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.
July 2016 - It can be obtained from Mouser Electronics new trinkets Adafruit. These trinkets are...
This circuit allows you to read resistors up to 20 M ohms, converting it into voltage. The circuit is...
In fact, this is the circuit also known as “vox”, that is, a relay triggered by sound. This...