Found in Popular Electronics from the 90's, this circuit uses 555 integrated and common transistors to generate two tones determined by push buttons S1 and S2.
This circuit was found in a telecommunications publication of 1978. Its purpose is to obtain...
This setting appears with variations at various points on this site. This is from a Motorola...
This circuit is from a 1967 documentation. The circuit uses germanium transistors and has an...