The figure shows how to couple two CMOS circuits using an optical coupler to achieve isolation. The circuit is from a Radio Electronics from August 1992.
This remote control receiver via the power grid appears elsewhere on this site from other sources....
This General Electric circuit is from the 80's. The PUTs used in this meter are not common...
This circuit was found in a documentation from the 50's. The transistor is made of germanium and...