We found this circuit in an American documentation from the 90's. The circuit makes use of general purpose NPN transistors. The 0.005 uF (5nF) capacitors determine the frequency.
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...