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Very Short-Range Carrier Wave AM or FM Transmitter Help

Discussion in 'Electronic Design' started by Joe G., May 7, 2005.

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  1. Joe G.

    Joe G. Guest


    I am trying to find or design a circuit that would transmit a carrier
    wave (at any fixed audio frequency) radio signal on the AM or FM band
    only a few inches (anywhere from 2 to 12 inches). The AM or FM
    frequency must be able to be changed across the band by changing a
    resistor, capacitor or ? It also needs to as small and simple as
    possible. Audio quality isn't important. Audio frequency it transmits
    isn't important.

    What I am trying to do is something like RFID they are using in
    inventory control, except mine must transmit less than 12 inches and
    just transmit a carrier wave at a fixed audio frequency. Since I have
    less than 16 objects I have to identify, I planned to tune each of the
    16 units to a different AM or FM frequency. Then I could identify
    which of the 16 units I had based on the frequency.

    I am also open to other ideas that would accomplish the same thing. I
    am trying to keep the costs low, so I didn't think standard RFID
    equipment would be an option.

    If you have any suggestions, even just a word or two of sdvice, it
    would be greatly appreciated.

    Thanks a lot,

    Joe G.
    Reno, PA

    my email is:
    ru4him at
    gmail dot com
  2. Luhan Monat

    Luhan Monat Guest

    Do you need the device to tranmit continuously? That would make battery
    life a problem. Comercial RFID's are passive devices powered by the
    unit scanning them.

    If you transmit on different frequencies, you would need a separate
    reciever for each one. Better, to have a unique tone on each but
    tranmit all of them at a common frequency.
  3. You can do that with Long Range RFID systems but the RFID readers are
    $3k~$5k USD

    Look up Alien RFID uses the 900MHz band

    The tags are cheep but the Min order qty for tags is 12k pcs

  4. Do you really need to use RF, or can you go with Infrared?. If so,
    infrared would be a lot simpler. You could build a pulse-width
    modulator, have it demodulated at receiver, feed into poor man's A/D
    converter, and get one of 16 outputs.

    If have to use RF, you're going to have to build transmitter/receiver
    pairs, and while that's not terribly difficult, especially at such
    short distances, it's not something you want to do in a weekend.
    First, you should look up ring oscillators on Google. Connect an odd
    number of inverters (3 or more) in a ring and think about what signal
    looks like. :) This will give you cheap oscillation. You can put low
    pass filter/amp on output to get nice clean sine wave.

    If you don't mind going straight analog, you might try Colpitts
    oscillator. There are several variations. Pick the one that is least
    intimidating. Are you can purchases off-the-shell crystal oscialltors
    that generate very stable fixed frequencies. Then you will only have
    to worry about the receivers.

    For each receiver, you will need a tuned tank circuit at the antenna,
    followed by an amplifier, followed another signal-conditioning stage
    (high input impedance, low output impedance - look up emitter follower
    on Google), followed by half-wave rectifier, then level detector that
    will indicate whether carrier is present or not.

    IMO, making 4 transceiver pairs this way just to do what you're trying
    to do is a bit much. But if you want to use the same circuit for each,
    you will have to modulate the signal somehow. In other words, if
    you're going to make these circuits yourself, a little math is
    unavoidable. :)

    You can also call up companies that make key fob transmitters and see
    how they do it.

    -Chaud Lapin-
  5. Joe G.

    Joe G. Guest

    Thanks for the input.

    Your comments shed new light onto the project. I am not an expert at
    these things, and your experience really helps.

    Thanks again!

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