This basic relay configuration was found in a July 1994 Electronics Now. The relay is supposed to be sensitive, and the times are changed by changing C1.
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...