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Cheap wireless (RF) receiver/transmitter solution?

Discussion in 'Electronic Basics' started by Phantoz, Dec 14, 2004.

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  1. RFI would be minimal in this case because the master could poll all the
    slave units avery ten or more seconds, and very little time would be
    spent transmitting.

    But I would consider another method. Since every student seems to have
    a cell phone, have them call a number and enter their ID, and then press
    a key, which can be 1 to 4, or 1 to a hundred for that matter. As soon
    as the student completes, it hangs up. You could service a lot of
    students with just a few IVR lines.
  2. But they're all on about the same freq, and would interfere with each
    other if they were used all at the same time.
  3. krose

    krose Guest


    You might checkout this solution: Classroom Performance System (CPS).
    I does exactly what you are describing. This website is the parent
    site for the product, but they use resellers for most of their

    You can search for "Classroom Performance System" and find other
    resellers. In my search I found some textbook publishers are reselling
    this product. If you all adopt their text, they might be willing to
    cut you a deal on the technology side.

    This is not an endorsement for the product, only a possible solution!
    Hope this helps,
  4. John Fields

    John Fields Guest

    I disagree. Your opinion as to what's important or not is just that:
    an opinion, and may or may not be accurate, depending on the
    needs/wants of someone else. In this instance it may well be that a
    multitude of RF transceivers operating in a packet switching network
    is the way to go, considering the costs involved with retrofitting an
    existing structure with a hard-wired network, maintaining that
    network, and upgrading or replacing it as time goes by.

    As far as pollution from RFI goes, unless you've got evidence to the
    contrary, I'm sure that devices capable of pumping out
    micro/milliwatts into a specific band and operating at duty cycles
    of, say, a few minutes of RF out over an otherwise silent 24 hour
    period aren't going to do much to raise the noise floor.
  5. Rich Grise

    Rich Grise Guest

    And who writes the software that parses all these IDs coming in to your IR
    receiver simultaneously, i.e., all on top of each other?

  6. d.wills

    d.wills Guest

    Well, if I were doing the project, then I would. Or maybe the same
    undergrads that you would use for your bushbutton idea. The reason I thought
    that some type of visual feedback was necessary for the students was that
    if some responses clobbered each other, the student would see that his/her
    response wasn't accepted and would simply push the button again. A Microchip
    PIC 16F877 attached to almost any IR detector and a few other components,
    programmed using the free JAL language could store dozens of respones and
    could send them to the PC when polled (or the PIC could initiate the
    transfer when the PC is ready.)

    Note that the op's only requirements were: "only
    requirements are that the devices have some sort of unique signature to map
    to a particular student (though simple attacks to this like giving their
    device to another student are not an issue atm), as well as that the device
    allow the student to choose one of four options, and work at an acceptable
    range (say, 20-25 meters). I would like to translate the results real time
    to some comodity pc hardware."

    As far as I know from this, only one student is answering each question. If
    all (up to 100) need to answer at once, just increase the number of
    receivers. If they all receive send a serial stream to the pc, and the
    students are likely to respond at different speeds, then the likelyhood of
    frequent problems is small, but some students will have problems almost
    every time. Any normal PC should be able to handle the serial stream easy
    enough and if all id's come in as something like FF12345678A where 12345678
    is the serial number and A is the answer with a header of FF to signal valid
    data, then it would probably be close enough to s finished product that some
    tweaking would be all that is necessary to complete it. OK, you're right, a
    checksum of some sort is in order to validate the incoming data stream, but
    I don't think that would be too hard.

    Note that I never said this was the best way to solve his problem, just
    another way. Since people were sending out ideas and the OP hadn't
    necessarily spelled out all of his requirements, I was just bouncing around
    an idea.

  7. Rich Grise

    Rich Grise Guest

    The problem is, they'll be coming in like this:



    Where the '1's are IR remotes with the LED "on", and the '0's are the
    IR remotes while the LED is off.

    How does your sensor distinguish which photons are coming from which
    channel during the times that _any_ transmitter is transmitting an IR

  8. d.wills

    d.wills Guest

    Your point is well taken, but the sensor doesn't need to distinguish between
    photons from different ir remotes, it just needs to determine whether or not
    it has a valid response from any remote. I would place the ir sensors on the
    front wall with baffles in place to block all (most) signals but those
    coming from the appropriate row. This isn't too hard to do. Now the sensor
    will receive an incoming signal(s). If the sensor reads a proper header (
    example - "1111000010100101"), then count the next several bytes. Receive
    the serial number, answer and check sum, and then check the input. If the
    signal gets clobbered by another remote, the checksum bombs and the receiver
    rejects that particular input(s). This is why the visual feedback is
    necessary (think bingo type board with one light per student, reset each
    question). A student will push their button, see that their answer wasn't
    accepted, and push again.

    If we limit the number of remotes per sensor (my guess is anything less then
    10 would be ok), and questions are displayed to the crowd or read to the
    crowd, and there are 2 to 4 possible responses, I've got to believe that the
    responses will come in over a 10 to 20 second time period per question,
    making the likelihood of competing entries even smaller. A quick check on
    standard baud rates for ir communication via remotes seems to be in the
    100-2000 bps range. Although not particularly fast, it should be fast
    enough. Yes, I know I am assuming a lot here, but I am envisioning a normal
    classroom setting with some students getting the answer much more quickly
    then others. With these assumptions, most student responses would be
    received the first time, with a few here and there that would have to
    re-enter their answer per question

    Again, not a perfect solution for all situations. I.E.: if only one student
    is to answer at a time, perfect and cheap. If all students are taking a
    test, but the answers are not based on the quickness of a response, it will
    work well for most students for most answers with cost and expandability
    still being protected. If this is a Jeopardy type game speed is of the
    essence, then it doesn't work at all.

    - Doug
  9. Eric R Snow

    Eric R Snow Guest

    I see advertised in Nuts and Volts small transmitters and recievers.
    And small transcievers. It seems to me that each device could be
    polled by a computer and when all devices have answered the computer
    tells the teacher. I know there are wifi setups that let many people
    connect at the same time. I believe these all ask for data from
    specific devices that it recognizes to avoid interference. Sorta like
    the way cell phones work.
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