Indicated for the protection of three-phase systems, this circuit was found in an American documentation of the 70's. The circuit can be implemented with modern 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...