Of course, the current is extremely small, but a voltage of 300 V can be obtained from 9 V of a battery. The circuit is from an Electronics Now of May 1994. C1 adjusts the operating point.
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...