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Curious to see tiniest magnetic signals in your environment? And make a tour through a number of concepts in analog electronics? Building a fluxgate magnetometer from scratch sensitive enough to see variation of the earthfiedl or montitor cars passing by on the neighbouring street... is a nice project to quench this curiosity.

This is a project to explore a method to measure very small magnetic fields, which was invented in 1937 by Friedrich Förster, known under the names Förster-probe or Fluxgate sensor. I describe a DIY project to build a fluxgate sensor with nT sensitivity or better than ( 1/50000 of the earth-field). In large part this is an educational journey visiting the fields of magnetic properties of materials, distortion free sinewave generation, signal analysis, phase synchronous detection and regulation feedback loops.
The final ciircuit proved to be sensitive enough to monitor the daily variations of the earthfield as well as the additional "noise" due to human activities, e.g. cars passing by at a 25-30m distant street, road works there parking lot  occupancy and other not completely identfied signals.

Further I discovered that there exist a one-chip sloution with an analogous functionality and a comparison between the two was made.


(Schematics update 21:05:25: Forgot the level shift between output  U8 to input U6A. This is now included (D5 , R40).