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Switching between RFID antennas - one reader 100 antennas - how?

Discussion in 'Electronic Design' started by [email protected], Mar 21, 2007.

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  1. Guest

    I would like to connect multiple antennas (more than 100) to a RFID
    reader and switch between these antennas using a microcontroller. Can
    anyone point me to the right direction what type of switch can be used
    for this?


  3. Guest

  4. It should be ok for 125 kHz, but the ON-switch capacitance is large,
    so you're going to lose some signal at 13.56 MHz.

    I've had some good success with discrete switches using this part:

    You could also try the many flavors of T-gates (analog switches) like
    HC4051 DG221B .... etc.

  5. Wim Ton

    Wim Ton Guest

    Depends if you want to use the antannas for receiving or transmitting . A
    few 100 mA and 10s of Volts may be a bit much for a CMOS switch.

  6. Genome

    Genome Guest


    I know nothing about RFID but if you've got some used bike seats you'd like
    to sell??????


    Anyway, if your tags were a bit clever isn't there some way you could random
    adaptively time division multiplex some sort of thing that goes like.


    'What's the answer.... boys?'


    'Me, me, me'


    'Garbled, shut up for a random time'


    'Sulk, purlease whip us'

    'What's the answer, boys?'





    Transmitter (To Himself)

    'OK, I will now shut up for a bit longer, hurry up and whip me again'


    'What's the answer, boys?'



  7. John Barrett

    John Barrett Guest

    100 antennas for one reader is getting a little outa hand in my opinion --
    you might consider independent readers with 4-8 antennas + a microcontroller
    (which you need anyway to handle antenna switching), and interconnect with
    ethernet or multi-drop serial. i'm assuming the antennas will be at least
    somewhat seperated, and you are creating a huge headache backhauling 100+
    coax feeds, not to mention coax losses in the longer runs.

    there are schematics and code online for building MCU based RFID readers for
    just a few dollars in parts, and adding an 8 channel multiplexor to the
    front end of one of those should be simple enough. RS-485 is easy enough to
    cope with for the multidrop connection, or get an MCU with on chip ethernet.

    Personally -- I would consider some of those 900mhz data radio modules for
    the backhaul link -- thats the setup we are doing for a warehouse RFID
    system -- 4 antennas per reader with 900mhz RF backhaul -- 50 readers for a
    total of 200 antennas covering 20 dock doors, 5 fork lifts, and 25 key
    transit points throughout the warehouse. We had some real fun setting up so
    that the readers from one dock door didnt pick up traffic from adjacent
    doors :)
  8. Guest

    Thanks guys for you suggestions, a bit more background on what I am
    trying to build: I try to identify chess pieces on a 8x8 chess board.
    The pieces have an implanted RFID glass-tag which I want to read from
    below the chess board. The board is just a few millimeter thick and
    made of plastic. One solutions of course is to move the antenna on an
    simple xy slegde but I want to aviod moving/mechanical parts. I also
    want to use RFID and not any of the alternative techniques like Reed-
    contacts, etc.
    So, the 64 antennas are on a regular 8x8 matrix, one below each field
    which is 5x5cm. What ever the antennas pick up will be processed by a
    microcontroller this gives the option to resolve ambiguities because
    the MC knowns at least most of the time where the pieces stand and
    only the piece that has been moved needs to be found. However, the
    main thing I am worry about at the moment is how the MC switches the
    antennas. One of the few things I konw about RFID is that the antenna
    when in resonance carries up to 100V which is too much for most analog
    switches and multiplexer.

  9. John Barrett

    John Barrett Guest

    thats too close -- the antenna in any square will likely pick up the chips
    in ALL the pieces !! not a bad idea in general concept -- but off the shelf
    RFID hardware has too much range for this particular application -- you need
    something much more tightly coupled and lower power -- it can still be based
    on RFID technology, but scaled down for the millimeter ranges that you are
    talking about -- I would SERIOUSLY look at the MCU based rfid reader
    schematics out there -- at least you have the circuit so you can tune the
    power such that you only activate the chip in the correct square. The
    antenna coils need to be scaled back -- less turns, such they create less
    field to activate adjacent chips, and only activate the closely coupled chip
    in a given square.

    Since you are passing AC signals to the antennas, you could use 16 triacs to
    create the 8x8 matrix switch -- 8 for each row, 8 for each column -- the
    power should be low enough that the partially connected antennas will not
    activate their chips.

    Here is a website with an MCU based RFID reader including source code for
    the MCU.
  10. John Barrett

    John Barrett Guest

    Just an additional thought -- use ferrite cores for the antennas to direct
    the field more vertically -- like a stick antenna -- short piece of ferrite
    with a few turns of wire around it.
  11. Joerg

    Joerg Guest

    Just an additional thought -- use ferrite cores for the antennas to direct
    the field more vertically -- like a stick antenna -- short piece of ferrite
    with a few turns of wire around it.

    I'd try it with pot core halves. Done that in the past, worked great and
    it's cheap.

    Yve: At such short distances I'd operate it at lower signal levels so
    you don't run into lots of volts. Also, you can limit that with diodes
    if needed.
  12. Iwo Mergler

    Iwo Mergler Guest

    RFID antennas are not 50 ohm, so the standard RF switching
    gear won't do. On the other hand, a cmos switch would
    probably be OK.

    It will be very lossy, but I understand that short range is
    actually a benefit in your case.

    For your chessboard, consider using a matrix arrangement.

    Using core material and smaller coils is probably a good idea.

    Kind regards,

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