be implemented with modern equivalent. Next to the...