This circuit uses a crystal phone and can be built with more modern transistors like the BC548. The original circuit is from a 1979 publication and is powered with only two AA or AAA batteries.
This configuration can be found at several points in this section and on this website. With the...
The circuit in the figure indicates when the input voltage exceeds a certain value adjusted by the...
This receiver was obtained in a documentation from the 90's and is of the super-regenerative type....