The transistors of this circuit found in an English 1980s documentation may be the modern BC548 equivalents. The circuit is battery powered and the earphone must be crystal.
This circuit is from a 1976 documentation. It makes use of isolated gate MOSFET which is already very...
This setting appears multiple times on the site from different sources. This is from a 2000 manual...
This circuit uses a current regulator from the 90's. This component is uncommon nowadays. The...