Have you ever wondered how a magnetometer works? We sure have, which was why we were happy to stumble upon this article on simple homebrew fluxgate magnetometers.
As [Maurycy] explains, clues to how a fluxgate magnetometer works can be found right in the name. We all know what happens when a current is applied to a coil of wire wrapped around an iron or ferrite core — it makes an electromagnet. Wrap another coil around the same core, and you’ve got a simple transformer.
Now, power the first coil, called the drive coil, with alternating current and measure the induced current on the second, or sense coil. Unexpected differences between the current in the drive coil and the sense coil are due to any external magnetic field. The difference indicates the strength of the field. Genius!
For [Maurycy]’s homebrew version, binocular ferrite cores were stacked one on top of each other and strung together with a loop of magnet wire passing through the lined-up holes in the stack. That entire assembly formed the drive coil, which was wrapped with copper foil to thwart eddy currents. The sense coil was made by wrapping another length of magnet wire around the drive coil package; [Maurycy] found that this orthogonal of coils worked better than an antiparallel coil setup at reducing interference from the powerful drive coil field.
Driving the magnetometer required adding a MOSFET amp to give a function generator a little more oomph. [Maurycy] mentions that scope probes will attenuate the weak sense coil current, so we assume that the sense coil output goes right into the oscilloscope via coax. Calibrating the instrument was accomplished with a homebrew coil and some simple calculations.
This was a great demo of magnetometry methods and some of the intricacies of measuring weak fields with simple instruments. We’ve covered fluxgate magnetometer basics before and even talked about how they made pre-GPS car navigation possible.
Am I the only one seeing a dead cockroach lying on the table?
Free protein, yummy!
The brain searches for patterns, a dead bug is a give away of an infestation which means food and health are in danger, so better be wrong many times than not seeing it.
No.
Wow that’s way simpler than I expected. I feel like I can make one with spare cores I have laying around! So the MEMS magnetometers have 3 such coils in 3 axes which sense current changes (only they are made on the sillicon)
There are many types of MEMS magnetometers, fluxgate is not particularly common. I think most use either Hall effect or magnetoresistance.
You almost lost me at “binocular ferrite cores”.
(Why would binoculars need ferrite cores? B^)
It is obvious: you don’t want the Earth’s magnetic field to interfere with the light electromagnetic field. Thus, without the ferrite elements, the image through the glasses will be disturbed by the variation of location, sun activity, time of the day and position and distance from civilisation (where are a lot of electromagnetic fields from electric engines and electronics). This is why engineers glasses in the beginning of the 20th century were thick, until the ferite cores were manufactured easier and with more details as the quality of the stock improved. Perhaps it is not a secret that there were some recent attempts to use them on contact lenses, for the same purpose, but for the moment is easier to were an empty glasses frame with the contact lenses, thus having a cheaper solution for the moment.
Nice article. I did Mine Warfare in the Navy for 14 years. Magnetic current was a huge part of my job. This brings back memories. :D