This circuit, which can be used as a shield, was found in General Electric documentation from the 1980s. The coupler uses a photo-SCR. The circuit admits equivalent components.
Found in a 1965 Radio Shack documentation this oscillator can be used with modern general purpose...
In the figure we have the way to use a single control voltage to act on two varicaps...
Found in a 90s documentation, this switch that is also sensitive enough to operate capacitively uses...