Found in American documentation from 1968, this circuit has an acute cut-off of the frequency to which it is tuned and which depends on RC. Transistors may be modern equivalents of general use.
Mouser has a huge list of sensors for the most diverse applications such as automation, IoT,...
This circuit is suggested by the Texas Instruments Linear Circuits Applications, based on the FET...
We found this circuit in a Motorola publication from the 70s. The circuit can be implemented with...