This double tone generator uses a specific integrated circuit, having been found in a 1977 CMOS Cookbook. The transistors can be the BC548.
Found in na 1970 EDN this circuit has a parallel regulation providing a symmetrical output of 15 V....
This configuration found in a 1976 publication can be elaborated with more modern operational...
This ultra-simple meter uses an output transformer and a small speaker as a microphone. The...