This circuit was suggested by General Electric in 1970 documentation. It makes use of a photo-SCR or LASCR, an uncommon component today.
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...