This harmonic oscillator controlled by crystal was found in an American documentation of the 90's. The circuit uses transistors of the time. Current equivalents can be used.
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...