Using a 4046 PLL this circuit was obtained from the October 1994 issue of Electronics Now magazine. The circuit can be modified to generate signals at other frequencies.
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...