Detecting a magnet... from a distance.

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Externet
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Detecting a magnet... from a distance.

Post by Externet » Sun Jan 28, 2018 6:47 am

Hi. Let's raise a bit the activity here...

A magnet slowly spins in the next room. Or house. Or block.

Detecting its presence will depend
- on the field strength of the magnet,
- the sensitivity of the sensor,
- the gain of the amplifier,
- and perhaps something else.

What method would you implement, aiming to the longest range ?
- Abolish the deciBel ! -

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jwax
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Re: Detecting a magnet... from a distance.

Post by jwax » Wed Jan 31, 2018 2:35 pm

I'll bite. How about a million-turns coil, and a nano-voltmeter?
Question- would it be more or less sensitive with air core or iron core?

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Re: Detecting a magnet... from a distance.

Post by Externet » Wed Jan 31, 2018 5:22 pm

Hi John. That is exactly the plan. So far.
Because ----> https://www.amazon.com/TMR9001-Ultra-hi ... ometer+tmr
:shock:

Yes, iron core can work as well as mumetal.
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Re: Detecting a magnet... from a distance.

Post by Lenp » Wed Jan 31, 2018 10:10 pm

OK, Here's the deal
Get an old CRT TV, place it near the magnet, use wireless linking of a video camera looking at the CRT screen to your remote location, and observe the color distortion caused by the magnet to confirm movement. Use a hall effect switch, and a siren on the roof. OR.. we could work out a deal where I'll come to your place, watch out the window with a pair of binoculars, and press a button when it turns.
(Rube would be proud!)
Len

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"I must be on the way to success since I already have the junk". (Me)

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Re: Detecting a magnet... from a distance.

Post by jwax » Thu Feb 01, 2018 7:29 am

That price of $1,200 seems a tad high for that chip.
Should be: https://www.amazon.com/TMR2102P-Linear- ... 611&sr=8-1

Of course, lenp's idea is feasible too. :)

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Re: Detecting a magnet... from a distance.

Post by Externet » Thu Feb 01, 2018 8:09 am

Hi John. I talked to the developer/inventor of that expensive chip yesterday. He suggested trying first with the $30 ones from his company, as a way to evaluate if the expensive one would justify the results/needs/cost. I ordered a 10Km coil of AWG 40 before jumping to the modern sensors; will see next week what happens feeding a INA111.

Hi Len. Deflection on a CRT does not provide the sensitivity. Try yourself with your oscilloscope if no TV on hand. A strong magnet needs to be much closer than the expectations :sad:
As you said, "place it near the TV" -Near is not the point. :sad:
But, if there is a coil as oscilloscope probe, things may be very different. Then there is amplification inserted.
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Re: Detecting a magnet... from a distance.

Post by jwax » Thu Feb 01, 2018 8:57 am

So that $1,200 price tag is real? Hard to believe.

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Re: Detecting a magnet... from a distance.

Post by Externet » Thu Feb 01, 2018 9:49 am

:!: Yes, it is very real :!:
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Re: Detecting a magnet... from a distance.

Post by haklesup » Thu Feb 01, 2018 1:08 pm

How slow, too slow and it might as well be stationary as the flux will not couple well into most detectors. a DC magnetic field can be detected with a compass and if smaller than Earth's won't wiggle a needle much.

At some point the signal would drop so far into the noise it would become undetectable. Noise sources include the Earths own field, local geologic sources any ferromagnetic object nearby, any motor or generator in the same range as what you are trying to detect, moving vehicles, any CRT, power lines or transformer or other source of EMI.

Assuming you could have a detector to sense all of that, it would take quote a feat of DSP filtering to ferret out a specific signal unless the source were exceptional.

I recall rumor that the Gov't had a system that could see your Computer from emissions coming from a CRT. http://www.surasoft.com/articles/tempest.php I'm not sure I believe it is real or misinformation.

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Re: Detecting a magnet... from a distance.Hi

Post by Lenp » Thu Feb 01, 2018 1:48 pm

Years ago it was purported that they could tell what was being typed on an IBM Selectric typewriter by the line current pulses from the motor related to the mechanical movements. After the enginers went the limits with filter and suppression ideas, some tech hung a flywheel on the motor shaft an the problem was solved!

Just thought I would say That!
Another ponderable imponderable
Len

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Re: Detecting a magnet... from a distance.

Post by dyarker » Thu Feb 01, 2018 4:17 pm

I recall rumor that ...
Not a rumor (that it could be done).
Dale Y

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Re: Detecting a magnet... from a distance.

Post by CeaSaR » Fri Feb 02, 2018 5:31 pm

$1200, huh? But the shipping I saw at $4.95 is a steal! Might have to pick one up, then. :D

But seriously, I'd have to agree with Hacklesup that a large amount of DSP with many built in noise profiles would be needed to strip away down to the true unknown signal, otherwise you would need a super trained eye or ear (whatever the output might be) to be able to discern useful output from all the noise you'd pick up.

1200?? SMH

CeaSaR
Hey, what do I know?

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Re: Detecting a magnet... from a distance.

Post by Externet » Sat Feb 03, 2018 8:17 am

A good portion of noise could be erased by doing it differentially if in noisy environments. That is two sensors to reject common mode.
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Re: Detecting a magnet... from a distance.

Post by haklesup » Sat Feb 03, 2018 9:00 pm

A magnetic wave travels at the speed of light, and not very far when not coupled with an electric field. I don't think any analog circuit would have the response time to measure a differential. With audio the wave travels slower and it still took DSP to make it practical. With DSP the incoming signals can be acquired with an accurate time stamp making it possible for a processor to accurately subtract the signals.

DSP is fairly attainable, If the magnet you are trying to sense is moving in similar speed to the audio spectrum then you have many choices of PC and mobile device software for audio DSP you can substitute your sensor for the microphone with appropriate interface. If it is not moving then you need to move around with a very sensitive compass mapping out errors (geomagnetic mapping).

I've played with this one. Its not free but it's also not crazy expensive either http://www.spectraplus.com/

you would probably want X, y and Z sensors if the direction if the field is important.

NI has tons of community support http://sine.ni.com/nips/cds/view/p/lang/en/nid/12050

Search on Magnetic Field probe, you may find something more complete than the chip. I found diagrams in the image search that looked like something buildable. Anything affordable is low sensitivity. What range of field do you think you want to measure.


I bet the state of the art is on the nose of a navy sub. You are separated from common noise down there and ships are big magnets.

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Re: Detecting a magnet... from a distance.

Post by MicroRem » Sun Feb 04, 2018 8:23 pm

Ships are magnets for torpedoes.

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