I am working on a project where I am trying to create a magnetic tag that can be detected from $20 - 50$ cm away. Is there any sensor on the market capable of this?
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$\begingroup$ My answer has been reedited with the operational limitations of these sensors. Sorry for the delay. In short, very difficult to find such sensor that works at 20cm away or more. Why you are interested in visible magnetism? Can you not just use a sensitive Hall sensor or a security tag detector? $\endgroup$– Markoul11Commented Nov 20, 2021 at 13:24
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$\begingroup$ The aim of my project is to create a tag that is very inexpensive, doesn't need electricity. I found that RFID tags can do something similar to what I am trying to make, but I am trying to make the tags so they cost no more than 0,01$ a piece. However I am finding that its probably not possible with magnets. $\endgroup$– BubuCommented Nov 20, 2021 at 14:00
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$\begingroup$ I see. Why you don't use just an optical tag like QR code? With a zoom camera from example a cell phone you could detect and identify from the distance you are interested in? $\endgroup$– Markoul11Commented Nov 20, 2021 at 14:33
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$\begingroup$ The tag has to be detectable through fabric or plastic $\endgroup$– BubuCommented Nov 20, 2021 at 14:36
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1$\begingroup$ Maybe shining the QR tag under the plastic or fiber with an infrared light can it made visible to a cell phone camera? exo.net/~pauld/activities/thermodynamics/… science.nasa.gov/ems/07_infraredwaves I always used my cell phone to see if the IR TV remote control is working. Anyway, good luck with your project! $\endgroup$– Markoul11Commented Nov 20, 2021 at 14:45
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You maybe interested in looking here:
Magnetic field gel based viewing film
or for more advanced applications the ferrolens (commercial product named Ferrocell).
However these optical magnetic sensors need more likely to be in physical contact with the magnetized matter and have a sensitivity at about 400 Gauss (i.e. 40mT) in average magnetic field strength. I doubt you can find such an optical magnetic sensor at 20cm-50cm that works at that distance range from the magnetic source? Unless we are talking about a very strong magnetic field source as for example in MRI where you can have 1T (10,000 Gauss) strength and above fields.
Note: MRI field strength value indicated above, refers to the Βr residual magnetism value thus field strength on contact, zero distance from the magnetic source which in a multi-poles MRI configuration can be translated into 0.3T (3,000 Gauss) and above field strength on air many cm away from the source.
You may try this product here the mini-Ferrocell which has a sensitivity of 100 Gauss (10mT) which translates to a field strength on air a few cm away from a normal N42 grade Neodymium dipole magnet with Br value on contact about 5,000-6,000 Gauss (i.e. Br 500 to 600 mT). If you increase the strength of the magnetic source on contact, say for example at 1T (i.e. 10,000 Gauss) Br value, you should gain more distance separation and still get a visible response from this sensor but no more than 8 cm distance at maximum in my experience.
Nevertheless, all these should not be applicable in your case since your magnetic tag as you described it should have no more than a few mT, Br residual magnetism strength on contact.
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3$\begingroup$ Your links are good, I was not aware of these devices. I think somebody downvoted your answer because you have not quoted from the links, so that people could understand without going there. (if you have noticed in my answers I give extensive quotes from the links supporting them, saves writing ). The first sentences in the links would be enough. $\endgroup$– anna vCommented Nov 20, 2021 at 12:55
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3$\begingroup$ @annav Thanks for the suggestions. The answer is now reedited with the operational limitations of these sensors. Ευχαριστώ :) $\endgroup$ Commented Nov 20, 2021 at 13:16