This circuit synchronized by the power grid can be used as a shield. We found it in an 80's Electronic Engineering. R1 can typically be 4k7.
This digital instrument input circuit was found in the September 1989 issue of Radio Electronics...
Found in an Electronic Design of 1964, this circuit operates with control voltages from 0 to 12 V...
This two-way switch or control shows how to control a lamp from two points. This version is from a...