I am playing around with RFIDs and want to detect when an android phone is about atleast 10-20 meter away. I planned to use an RFID tag but need the tags pin broken out so that some interrupt line goes high or low to signify that a read operation is in progress. However I am unable to find any RFID tags that have their pins accesible. Can someone give me some idea as to how to proceed. Also is the range of 10-20 meters realistic with passive RFID tags?

  • \$\begingroup\$ What makes you think that RFID tags have pins? \$\endgroup\$ – Dave Tweed Feb 23 '14 at 4:51
  • \$\begingroup\$ Well I though they would be using some egneric memory chip which would have pins \$\endgroup\$ – user2578666 Feb 23 '14 at 5:43
  • \$\begingroup\$ It doesn't have that range either - more like 10cm \$\endgroup\$ – pjc50 Feb 23 '14 at 10:34
  • \$\begingroup\$ 10-20 meters? Bluetooth is a better option for that \$\endgroup\$ – Passerby Feb 23 '14 at 19:48

There are multiple classes of RFID devices. The more legacy devices use a frequency of 13.56 MHz (worldwide) and are operate on the principle of inductive coupling. The range of these devices is typically on the order of a few cm. This is the type of RFID tag that might be implanted in your dog or used for a "smart card" building access.

A second class is called UHF RFID and operates at much higher frequencies. This ranges from 868 MHz (Europe) to 915 (USA) to 960 MHz (Japan) across the global ISM bands. These tags operate on the principle of far-field electromagnetic coupling. These tags are larger (physically about 1/2 wavelength in size) and can operate up to 20m. However, this requires a specialized reader that can output 1W of RF power --- Much more power than a typical mobile phone can generate!!

There are other classes of RFID technology, but I am ignoring them and focusing on the two main technologies.

For the longer range (several m) UHF RFID devices, the process and protocol of being read requires several rounds of exchanges. A reader will tell the tags in view that a round is beginning. Tags will generate a random slot number. The reader will advance through the slots. If the slot matches the tag's chosen slot number, the tag will send an acknowledge. If the reader receives the acknowledge the handshaking will begin, and eventually after a couple more exchanges, the tag will return its identification number. At this point, the tag is considered 'read' and the reader moves on to its next slot or round.

For your project it is difficult to trigger the interrupt when the tag is being read. It would be simpler to use the reader itself. However, there are some chip manufacturers that are using I2C to allow for other sensor data and tag programming. For example, look at the Monza X series: [http://www.impinj.com/Monza_X_Development_Kit.aspx][Link 1]

I think your best bet is going to be:

  1. Use Bluetooth Low Energy (BTLE) instead
  2. Use a reader with a Low Level Reader Protocol (LLRP) link between the reader and the phone
  3. Work with a RFID chip with an I2C bus

EDIT: [This is a spec sheet][Link 2] for a current chip that cites 20m read range. There have been some improvements since then with regard to lowering the RF sensitivity, but I am not sure what the latest numbers are for this year.

And [one more example][Link 3] from a few years ago showing 100 feet read.

Links: (I don't have enough reputation to link to things it seems) (1): http://www.impinj.com/Monza_X_Development_Kit.aspx

(2): http://www.impinj.com/Documents/Tag_Chips/Monza_4_Tag_Chip_Datasheet


  • \$\begingroup\$ I understand you trying to amend my answer in light of you not being able to leave an answer (maybe a glitch) but can you provide a link to the passive tags that can be activated and read at 20m? \$\endgroup\$ – Andy aka Mar 25 '14 at 21:33
  • \$\begingroup\$ I added in a link to a monza (not affiliated) chip that cites the 20m on their spec sheet. Its mainly a function of RF sensitivity: the lower the sensitivity, the farther the operating range. \$\endgroup\$ – Stwerp Mar 26 '14 at 16:44
  • \$\begingroup\$ i can't get the 2nd link to work dude. Are you sure they are passive I.e. energy harvest the rf power received to provide enough power for the chips? \$\endgroup\$ – Andy aka Mar 26 '14 at 17:31
  • \$\begingroup\$ I'm sorry its not working. If you google "Monza 4", the spec sheet should show up on impinj's website. Yes they are fully-passive. Modern tags have a sensitivity of -20 dBm (10uW RF). For a reader broadcasting +36 dBm EIRP (4W RF), the Friis transmission model predicts a read range of a little over 20m. For semi-passive, the read range is limited by reader sensitivity and can be many, many more meters. In other words, the limiting factor for passive devices is the turn-on power threshold of the device and not the reader sensitivity. \$\endgroup\$ – Stwerp Mar 26 '14 at 19:05

Ten to twenty metres is unrealistic from a passive tag unless it has an unrealistically large energy harvesting coil to receive enough power to energize its circuits. Of course if the energy transmitted by the static tag detector is unrealistically high then this can help but I'm talking about hundreds of volts across a coil several metres in diameter. How much room have you got?

  • \$\begingroup\$ The reader is the android phone. so the energy on that side isnt too high. I think 10-20 meters isnt possible then. Is there any other way to locate the presence of an android phone with a circuit which has very low power consumption \$\endgroup\$ – user2578666 Feb 24 '14 at 2:22
  • \$\begingroup\$ That sounds like a different question - maybe start a new question? \$\endgroup\$ – Andy aka Feb 24 '14 at 9:45

As the communication range of HF RFID and NFC is limited to a couple of meters (inductive coupling is only possible within the near field of the antenna coils and the frequency is at 13.56 MHz), the only way right now is to use a very high sensitive receiver to detect an NFC device (eavesdropping see here).
Bidirectional communication is not possible over such a range. In such cases you have to use UHF RFID technology.


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