This circuit was found in an English publication from the 70s, an article that teaches how to calculate instruments. The circuit is still current and can be built with an analog indicator of 100 uA.
This is one of the many hysteresis comparator circuits the reader may find in this section and on this...
This circuit is from a Radio Electronics from February 1971. The SCR is load controlled and the...
Found a documentation on PCs from 1996, this circuit can be used as a shield. The relay is rated at...