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RFID experiments

Radio-frequency identification (RFID) is a way to use electromagnetic fields to send and receive data wirelessly. The system consists of two parts: reader and a tag.  Tags can be passive or active. I think the most popular are passive tags. Meaning, there are no batteries needed, the power comes from the reader. The reader constantly sends out an interrogation signal and when a tag absorbs the energy and powers up, it radiates back information from the embedded chip.

Then it divides further - different frequencies, generations, encryptions etc.

Also one popular part is NFC (Near Field Communication) which has better security and other improvements. Latest phones usually come with NFC read/write capabilities built in.  So you can pay with your phone or touch phones together to share information. A lot of possibilities.


RFID/NFC is quite popular in our commercial world.

  • Anti-theft – stores use it to stop people stealing stuff.

  • Tracking people - putting tags inside shoes to track people, some festivals or nightclubs but them inside wristbands.

  • Payment – all kinds of simple payments systems or paying with a phone

  • Transportation - tag on a car so you will be charged automatically etc.

  • Security – opening doors, gates etc.

  • Public transport

  • Passports and other cards – rumored bombs that only explode when there is a US passport nearby.

  • Animal identification

  • Sporting events – games, lap times etc.


Homemade 125 kHz FSK tag reader


So of course there are two ways to approach this problem. The first way is to build your own goods, second way is to buy necessary stuff. I went with the third way – buying stuff and meanwhile building my own stuff.

Went with scanlime’s “World's simplest RFID reader” design and there is also an Arduino implementation of the same thing.




FSK-RFID-reader-v2 (1)

Lately I like to build these “development” beds, where it would be easy to add/remove stuff and also it adds rigidness without having a case.

Blue Plexiglas is pretty hip, got to unleash my artistic skills....

Arduino generates a 125 KHz carrier.


Antenna design.

At first I went pretty loose on the antenna design luckily/obviously that did not work.



The coil needs to resonate at 125 KHz. I chose a random capacitor - 10nF. Working out the inductance gave me 162µH.

If that is done we need to calculate the coil's dimensions that correspond to 162µH.



I used an old bottle with a diameter of 6.9 cm. With that diameter I needed to make a coil with 33 turns.

I also experimented with various diameters and capacitor values – weirdly enough all of them worked.

The biggest problem is that I do not have the right tags. The system detects a tag but it does not decode it (yet?). But it does detect a tag so that is a win I guess.

Here is it in action:


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