Using a Standard Coil for NFC Tag Implant Reading

A few months ago Hackaday covered the xNT crowdfunding campaign which aimed at making an NTAG216 based NFC implant for different purposes. I actually backed it, found that standard NFC readers don’t perform well and therefore decided to try using a standard coil as an antenna for better reading performances.

Most NFC readers typically only have a small sweet spot where implant reading is possible. This is due to what we call coupling factor which depends on the reading distance and reader & NFC tag antenna geometries. Having a smaller antenna diameter increases the coupling factor and makes implant positioning easier.

In my detailed write-up you’ll find a good introduction to impedance matching, a process where a few passive components are added in series/parallel with an antenna to bring its complex impedance close to a RF signal transmitter’s. This usually requires expensive tools but allows optimal power transmission at a given frequency.

You may find our xNT coverage here.

An attempt to replace multiple RFID cards with a single hacked-together tag

rfid-multipass

It’s kind of a convoluted title, but [Hudson’s] attempt to replace multiple HID Prox cards with one AVR chip didn’t fully pan out. The project started when he wanted to reduce the number of RFID access cards he carries for work down to just one. The cards use the HID Proximity protocol which is just a bit different from the protocols used in most of the hobby RFID projects we see. He ended up taking an AVR assembly file that worked with a different protocol and edited it for his needs.

The device above is the complete replacement tag [Hudson] used. It’s just an AVR ATtiny85 and a coil made of enameled wire. The coil pics up current from the card reader’s magnetic field, and powers the chip through the leakage on the input pins (we’ve seen this trick a few times before). The idea he had was to store multiple codes on the device and send them all in a row. He was able to get the tag to work for just one code, but the particulars of the HID Prox reader make it difficult if not impossible to send multiple codes. The card must send the same code twice in a row, then be removed from the magnetic field before the reader will poll for another combination.

RFID emulator card includes a learning mode

rfid-emulator

This RFID card has a lot of nice features. But the one that stands out the most is the ability to learn the code from anther RFID tag or card.

You can see that the board includes an etched coil to interact with an RFID reader. This is the sole source of power for the device, letting it pick up enough induced current from the reader to power the PIC 12F683 seen on the upper left of the board. The underside of the PCB hosts just three components: an LED and two switches. One of the switches puts the device in learning mode. Just hold down that button as you move the board into the magnetic field of the reader. While in learning mode a second RFID tag is held up to the reader. It will identify itself and the emulator will capture the code sent during that interaction. This is all shown of in the video after the break. We wonder how hard it would be to make a version that can store several different codes selected by holding down a different button as the emulator is held up to the reader?

If you want to build your own card reader too here’s a project that does it from scratch.

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Wristband RFID unlocks car door and starts engine

[João Ribeiro] is an electronics engineer by day, but in his free time he likes to ply his trade on everyday items. Recently he’s been integrating his own microcontroller network to unlock and start his car via RFID. In addition to the joy of pulling apart the car’s interior, he spent time designing his own uC breakout board and developing an RFID reader from a single chip.

He’s working with a 1988 Mercedes that has very little in the way of electronics. It sounds like the stock vehicle didn’t even include a CAN bus so the prelude to the RFID hack had him installing a CAN bus network made up of two microcontrollers. One reads the velocity and RPM while the other displays it on the tachometer. When he began the tag-based entry system he used an RFID reader module for prototyping, but eventually built his own reader around the TRF7960 chip. This included etching his own receiver coil which was mounted in the side-view mirror bracket. To unlock the doors he holds the bracelet up to the mirror and the vehicle lets him in. The video after the break starts with a demonstration of the completed project and moves on to some build videos.

We certainly like the idea of using a bracelet rather than implanting the tag in the meaty part of your hand.

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Configurable RFID tag from 7400 logic chips

This soldering nightmare is a configurable RFID tag which has been built from 7400-series logic chips. The beast of a project results in an iPhone-sized module which can be used as your new access card for security systems that uses the 125 kHz tags. The best part is that a series of switches makes the tag hand programmable, albeit in binary.

Of course this is an entry in this year’s 7400 Logic Competition. It’s from last year’s winner, and he’s spent a lot of time documenting the project; which we love. We were surprised that this many chips can be powered simply by what is induced in the coil from the reader. This is just one of the reasons the 7400-series have been so popular over the years. After working out the numbers, a 64-bit shift register was built to feed the tag ID to the encoding portion of the design. There were many kinks to work out along the way, but once it was functional a surface-mount design was put together resulting in the final product shown off in the video after the break.

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Store your RFID transit card inside your cellphone

Check it out, this is a Boston transit pass — or at least the parts of it that matters. [Becky Stern] got rid of the rest in a bid to embed the RFID tag inside her cellphone.

The transit pass, called a CharlieCard, started out as a normal credit card shaped tag which you might use for access in the workplace. She unsheathed the chip and its antennae by giving it a generous soak in acetone. In about thirty minutes the plastic card looks more like paper pulp, and you can gently fish out the electronics. These are now small enough to fit in the back cover of a cellphone much like those inductive charging hacks.

[Becky] put hers in an iPhone. But the idea comes from [Dhani Sutanto] who used the same technique to extract the coil from a London transit pass. He then embedded the hardware in a resin cast ring.

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RFID jukebox for the kids

[Dominik] built a fun musical toy for his daughter [Anna]. It’s a jukebox that lets her play her favorite tunes using RFID tags to select between them.

The project is simple, yet robust. The enclosure is a wooden craft box that you can pick up for a couple of bucks. Inside there’s an Arduino with a Wave Shield which handles the audio playback. An RFID reader takes input from the set of card-tags he procured. An internal Lithium battery powers the device, with a USB port for charging.

Sure, those guts have some cost involved in them. But there’s no LCD which can be broken, and we thing the boards will hold up well to abuse if mounted correctly. Plus there’s a lot of future potential here. When we saw the cards we thought of those toys which make the animal sounds (“what does the cow say… mooo”). This could be used for that with really young children. Then repurposed into this jukebox as they get a bit older. If you put the guts in a new enclosure it will appear to be a brand-new toy, right?

See a demo of the project in the clip after the break.

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