[HN Gopher] Build a DIY magnetometer with a couple of seasoning ...
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Build a DIY magnetometer with a couple of seasoning bottles
Author : nullbyte808
Score : 50 points
Date : 2025-11-29 12:04 UTC (8 days ago)
(HTM) web link (spectrum.ieee.org)
(TXT) w3m dump (spectrum.ieee.org)
| mcculley wrote:
| Could one use something like this from the surface to detect
| steel submerged under 20-40 feet of water?
| sllabres wrote:
| I think, not from the surface, but have a look here [1], where
| the author referenced from the IIEE article has build a
| submergible sensor and detected (a know) boat.
|
| [1] https://alexmumm.de/pgProtonMagMarine_en.htm
| greggsy wrote:
| How is this different from the magnetometer accessible in a phone
| through and app like Phyphox?
| fudgybiscuits wrote:
| You learn a lot more making this.
| sllabres wrote:
| The sensitivity When I play with phypbox [1] there is a
| sensitivity in the uT range. From the web page [2] the device
| build has a 0.1 nT resolution and 50 ppm absolute accuracy.
|
| [1] https://phyphox.org/download/
|
| [2] https://alexmumm.de/pgProtonMagnetometer_en.htm
| RossBencina wrote:
| The magnetometer in your phone is a MEMS sensor which measures
| mechanical deflection of a current-carrying element. The
| deflection is caused by the Lorentz Force, i.e. force induced
| by an electron current flow in a magnetic field (in this case,
| the earth's magnetic field).[1] The magnetometer in the linked
| article senses (EDIT: corrected, hopefully) oscillation in the
| magnetic field of protons, a result of Larmor Precession[2].
| Remarkably, the oscillation frequency is proportional to the
| ambient magnetic field strength, and the frequency is in the
| audible range. The circuit works by rotating protons in the
| fluid so that their magnetic axis align, this results in a
| synchronised bulk magnetic field oscillation that is large
| enough to be sensed by a simple tuned amplifier circuit.[3]
|
| Further, the magnetometer in your phone is a 3-axis device that
| measures the orientation of the magnetic field, whereas the
| magnetometer in the linked article detects only the strength of
| the magnetic field (in fact, is tuned to detect only a single
| strength/precession frequency).
|
| [1] https://en.wikipedia.org/wiki/MEMS_magnetic_field_sensor
|
| [2] https://en.wikipedia.org/wiki/Larmor_precession
|
| [3] https://en.wikipedia.org/wiki/Proton_magnetometer
| dvh wrote:
| If I may recommend, replace output LM386 stage with any dual
| opamp (e.g. another NE5532 or TL072, slightly different schematic
| of course), they can drive 32 ohm headphone speakers without
| issue and have significantly (~100x) lower white noise.
| RossBencina wrote:
| The schematic in the linked article shows an NE5532.
| dvh wrote:
| Only in the first two stages. Output stage is LM386 which
| will be the source of the most of the noise. Replace the
| LM386 with another NE5532 (but modify the schematic of
| course, LM386 is single audio amp and has different pinout)
| ErroneousBosh wrote:
| You can drive even 8 ohm headphones to unpleasantly loud levels
| with any opamp and a pair of transistors to beef up the output,
| along with a resistor to sort out the biasing. I did something
| like this as a headphone driver amp for "desktop mobile" radios
| used as part of a communications centre for a large festival.
| Motorola had a device that would do it, for about 500 quid
| each. I built the thing in the PDF at the bottom (I must have
| rerendered this at some point, it was definitely not done in
| 2022, more like 2012).
|
| Using cheap bag-of-1000-for-a-fiver Chinese transistors off
| eBay I was able to get incredibly quiet output, to the point
| that I needed to add a muting gate because the radio was
| objectionably noisy. I notice that the exact transistors are
| not mentioned but any small-signal NPN and PNP ones will do - I
| used BC548 and BC558s, like I use in everything.
|
| It will be way quieter and way more stable than an LM386.
|
| Edit: I'm a lot better at drawing things in Kicad these days,
| and would have left the capacitors at the input a lot tidier.
|
| https://onlyfandans.com/headphone.pdf
| jacquesm wrote:
| Note the first comment.
| metadat wrote:
| I want to see pictures of the device and ideally a video of it in
| action. It would be stimulating.
| notaurus wrote:
| Hmmm.
|
| > the listening circuit must also be tuned to resonate at the
| expected frequency of proton precession, which will depend on
| Earth's magnetic field at your location
|
| > the frequency of these tones matches the magnetic field at my
| location to about 1 percent
|
| I don't doubt the physics, but I'm not sure about the experiment
| design. Being able to hear the correct frequency may just mean
| you've built an oscillator and tuned it.
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