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
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
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
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://
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
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...
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?
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 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
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.
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.
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.
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.
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.
I've been using such capacitors for various tasks: 0.2 - 0.47uF /400V Mil SPC.
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.
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
I read in sci.electronics.design 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.
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
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.
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 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.
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?
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.