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 configuration was found in an American publication from the 90s. Operators admit equivalents just...
The circuit shown in the figure has basically the same configuration as any TTL oscillator, with...
This circuit with the PLL567 serves as a decoder for PWM modulated signals of frequencies up to...