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Mains powered very low power transmitter - need help please

Discussion in 'Electronic Basics' started by nina.p20, Aug 16, 2005.

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  1. nina.p20

    nina.p20 Guest

    I need to build a mains powered FM transmitter, for relative short
    distances, about 100 - 200 meter. Going to use them in a kindergarten
    and they have to be very audio sensible, and at the same time to work
    7/24... Frequency - about 100MHz..
    I'm aware of the safetly rules, they will be placed near the ceiling;
    I've seen several devices as "surveillance or spy devices", and I mean
    exactly the same devices, but for other purposes. The devices - as I've

    understood are powered by a capacitor (250V), a diode bridge, a
    regulator and an electr. cap. for filtering.
    One of the most important things - they use no ANTENA, but how this can

    be made is "misterious" for me. They claim they use the MAINS wires as
    I'll be very thankful for any help, idea or circuit :)
    Regards, Nina
  2. Such devices use an ingenious, but not mysterious, bit of "trickery".

    You basically have a mundane modulated oscillator powered as you
    say from a pair of wires that have 60 Hz power on them. The trick
    comes in when you want to get the signal output not into the open
    air where anyone can get at it, but to a specific intended receiver,
    and the trick is named "filtering".

    The FM output is coupled back on to one of the wires through a
    capacitor (or more complex circuitry for "zap" protection) that has
    low reactance at the modulated frequency, but high at 60 Hz so that
    60 Hz power doesn't get back into the transmitter. As far as the
    device is concerned, the incoming power line and "antenna wire" are
    separate because its rectifier etc "looks" at the power line through
    a filter that easily passes 60 Hz but blocks everything else, while
    the modulator output "looks" at the AC line through a filter that
    blocks 60 Hz but easily passes the modulated signal.

    The receiver uses the reverse of this trick; it's an FM receiver
    whose power supply "looks" at the AC line through a filter
    (rectifier etc) that easily passes 60 Hz but blocks everything else
    and whose FM input "looks" at the same wire(s) through a filter that
    blocks 60 Hz but easily passes the modulated signal; as far as it
    can tell, the power and signal come in on separate wires.

    BTW, this works best on a given branch of the power line; often
    people try to use them on branches fed by separate circuit breakers
    and run into trouble because the breakers or GFCIs between separate
    branches may block or dump RF to ground. YMMV.

    Mark L. Fergerson
  3. nina.p20

    nina.p20 Guest

    Thank you very much Mr. Fergerson,
    I'm not sure I've got the point: What I'm looking for is a WIRELESS
    transmitter, and I think you ment some kind of "Mains Intercom"...
    I think so because the receiver's trick...
    Please, take a look here :
    This is an example of more or less what I need, just using it as a
    monitoring device, a wireless baby-sitter :)
  4. Lumpy

    Lumpy Guest

    100+ meters IS a relatively long distance for
    those low power xmitters.

    Why do you want to BUILD one instead of purchasing
    the one(s) listed in the link you posted?

    [end technical comments, begin social comments]

    What is the purpose for your transmitter?

    What kind of surveillance on the kindergarten
    kids are you doing? Are you leaving them

    Where do the parents/school board
    stand on this question?

  5. nina.p20

    nina.p20 Guest

    Well it seems that instead of getting some electronical help...I'm
    getting something else.... LOL
    I don't buy the devices, as they are expensive, and may get problems
    ordering them (custom)/ The range is up to 100m because there are walls
    that may do some kind of "screeneng". The devices are not for me, just
    been asked to build. They are suposed to monitor the kids when
    Now, maybe I'll have a professional answer? Have I passed the test? :D
  6. John Fields

    John Fields Guest

    Trust me, you will _not_ be able to build anything that works, (that
    is, construct it from a schematic and a bill of materials) so if you
    have any hopes of putting a functioning system together, you'll have
    to buy something which has already been built and install it in the
    premises you want to monitor. And hope for the best. Radio is
    tricky, and if you don't know what you're doing, chances are you'll
    fail. There may be a problem with the range you're trying to get,
    since that's a little far for an incidental radiator in the US and
    there are limitations on the strength of the signal the transmitter
    is radiating, regardless of its power rating.

    The transmitter you're talking about uses the mains wiring _as_ the
    antenna, instead of a dedicated antenna, by connecting the signal
    generated by the transmitter to the mains wiring and letting the
    mains wiring do the radiating.

    If you need a range of 100m, then you need a range of 100m
    regardless of what the walls are doing, so if you're trying to
    include a fudge factor because the walls "may" be doing something,
    you need to find out exactly what the walls are doing.

    All that aside, I assume that you're going to want to monitor the
    audio from the kindergarten at some distance away from the
    kindergarten. How far away is that, and will the receiver always be
    in the same place? If so, you may be able to get away with a lot
    less power by using a couple of directional antennas pointed at each
    other. How about some more details on the monitoring station?
  7. Lumpy

    Lumpy Guest

    Well, maybe you will, maybe you won't.
    This is usenet. You may get a right or wrong
    answer or even one that is deliberately

    You're looking for a professional answer,
    so that you can take an amateur approach.

    It's like asking how to build your own locks
    for the doors. Or how to print your own
    textbooks for the kids. It's a pretty sure
    bet that buying a professionally designed
    and built system will be less expensive
    and have less problems than you trying
    to reinvent the wheel.

    If you've got kindergarden kids sleeping
    in a building with someone monitoring them
    100-200 meters away, in another building,
    then I suggest that "how to build" is the
    least of your worries.

  8. Don't "Mister" me, I work for a living... ;>)
    Giving that link with your first post would have been helpful.
    You said before:
    Why do you consider this mysterious? It's done as I wrote before,
    by coupling the output of the transmitter to the power line through
    a capacitor that won't pass the power frequency. As far as the
    transmitter is concerned, pretty much any decent conductor makes as
    good an antenna as any other as long as it's more than a few
    wavelengths long. And carrier-current systems, which is what I
    described, do tend to radiate a bit.
    From reading your posts, it appears that somebody wants you to
    covertly provide them with something that they could buy off the
    shelf, except doing that would leave a paper trail leading back to
    them. If I were in your shoes I'd be very suspicious.

    Besides, from reading the specs, are you sure you could tune the
    thing as tightly as the commercial unit and get it to stay there
    without constant tinkering? This isn't a matter of soldering parts
    to a board and expecting it to work perfectly right off the bat.
    Narrowband FM transmitter/receivers can be cranky beasts.

    I notice prices aren't listed; have you contacted them? If you
    have, have you been offered at least as much to build something that
    will do the same thing?

    Mark L. Fergerson
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