Cheap wireless (RF) receiver/transmitter solution?

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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.

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.

-Doug

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

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. ers