We found this circuit in an old oscilloscope publication. With it, it is possible to observe the B / H curve (hysteresis) of an inductor in a common oscilloscope.
The circuit is simple, as it uses only one transistor and a relay that can be MC2RC1 or MC2RC2,...
We found this circuit in a 1990 documentation. It works with the power of the digital circuit...
The filter shown in the figure can operate with frequencies up to several tens of megahertz thanks...