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 appeared in the July 1969 issue of Radio Electronics magazine. The sensor is an LDR....
For signals up to about 500 kHz modulated in amplitude and transmitted by the power network, the...
This circuit was found in the March 1984 issue of Radio Electronics magazine. The sensor is a...