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.
We found this configuration of band rejector filter or band rejector with operational 741 in a...
This oscillator can use any CMOS inverter. The circuit is crystal controlled and was found in the...
This circuit was found in a 1967 US documentation. It shows how to build a gain control by digital...