I'm trying to create a double-sided NFC cards, where the two sides read different things. So depending on which side you hold onto your phone, you get a different result. I've tried with a sheet of aluminum in between, but it blocks reading NFCs altogether. When I hold a sheet of aluminum on top of an NFC card, the reader can't read it at all - no matter if there's another card on top of it or not. Is there a way to block NFC signals going to the card in the back without disabling any NFC functionality?
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1\$\begingroup\$ Draw a picture showing the coils please. \$\endgroup\$– Andy akaCommented Dec 14, 2022 at 16:53
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1\$\begingroup\$ My EE knowledge stops at making a blinking LED with a basic circuit. The NFC cards refer to en.wikipedia.org/wiki/MIFARE#MIFARE_Ultralight_family Presumably they look like idcardsdirect.co.uk/pub/media/catalog/product/cache/… on the inside. \$\endgroup\$– ReactormonkCommented Dec 15, 2022 at 12:31
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3 Answers
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NB: you're trying to do something that is kind of in conflict with the physics of why people chose to build NFCs the way they did (namely, using the NFC tag's coil as secondary winding in a transformer to which the primary side is the reader).
So, things get a little involved and you need to deal with the magnetic fields.
You'd use a steel sheet to effectively block the near-field coupling on which NFC tags are based. To still allow for the tags to work, you'd add one of the soft-magnetic spacer materials you can buy which allow one to stick a tag onto a ferromagnetic item.
This spacer inherently needs a non-negligible thickness; so if that's not acceptible to your "NFC card" (whatever that means in practice!), then your proposed device is physically impossible, I'm afraid.
EDIT: Tomáš' Answer shows how to do this.
answered Dec 14, 2022 at 16:46
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\$\begingroup\$ Thanks for the hints, I'll experiment with that. The resulting cards can be reasonably thick, so that shouldn't be an issue. \$\endgroup\$ Commented Dec 15, 2022 at 12:32
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\$\begingroup\$ whatever "reasonably" means :D \$\endgroup\$ Commented Dec 15, 2022 at 12:45
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\$\begingroup\$ I think if it starts being thicker than 1.5cm, it'll get interesting. Gotta experiment with different thicknesses. \$\endgroup\$ Commented Dec 15, 2022 at 14:14
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\$\begingroup\$ You could also try a ferrite sheet instead of steel, Say, a fridge magnet. \$\endgroup\$ Commented Aug 21 at 11:21
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I created dual side NFC tag this way:
- NFC Tag
- Ferite foil
- Aluminimum foil
- Ferite foil
- NFC Tag
Tested with iPhone8 as NFC Reader
Used components:
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\$\begingroup\$ oh neat! I'll have to revise my answer to point to this! \$\endgroup\$ Commented Aug 21 at 11:22
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I can see that this question is old enough and already answered, but I'd like to leave a solution that might help with a slightly different setup.
You could have tried to design the two NFC antennas next to each other, unlike to opposite sides, far enough apart that you could communicate with them separately.
