This circuit makes use of an optical coupler to isolate the telephone line. The circuit is from 1970s documentation. The components admit equivalents.
The inverter circuit shown in the figure can be used to power a fluorescent lamp in an emergency...
This circuit was found in 80's Popular Electronics. The circuit uses an infrared-sensitive phototransistor...
Found in documentation from the 1980s, this circuit generates signals that can reach a few...