This circuit makes use of an optical coupler to isolate the telephone line. The circuit is from 1970s documentation. The components admit equivalents.
This circuit was found in a 1960 transistor manual from Sylvania. It measures the intensity of...
I found this circuit in an old Argentine publication from 1973. The components are no longer...
Found in Ferranti's 1969 documentation, this circuit can be mounted with a modern equivalent...