This high sensitivity sensor was found in a 1967 General Electric SCR manual. The circuit admits equivalents by triggering a load according to the used SCR.
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...