Qucs is a GPL circuit simulator. And if you want the GUI option, you might want to try out QucsStudio, which uses Qucs under the hood, and is free to use, but binary-only.

This multiplexer switch circuit is built with the IC 4066 and was designed to switch multiple signal sources into a single common output and can be used as a replacement for the conventional ...

This last principle is used in the sensitive switch installation you will be presented next. Photocell is placed in a bridge circuit and a comparator is used as detector bridge balanced. The ...