This circuit makes use of an optical coupler to isolate the telephone line. The circuit is from 1970s documentation. The components admit equivalents.
This sophisticated audio generator based on the XR2206 IC was featured in the May 1980 issue of...
This circuit measures the modulation of a 11-meter transmitter, having been found in a 1989...
We have other similar setups on this site. This one is from September 1976 Radio Electronics. The...