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Mains powered very low power transmitter - need help please

Discussion in 'Electronic Design' 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. I read in that nina.p20 <>
    wrote (in <>) about
    'Mains powered very low power transmitter - need help please', on Mon,
    15 Aug 2005:
    This is a VERY DANGEROUS technique, and using it in a kindergarten
    terrifies me. PLEASE DON'T. Putting stuff on the ceiling doesn't stop it
    catching fire.

    It's also illegal to transmit on 100MHz without a broadcaster's licence,
    especially over such a range as 200 m. Think how many FM broadcast
    receivers are in a circle 400 m across round your site.

    Can you please tell us exactly what you want to do, and we will try to
  3. nina.p20

    nina.p20 Guest

    Thank you again Mr. Woodgate for responding,
    I'd like to split my question to 2 parts:
    First, I'm really interested to understand how those devices are made,
    just for my information and knowledge.
    Secondly - what I really need is a wireless babysitter, and cheap
    I've Mentioned this frequency because it's receptable on a domestic
    receiver, and the range may be up to 100m. As I have some experience in
    electronics, (well, being supervisor for the last 27y of an electronic
    lab - not RF - and now retired) I'd like to build such devices. I can
    make some changes to the receiver to work at higher frequency, let say
    110 - 115 problem. The problem is that the devices I've build
    so far (as mentioned in my first post) are very unstable, noisy (can
    hear the 50Hz ripple louder than the picked sounds). I'm aware that
    there will be no chance to get a licence, and about the fire danger -
    at the place it's been intended to be placed there is no chance, as
    it's a concrete 3m high wall - far away from any dangerous flamable
    TIA for further help :)
  4. I read in that nina.p20 <>
    wrote (in <>) about
    'Mains powered very low power transmitter - need help please', on Tue,
    16 Aug 2005:
    Buy one: you couldn't make one for less than the purchase price AND they
    work in a permitted frequency band as well as being safe.
    I'm sorry, but your message indicates to me that it would be inadvisable
    to advise you further. 110-115 MHz is outside the range of broadcast
    receivers because it is used by aircraft systems, including landing
    systems, so you would stand a good chance of causing an aircraft crash.

    Also, ANY fire is dangerous, even if it's only the electronic equipment
    that burns up. Burning bits drop on people below.

    If you didn't learn discretion in 27 years, I stand no chance of
    teaching you now.
  5. Naveed

    Naveed Guest

    Why dont you just get a baby monitor or walkie talkie and hack/midify it to
    suit your needs? They tend to have channel select switches so you can
    select the best one to suit your needs and you dont need a license for it.

  6. nina.p20

    nina.p20 Guest

    Hello Mr. Woodgate,
    Thank you very much for responding.
    Well, I've been for the last 20 years the safety superviser of my
    company too (over 5000 workers), but this really is not the point.
    As I don't want to produce such disasters as you wrote, and don't want
    to be blamed that the next earthquake or other disasters happened
    because my HUGE few mW wireless baby-sitter, I'll still ask for
    asistance - just for my knowledge, but I'll follow your advice and try
    to find a ready made one... :) I think you may be right...
    Thanx a lot for the time you've spent for writting me,
    Best regards,
  7. nina.p20

    nina.p20 Guest

    Thank you for responding Naveed,
    I've bought 2 pairs (1) of walkie talkies for this purpose.. one works
    on 27MHz and the second on 49MHz. There are 3 problems with this
    request : 1 - I've asked for mains powered devices - transformerless, 2
    - their range is up to 30m (I think I can modify this, building an RF
    AMP stage....but I'm not sure it's legal) and 3. - to encrease their
    sensitivity (they are supposed to hear a wisper from 3-5 m) I have to
    buid a high gain audio amplifier. The best part is they are crystal
    I'm just curious how devices like the ones in
    h**p:// work. Mains, safe(?) and NO EXTERNAL
    ANTENA !!! I'm asking just for my knowledge, and trying to find answers
    for that anywhere ... no success so far.
    I've build a long time ago surveillance devices, but never succeeded to
    make them work from mains, getting very high 50Hz humming without any
    antena, and with a simple wire short antena, it became unsupportable
  8. Naveed

    Naveed Guest

    Is this system your trying to put together supposed to be covert so the
    people your monitoring don't know they are being monitored? If it is then
    there will be legal issues you may need to deal with. Otherwise you could
    just get a baby monitor (they have very sensitive microphones) and you
    should be able to filter any hum pickup at the receiving end. If you want
    it transformerless to reduce the mains humm then you can use a switch mode
    power supply and add some extra filtering and a shield.
  9. Naveed

    Naveed Guest

    I've just looked at the website. Don't know much about how its done. Why
    don't you buy some and take a look for yourself?

  10. nina.p20

    nina.p20 Guest

    The device is supposed to monitor large rooms, not spying on people !!!
    I need it transformeless to reduce the size, it's very important for
    several reasons.
    A swich mode PS is very big for my needs....
    The website I've sent is just for an example for the "transformerless
    transmitter" posibility, and I don't buy it for 3 reasons:
    1 - It's very expensive for my needs,
    2 - I'll really mess up with the custom authorities trying to buy a
    3 - still hope that a "good soul" may give me (maybe for some money...)
    an idea - or better - a schematic of this kind...

  11. Transformerless, mains-powered devices are tricky to
    design and critical to select parts for and demanding to
    build to maintain the kind of safety needed in your
    application. Perhaps you should explain why you think
    you need transformerless mains-powered supply for this
    Your requested range (100-200m indoor/outdoor?) greatly
    restricts the kinds of solutions available to you. And in
    many places makes it practically impossible to do without
    a license to use the appropriate frequency spectrum.
    Kindergarten children should generate enough noise to
    be picked up by the average inexpensive electret mic.
    Tying to detect individuals or conversations at a distance
    especially indoors, is a violation of the known laws of
    physics which nobody has yet solved.
    They likely use a large capacitor to drop the mains power
    to what is needed by their circuitry. This is an unconventional
    method and requires special capacitors that are capable of
    handling mains power directly. NOT your average kind of
    circuit or components. It is also possible that they could
    use small ~ 1-inch (2.5cm) transformers for power conversion.
    Specialized circuit design, component selection, and
    construction, and submission to safety testing agencies
    for certification to allow sale in most countries.
    They simply use the mains wiring for the antenna by using
    small capacitors to couple the RF to the power line. Note
    that these capacitors also must be special high-voltage,
    non-flammable ones for safety.
    Demonstrating that you are asking about a very specialized
    and demanding kind of circuit design. It is difficult to design
    something very sensitive to audio/microphone signals which
    is in such close proximity to high-voltage mains power. And
    even more difficult when done with a transformerless power
  12. Guest

    Sir, what you would best consider is a trip to the local radio shack
    store or similar and pick up a 110V carrier current intercom system.
    (~60$ per pair) They use FM on low frequency carriers injected back
    into the 110V line using a L-C network that passes 110 to their power
    transformer or these days a chip with a special power FET to current
    limit into a few mA DC supply , and yet provides a proper impedance for
    the low frequency RF. They have a sensitive mike and can be locked on
    transmit, and used to come in a 4 channel flavor. We used them to
    listen to my father all night when we brought him home from the nursing
    home. Only glitch is you must often have the receiver on the same pole
    transformer or often on the same phase, however coupling modules are
    available from on line shops that sell household control devices (X10
    for example) that use the same principles for remote controling lights
    and coffemeakers. frequency is typically 50 to 450 Khz in 4 bands.

    They would work very well for your purpose, are small enough to be
    covert and are UL compliant. I'd much rather have one of those in my
    kindergarten as it cuts down information transmitted to people who
    should not hear it. As a former teacher, I can say having classroom
    conversations get out to the wrong people on a simple FM carrier is
    REALLY BAD for both the teachers and the students. It can ruin

    I can however understand why the supervisor of a nursery would want to
    monitor it these days. However with web and cell phone communications
    these days, having a unencrypted carrier up stands a good chance of
    attracting some really bad people through word of mouth.

    for a example of how the off line low current transformerless PSU
    works, go to and take a look at a LR6 IC.

    the keywords to search for on using power lines as a transmission line
    are "carrier current" carrier current is also used by utilities for
    controlling substations and on subways, its a proven reliable

    This National semiconductor ap note (outdated) is a good tutorial
    for data over powerline, and its coupling network will explain your FM
    using power line antenna question.

    And before someone docks me for telling this fellow this, he would be
    much better off on carrier current then 109-138 aircraft areas thus
    not messing up a localizer/omni/glideslope or marker signal at the
    local airport. 76-88 is used for industrial remote control and other
    telemetry links so he doesnt want to be there either. Either side of
    the FM band is a BAD place to be. Also CC limits the receive
    possibilities to 100 meters or so.

    Steve Roberts
  13. nina.p20

    nina.p20 Guest

    Thank you Mr. Crowley,
    1. I need them transformerless as they were intended to be connected on
    a wall without a mains socket, and without drilling on this wall
    connected to the mains wall box.
    2. The device (s) have to transmit FROM the room to the yard - or
    inside the building, so there are some walls included. 100m indoor is
    3. Sensitivity is needed for the sleeping time. Any unusual sound, as a
    sound made by a kid waking up must be transmitted BEFORE this kid is
    waking all the room.
    4. I've been using such capacitors for various tasks: 0.2 - 0.47uF
    /400V Mil SPC.
    5. Coupling RF output to mains by let's say 0.1nF/400V won't do the
    job, as without being isolated by transformer it may get a large AC
    potential and burning the device.
    6. Somehow you are under the impression that I'm trying to build some
    kind of very sophisticated spying device, and just trying to hide it
    under the cover of a "naive" device.
    IT'S NOT SO !!! If I would like to try to do such a surveillance
    device, I would ask for exactly what I need, without fear and without
    hidding !!! The worst answer I could get is that this group is not
    dealing with such devices, that it's illegal etc. Without many
    philosophic thoughts about safety, legality and so on.

    I've asked for a circuit that IMO is very simple - for ppls that work
    with RF and have good will and good knowledge. If there is no such a
    person, or if the moderator thinks that I'm trying to do illegal
    thinks, he/she can simply delete my request...
  14. nina.p20

    nina.p20 Guest

    Thank you very much Mr. Roberts !!!
    The information you've sent is invaluable for me :)
    I've already ordered some pieces of LR and hope that they will answer
    to my curiosity... :)
  15. Ben Bradley

    Ben Bradley Guest

    So as soon as someone hears a child awake, they're going to run 100
    meters before the child wakes up all the others? Or is Carl Lewis on
    X(c) = 1/(2pi*f*c) =1/(2*3.14*60*0.1e-9) = 26,539,278 or 26 megohms.
    Current with 120 volts across it is I = E/Z = 120/26,000,000 = 4.6
    The capacitors I've seen in X-10 units (putting 120kHz on the power
    line) are about 0.1 microFarads, so the numbers are 1000 times as
    great, and the current through the capacitor is about 4 milliamperes.
    The capacitor then goes to a low-impedance winding of a transformer
    designed for the specified frequency (the RF frequency, not 60 Hz),
    which couples the RF into the power line (which would have an
    impedance that varies a lot with frequency, but is fairly low
    regardless, on the order of 1 to 10 ohms). So yes, you use both the
    capacitor and a small RF transformer (NOT a relatively large filament
    or "wall-wart" sized 60 Hz power line transformer - the RF would be
    strongly attentuated through it anyway) to couple the signal to the
    power line. The X-10 RF transformer I recall looked just like the tiny
    IF transformers in old hand-held transistor radios, hardly bigger than
    an electret mic element itself.

    My soulution would be an electret microphone with 100 meters of
    cable going wherever your monitor is, going to an electret 'bias
    supply' and preamp, as in a computer soundcard mic input, minidisc mic
    input or old portable cassette recorder mic input and preamp.
  16. I do not understand your circumstances. How do they connect
    to the mains power?
    100m may be too far for many no-license rf channels in most
    countries. Since we don't know where you are, we can't even
    make any educated guesses.
    Leaving multiple children sleeping unattended seems like a bad
    idea to me, especially at the distances you are specifying. But if
    you really want that kind of sensitivity and selectivity, you will
    need to move the microphone closer to the kids. This is a law of
    acoustic physics that remains unbroken.
    400V isn't really sufficient for safety even for the North American/
    Japanese 100-120V. 600V and special flame-proof components are
    what is more commonly used in this application. And of course
    you need to double that if you are in a 230-240V mains territory.
    As Mr. Bradley observed, impedance matching (and isolation) with
    an RF transformer seems like a popular way of solving this problem.
    Our concern is that you are asking about a potentially dangerous
    circuit and you don't seem to have enough appreciation for the
    hazards of this project. We are reluctant to help you do something
    possibly destructive or lethal.
    I would bet a good dinner that there are several examples of
    these kinds of mains-coupled RF circuits out there on the internet
    just waiting for you to search for them. And even direct-mains
    power supply circuits also. If you are as experienced in
    electronics as you claim, you should be able to find several
    examples of relevent circuits and be able to evaluate them to
    see how applicable they are to your requirements.
    We don't know whether it is illegal or not. We don't know
    enough details (not even what city/country you are in).

    There is no "moderator" this is a Usenet newsgroup which
    is available for reading and posting messages to everyone.

    You appear to be reading this newsgroup through the Google
    web interface. But Google is only a slow, indirect way of
    accessing some of the 100,000 Usenet newsgroups.

    You may find this helpful...
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