This circuit was obtained from an Electronics Now from the 90's. It generates the dial signal and the ring tone, serving for testing as well as for demonstrations.
This circuit uses old germanium transistors because it was found in an old magazine from 1973....
This logic level indicator also indicates the presence of pulsating signals through a third LED. It...
This circuit is from the 1990 Motorola Linear Databook, which can be developed with more modern...