Found in a Popular Electronics from the 90s, this circuit can be adapted to use modern operational amplifiers. The output voltage is the square root of the input voltage.
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...