We find this encoding circuit in an American 1980s amateur radio documentation. The circuit can be implemented with equivalent modern components.
This extremely simple circuit was found in an American publication called Electronics Handbook,...
This circuit was obtained from an English magazine from 1981. The magazine no longer exists, but...
This circuit is from a manual of FETs of 1977. The FET can be the BF245 or MPF102 and the relay is of...