This is the circuit for the HM-10ª, an instrument sold by Heathkit. The circuit is from a Radio Electronics from May 1963. The indication is by an analog instrument.
This circuit was found in American documentation from 1966, but it can be assembled with a modern...
This circuit consists of a bistable touch switch with the known CMOS 4013 double flip-flop. The...
This bistable and monostable Vox circuit was found in a 2002 documentation. The 556 is a double 555...