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.
This circuit is from Radio Electronics from February 1987. The frequency is given by the crystal that...
In this circuit, a light-sensitive SCR or Light Activated SCR determines the operation of an...
This 3-transistor circuit was found in a Mullard manual from 1960. The transistors are germanium of the...