What does NFC mean? If your bus card has no battery, where does the power come from? Bumping your phone into the router and connecting to WiFi are the same trick
during rush hour, I queued at the subway turnstiles. The guy in front stuck his phone to the sensor area, and the gate opened. I touched the physical bus card in my pocket that had been used for five years, and suddenly got stuck by a question I had never thought about before: this card has a plastic shell and no batteries at all. What is it using to talk to the turnstiles?
I got home, I rummaged through all the NFC devices at home—bus cards, community access cards, my wife's phone, routers. I figured out this 'tap' thing clearly. This has to be traced back to where electricity comes from.
card had no battery; electricity was "injected" through the air
I took apart a canceled old bus card. When I pried open the case, there was no button battery, only a coil of copper wire wrapped around it, plus a black chip smaller than a grain of rice. The source of electricity is hidden within this wire. Below the card reading area of the
turnstile is a larger coil. When powered on, it continuously changes the current direction at a frequency of 13.56 million times per second, creating a magnetic field with the same frequency fluctuations around it. When your card is struck, the coil on it happens to be immersed in this magnetic field, and the electromagnetic induction discovered by Faraday two hundred years ago starts working—current is induced inside the coil, rectified and stabilized, and then powers the chip. Simply put, the card reader and card form a hollow transformer: one side powers on, the other draws power remotely.
wireless charging for phones is related to the same family, but the power difference is several orders of magnitude. Wireless charging feeds milliwatts, NFC feeds milliwatts, and just enough for a chip to wake up and shout, "I have SIM number XXX."
Why can NFC only be used when sticking it on, and it stops working just ten centimeters away? Just do the math and you'll understand. A 13.56MHz electromagnetic wave has a wavelength of 300 million divided by 13.56 million, about 22 meters. The antenna coil on the card is only a few centimeters long, far from the scale to truly "emit" radio waves; the energy only curls around the coil as a magnetic field, decaying away by about ten centimeters. This isn't a flaw; it's precisely the design goal—it only takes effect after a single touch, and this distance itself is the safety boundary.
so in the word NFC, 'NF' stands for Near Field—the name already says the temper is written all over the face.
three ways to live: card usage, card reading, and two phones facing each other
NFC devices have three working modes, and daily life is almost entirely covered by the first two:
the first is card simulation , where the phone dresses itself as a card. Sticking your phone to the turnstile means the gate is like a transit card; A POS machine is essentially a bank card. There isn't a real bus card chip in the phone; it runs software simulation (called HCE on Android), and sensitive data is locked in the phone's security chip—which is why it's harder to copy when using a bus card than a physical card.
The second is read/write mode, the phone acts as a card reader to read other cards or tags. Next, let's talk about connecting the router to the internet when you touch it—this is the mode.
third is point-to-point , where two phones touch back to send a message—this is the early days of Android Beam. The 2011 launch event was very glorious, but in reality, very few people used it. Starting from Android 10, it was simply abandoned. So stop worrying about "two phones tapping to send photos"—that feature is already in the graveyard. Send files obediently via WeChat or Bluetooth.
also do the math of speed to avoid overestimating it. NFC standard speed levels come in three levels: 106, 212, and 424kbps. At the highest level, that's 53KB/s, so transferring a 1MB photo takes 20 seconds. It wasn't meant to transmit data; it was about 'credentials'—card numbers, passwords, access permissions, dozens of bytes, processed in milliseconds. This is completely different from the division between Bluetooth and WiFi. I did a full calculation of the short-range wireless network accounts at home in the about Bluetooth and WiFi pushing 2.4G.
the router connects to the internet with just a touch, and inside is actually a label
My friend's Huawei AX3 Pro, I saw "tap tap to connect to the internet" for the first time: his phone gently pressed against the top of the router, and WiFi connected, without even entering the password. At the time, I thought it was some kind of high-level near-field negotiation, but later I realized it was very simple.
routers with NFC have an NFC tag stuck inside the device—a erasable storage sticker pre-written with the WiFi name and password pre-written. The phone touched it, read the tag in read/write mode, received the credentials, and then went back to the normal WiFi connection process. It's essentially no different from copying the password to the guest; the only difference is that the copyist is the magnetic field, and the guest doesn't need to speak.
Huawei machines, this switch is turned off by default. You need to enable NFC in the webpage backend and system settings to take effect; The high-end models with a central hub in the Xiaomi BE6500 Pro also have this feature. By the way, after a round of internal Xiaomi and Huawei router , NFC connection is one of Huawei's signature moves.
compared to sharing QR codes, each has its own venue. Xiaomi series commonly used QR codes: Customers scan it, and Android phones recognize the password field in the QR code and connect directly. I wrote how to pull up that code. QR codes should be activated, aligned, and other for recognition; NFC is in the dark, just tap the screen once. On the other hand, anyone can scan a QR code, but NFC must be stuck within ten centimeters—if you have a mischievous kid at home, which is more reliable? A reminder: the password stored on the tag is plaintext. The router's built-in tag doesn't matter, but don't buy unknown third-party NFC stickers and stick them on the router—you have no idea what's inside.
access card simulation: the truth that property management becomes invalid every time you upgrade
In 2021, the access card issued by my community was the M1 (Mifare S50). This card has a classic structure: 16 internal sectors, each with 4 blocks and 16 bytes per block, totaling only 1KB of storage, and each sector is guarded by an independent key A and key B. The mobile wallet simulates access cards, essentially reading data from non-encrypted sectors on the card and copying it to the phone. If the property management is too lazy to change the default key (factory zero keys often remain unchanged for years), the phone reads it correctly—my card was successfully simulated, and I only took my phone when I went out.
good days end in 2024. The property management upgraded access control and replaced a batch of CPU cards, but the phone read all zeros. The encryption logic of the CPU card runs inside the chip, the key never comes out of the card, and the external data cannot be read. At most, a card number (UID) is copied, and the simulated gate does not recognize it. The simulated door card on my wife's phone became a souvenir from then on.
So, there is no standard answer to whether a mobile phone can replace an access card; it depends on what card your property management uses: the old M1 card is most likely simulated; Encrypting the M1 card requires luck, requiring two NFC phones to read the key, which is troublesome; CPU cards are basically giving up. If you really want to go all out with just one phone, you either have to consult property management or check if the community's access control is connected to platforms like Mijia. Speaking of which my smart door lock Bluetooth and never intended to use a SIM card from the start.
NFC tags costing just a few yuan each are lazy gadgets
Once you understand the principle, NFC tags become fun. A blank label costs a few yuan, and a common NTAG213 chip has 144 bytes of usable capacity, enough to write a short URL; The larger size has a NTAG215 of 504 bytes, enough to store one electronic business card; NTAG216 up to 888 bytes. Wherever the written content is posted, there is a physical button.
put up a photo on my entryway. My wife's iPhone has an NFC automation set up in the shortcut: scanning this tag triggers a "home" shortcut—turning on the entrance light and setting the air conditioner to 26 degrees. When you enter, just press your phone against the door frame, and the whole house comes alive. This setup integrates with Mijia's scenarios in the same way. I wrote about my Home mode setup , the difference is that the trigger changed from "Door lock opened" to "Phone touched tag." Mijia's official account has also released a 'bumper patch' feature, which can be used in Android phone wallets, following the same logic. As for wireless switches like Zigbee, they take a different path—lower cost but require a gateway. Zigbee for a complete comparison. NFC tags have zero power supply and zero gateways—just stick them on.
writing labels doesn't require professional equipment; just install a card reader app on your phone and write the website or text into it. Don't buy disposable locking labels; one wrong spell ruins the whole page.
Security boundaries and taking a personal stand: these things can be done, but don't expect
This matter needs to be clearly defined first: NFC is passive, waiting to read—don't touch it, don't read; The effective distance is ten centimeters; if you want to read through your pocket, you have to stick to someone else's body. If you move too much, you'll be seen as a pervert. The real risk window lies in the payment segment—UnionPay chip cards offer small amounts with no password, no password, and no visa (the "QuickPass" option where you tap without entering a password). The limit was 300 yuan per transaction in the early years, but since June 2018 it has been 1,000 yuan, and some merchants who haven't upgraded are still stuck at 300 yuan. It's really worrying—if you can turn off the small payment double exemption in the bank app with one click, the price is that you have to enter a password just to buy bottled water.
here's a reminder for beginners: don't use your phone to touch mysterious NFC stickers on the street, in shared bike baskets, or in elevators. You can write a URL as a tag, and if your phone touches it, it might pop up a phishing page directly. NFC itself won't cause phone poisoning, but it can hand over a note, and whatever the note says depends on who is posting it.
| what you want to do | NFC can you do it | how can you say |
| guest is even WiFi | my home | Huawei routers tap a bit, Xiaomi uses QR codes, and you don't have to read passwords |
| phones use them as access cards | check the card | old M1 cards can simulate encryption, encrypted cards rely on luck, CPU cards give up |
| swiping the subway, taking the bus, | wallet to activate a transit card, it's harder to copy on your phone than with a physical card | |
| sending photos to friends | not allowed | 424kbps cap, 1MB photo takes 20 seconds, point-to-point mode is discontinued |
| one-touch triggers scene | NFC labels plus shortcuts, and a physical button is stuck in the entryway. Here's a note about the HTML128__ |
order: if you want to experience NFC, first check your phone's settings to check if there's an NFC switch (available on mid- to low-end Android devices and all iPhones), then unlock the transit card in your wallet. This is the easiest step; To connect to the internet, you need a device with NFC first; Before the access card simulation, figure out the type of community card to avoid wasting effort. Behind this gesture is remote power supply plus a 10-centimeter whisper message, just a small matter.
