This high input impedance circuit was found in American documentation from the 1980s. The power supply must be symmetrical. The circuit applies to modern operational.
This circuit triggers the LED when the monitored battery voltage drops below 9 V for 12 V...
We found this circuit in a Radio Electronics from January 1967. The SCR is already the C106, which...
This power source with silicon diodes at a time when valves were still used was found in Radio...