Journal · Shop Notes

How NFC tags work, and why every OneKey has one

Shop Notes, from the Key Bored workshop · October 18, 2026 · 3 min read

An NFC tag is a tiny chip connected to a flat coil antenna, with no battery anywhere. When you hold a phone close, the phone's own radio field powers the chip, and the chip answers with whatever it stores, usually a web link. The whole exchange happens within a few centimeters and takes a fraction of a second.

Power from thin air, more or less

NFC stands for near-field communication, and it runs at 13.56 MHz. Your phone generates a small magnetic field at that frequency. When a tag's coil antenna sits inside that field, a little current flows in the coil, enough to wake the chip up and let it reply.

That's why tags can be so small and cheap, and why they last basically forever. There's nothing to charge and nothing to wear out.

Why it only works up close

The range is short on purpose. Most phones read tags within about four centimeters, and in practice you'll often need to touch. That makes every read feel intentional. Nobody scans your keychain from across a room.

On an iPhone XS or newer, tags are read in the background, so you just hold the top of the phone to the tag. Most Android phones read them whenever NFC is switched on, which it usually is.

What a tag can hold

Simple tags don't hold much. A common chip like the NTAG213 has 144 bytes for your data, which is plenty for a web address and not much else.

More advanced chips can do clever things. The NTAG 424 DNA can generate a fresh, cryptographically signed link each time it's tapped. A website can check that signature and confirm the tap came from that exact physical tag, which makes it very hard to fake by copying a link.

The metal problem

Metal and NFC don't get along. A metal surface right behind the antenna soaks up the field and detunes the coil, and the phone can't read the tag at all.

The fix is a tag built for metal. These add a thin layer of ferrite, a magnetic ceramic, behind the antenna. The ferrite steers the field away from the metal so the tag can be read from its open face.

We ran into this designing the KB OneKey Pro, which is almost entirely aluminum and brass. The answer was a shallow round pocket in the bottom of the brass weight for an on-metal tag, covered by a thin label. Our first version of that pocket was too deep and left only a sliver of brass above the switch holes, so we made it shallower and let the label stand a fraction of a millimeter proud. The case is designed with exactly that much room.

What it does in OneKey

Tap your phone to the bottom of a OneKey and it opens a page for that key. In the KB OneKey, the tag sits inside the body on the floor of the air chamber, so you'll never see it. For Batch 001, each key's page shows its number.

The tag has no battery, no GPS, and no way to track you. It only answers when a phone is held right against it.