This old version of analogue switch was found in a 1969 documentation. Components of the time are used, with emphasis on the CAG7 integrated circuit. It is worth as a reference.
This circuit from Popular Electronics powers a small speaker. The transistor can be of general use...
When the sensor X1 is opened, the relay is energized. When waiting, the current drained is very...
This circuit for measuring bridges was found in the October 1995 issue of Electronics Now...