This circuit uses an optical coupler called Raystor created by Raytheon in the 1960s. The circuit uses components from the era that can be changed. The current is low.
If the input voltage falls below 12 V the LED glows up. Other voltages can be monitored by...
ON Semconductor FFSP0665B 650V 6A SiC Schottky Diodes Available through Mouser Electronics, these...
This adjustable circuit from 0 to 200 nA was found in a 90's EDN. The components can still be...