Mains powered very low power transmitter - need help please

[nina.p20]

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

 

[John Woodgate]

I read in sci.electronics.design that nina.p20 <nina.p20@gmail.com>
wrote (in <1124173827.485384.84010@g43g2000cwa.googlegroups.com&gt about
'Mains powered very low power transmitter - need help please', on Mon,
15 Aug 2005:

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

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
help?
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[nina.p20]

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
one...
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 MHz...no 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
material
TIA for further help
Regards,
Nina

 

[John Woodgate]

I read in sci.electronics.design that nina.p20 <nina.p20@gmail.com>
wrote (in <1124179239.032777.239520@g47g2000cwa.googlegroups.com&gt about
'Mains powered very low power transmitter - need help please', on Tue,
16 Aug 2005:

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
one...

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'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 MHz...no 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
material
TIA for further help
Regards,
Nina

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.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Naveed]

"nina.p20" <nina.p20@gmail.com> wrote in message
news:1124173827.485384.84010@g43g2000cwa.googlegroups.com...

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

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.

Naveed

 

[nina.p20]

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,
Nina

 

[nina.p20]

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
controlled.
I'm just curious how devices like the ones in
h**p://www.eskan.com/mains.html 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
humming..
Regards,
Nina

 

[Naveed]

"nina.p20" <nina.p20@gmail.com> wrote in message
news:1124187027.343698.236820@g47g2000cwa.googlegroups.com...

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
controlled.
I'm just curious how devices like the ones in
h**p://www.eskan.com/mains.html 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
humming..
Regards,
Nina
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.

 

[Naveed]

"Naveed" <corlioni1976REMOVE@yahoo.co.uk> wrote in message
news:ddsh74$r0k$1@rdel.co.uk...

"nina.p20" <nina.p20@gmail.com> wrote in message
news:1124187027.343698.236820@g47g2000cwa.googlegroups.com...
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
controlled.
I'm just curious how devices like the ones in
h**p://www.eskan.com/mains.html 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
humming..
Regards,
Nina
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.
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?

Naveed

 

[nina.p20]

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
bug....
3 - still hope that a "good soul" may give me (maybe for some money...)
an idea - or better - a schematic of this kind...

Regards,
Nina

 

[Richard Crowley]

"nina.p20" wrote...

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,

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
device?

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)

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.

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
controlled.

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.

I'm just curious how devices like the ones in
h**p://www.eskan.com/mains.html work.
Mains,

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.

safe(?) and

Specialized circuit design, component selection, and
construction, and submission to safety testing agencies
for certification to allow sale in most countries.

NO EXTERNAL ANTENA !!! I'm asking just for my
knowledge, and trying to find answers for that anywhere ...
no success so far.

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.

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 humming..

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
supply.

 

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
lives/carreers.

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 http://www.supertex.com 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
technique.

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.

http://www.national.com/pf/LM/LM2893.html


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

 

[nina.p20]

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
enough.
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...
Regards,
Nina

 

[nina.p20]

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...
Regards,
Nina

 

[Ben Bradley]

On 16 Aug 2005 09:02:02 -0700, "nina.p20" <nina.p20@gmail.com> wrote:

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
enough.
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.

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
call?

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.

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
microamperes.
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.

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...
Regards,
Nina

-----
http://www.mindspring.com/~benbradley

 

[Richard Crowley]

"nina.p20" wrote ...

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.

I do not understand your circumstances. How do they connect
to the mains power?

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 enough.

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.

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.

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.

4. I've been using such capacitors for various tasks: 0.2 - 0.47uF
/400V Mil SPC.

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.

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.

As Mr. Bradley observed, impedance matching (and isolation) with
an RF transformer seems like a popular way of solving this problem.

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.

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'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,

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.

or if the moderator thinks that I'm trying to do illegal
thinks, he/she can simply delete my request...

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...
http://www.catb.org/~esr/faqs/smart-questions.html