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Mains powered very low power transmitter - need help please

N

nina.p20

Jan 1, 1970
0
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
antena....
I'll be very thankful for any help, idea or circuit :)
TIA,
Regards, Nina
 
M

Mark Fergerson

Jan 1, 1970
0
nina.p20 said:
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
antena....
I'll be very thankful for any help, idea or circuit :)

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
 
N

nina.p20

Jan 1, 1970
0
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 :
h**p://www.eskan.com/mains.html
This is an example of more or less what I need, just using it as a
monitoring device, a wireless baby-sitter :)
 
L

Lumpy

Jan 1, 1970
0
nina.p20 said:
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...

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
unattended?

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


Lumpy
 
N

nina.p20

Jan 1, 1970
0
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
sleeping.
Now, maybe I'll have a professional answer? Have I passed the test? :D
Nina
 
J

John Fields

Jan 1, 1970
0
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
sleeping.
Now, maybe I'll have a professional answer? Have I passed the test? :D
Nina

---
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?
 
L

Lumpy

Jan 1, 1970
0
nina.p20 said:
Now, maybe I'll have a professional answer?...

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
misleading.

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.


Lumpy
 
M

Mark Fergerson

Jan 1, 1970
0
nina.p20 said:
Thank you very much Mr. Fergerson,

Don't "Mister" me, I work for a living... ;>)
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 :
h**p://www.eskan.com/mains.html

Giving that link with your first post would have been helpful.
You said before:
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
antena....

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.
This is an example of more or less what I need, just using it as a
monitoring device, a wireless baby-sitter :)

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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