This circuit was found in a 1980s documentation, using a comparator from the time. The configuration can also be implemented with comparators or modern operational.
This circuit was found in a 1962 publication. At the time there were high voltage batteries like the...
This preamp version uses BC109 transistors that are older versions of the current BC549 in plastic...
This circuit is suggested by Motorola, using a 4N26 type coupler and an MC1733 type operative. However,...