The main feature of this circuit found in an old American publication is that it is powered by the very signal it converts. The transistors are of the time and can be replaced by current types.
This circuit that can only be used with simple resistive and inductive loads (never electronic) was...
This digital instrument input circuit was found in the September 1989 issue of Radio Electronics...
This circuit was found in an 80s documentation. The integrated circuit used is from that time,...