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?
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:
- Use Bluetooth Low Energy (BTLE) instead
- Use a reader with a Low Level Reader Protocol (LLRP) link between the reader and the phone
- 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
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?
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.