Another app note from MAXIM Integrated utilizing supervisory chip and LEDs to indicate voltage faults. Link here
Traditionally, supervisory ICs have been used to monitor voltage rails for undervoltage faults (UV). When a supply rail falls below a set threshold, the system is held in reset using the reset output signal to the microcontroller to prevent erratic behavior or catastrophic failure.
from Dangerous Prototypes https://ift.tt/2Jlxcyc