Using the 555 and 4020, this timer was found in the July 1994 Radio Electronics magazine. The relay is voltage-dependent, and the transistor is general purpose.
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...