This circuit was found in a 90's Electronic Design. The circuit can be adapted to use modern components. The power supply is made with 5 V which allows its use as a shield sensor.
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...