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.
Capacitor C and resistor R in this circuit determine the time the output remains at the high level...
The purpose of this circuit is to extend the range of applications from a Darlington pair to DC....
Found in an old English publication this circuit uses a dual gate MOSFET. Coils not specified....