Found in a Popular Electronics from the 90's, this circuit detects the approach of objects by the reflection of the radiation emitted by the LED. The relay is a 12 V sensitive type.
The integrated circuit used in this project is not common today, but it can be implemented with more...
Found in an American publication from 1969, this circuit can be adapted to be used as a shield....
This current meter with many scales was found in a 80s American documentation. The operational amplifier...