This circuit uses an unusual PLL, the 4046 CMOS. We found it in the October 1994 issue of Electronics Now. The circuit also uses a 4093 gate as a trigger.
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...