The purpose of this circuit is to monitor the output voltage of a power supply. The under and over voltage indicator was found in the August 1991 Radio Electronics magazine.
In the figure we have the way to implement a 5 to 45 V zener diode with 50 mA of maximum current...
This circuit is from a Radio Electronics magazine from December 1958. The indicator instrument is analog...
These integrated ones consist of class B amplifiers with gain fixed 24 internally for better...