This circuit is from 1964 documentation but can be easily implemented based on current components. Input waveforms and detection are shown next to the diagram.
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...