DTMF over Air - Page 1
Forum section: electronics / projects

author: @rt
author_rank: Super Contributor
timestamp: October 03, 2015, 06:01:23 am
content: Hi Guys,I have send and received DTMF over air from the ground up, and would like to improve the receiver.I have an AM receiver listening at 1700kHz, just off our AM commercial band here in Aus, and that frequency can carry DTMF just fine.This is a surprise to me because commercial radios don’t seem to feature DTMF modulation until they get into VHF.Beginning with a Motorola MC145436 DTMF decoder IC, it occurs to me a lot of the AM radio is an unneeded complication.The minimum requirement (I believe) would be:(1) An LC circuit tuned to ring at the desired frequency,(2) A diode to strip the carrier (though I’m not sure it could be omitted when the chip runs at only 3.57MHz),(3) “Perhaps” some transistor amplification if the signal is not strong enough for the IC to read the signal.(4) “Perhaps” an antenna and Earth connection coupled to L in the LC circuit if L is not a good enough antenna.Am I correct in all of this?Cheers, Art.

author: TSL
author_rank: Regular Contributor
timestamp: October 03, 2015, 08:30:42 am
content: Don't be transmitting anything without the appropriate license or you could get your kit confiscated and be hit with a heavy fine.For what you're doing a LIPD license is probably appropriate.Go here for more info....http://www.acma.gov.au/Industry/Spectrum/Radiocomms-licensing/Class-licences/lipd-class-licence-spectrum-acma

author: @rt
author_rank: Super Contributor
timestamp: October 03, 2015, 02:09:21 pm
content: Ok.. I have a midrange pic making the tones, and dedicated Motorola MC145436 decoding at the receiving end.I have decode routines for dsPic, but don’t mind the chip doing it.It’s works fine audio coupled through an AM radio (I shifted it’s range to cover 1700kHz).I’m sure the transmitter is fine, but having trouble with my own minimal receiver.Yes I’m trying my best with radiation to keep it on my property.I know my AM radio won’t tell if ACMA can see it on their spectrum analyser or whatever,but to be honest, I have open air projects that make more noise on 10MHz than this,and that isn’t even trying to be a radio transmitter :O

author: sarepairman2
author_rank: Frequent Contributor
timestamp: October 03, 2015, 07:45:30 pm
content: this guy can do dtmf over air

author: helius
author_rank: Super Contributor
timestamp: October 03, 2015, 08:11:58 pm
content: You might get some ideas from sites like this:http://www.genave.com/dtmf.htm

author: Someone
author_rank: Super Contributor
timestamp: October 03, 2015, 10:42:35 pm
content: Quote from: @rt on October 03, 2015, 02:09:21 pmYou really do not want the huge fines that can be imposed, overlapping with radionavigation is a quick way to get noticed so stick to the 1800-1825 amateur band and check your other (un)intentional radiators are clear of any infrastructure. People get pulled up on this stuff all the time.

author: Howardlong
author_rank: Super Contributor
timestamp: October 03, 2015, 11:22:39 pm
content: Quote from: @rt on October 03, 2015, 02:09:21 pmInvest in getting yourself a ham ticket, please. From what you're saying and the way you're saying it, you're inviting an unexpected knock on the door.

author: @rt
author_rank: Super Contributor
timestamp: October 04, 2015, 01:18:22 am
content: sarapairman, I think it might be a matter of speed and timing giving the appearance of simultaneous sounds like software!I’d be lying if I said the emission was not always on my mind, and it might make sense to move up a little into an amateur band.1700kHz is very close, but it’s an advanced amateur band. Don’t get me started on the licensing :rolleyes:When I did my first license looking to continue study for advanced to facillitate radio experimentation(which is the Government’s view of what the hobby is all about) it meant that I had too much time on my hands!It’s far more virtuous to buy a commercial black plastic box and plug an antenna into it.I’m not using much power when modulating anything (variable PSU).There are AM and FM transmitter kits for kids, and for sending to radios in vehicles,so I figure there’s a point where my stuff is going to look like one of those if it’s noticeable at all.Do people with Tesla coils and Slayer Exciters get knocks on their door,or is it just because I’m modulating the carrier that draws attention here?It was actually me asking questions about interference before ever getting started.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 04, 2015, 02:23:08 am
content: Legalities aside, I don't recall seeing anything about signal strength or QRM/QRN?There are (or at least used to be) complete AM receivers in a TO-92 plastic 3-pin transistor package.You just add the tuned L/C circuit at the input, and take the demodulated AM signal output from the other end.ZN-414, apparently superseded by TA7642 which I found on Ebay for US$1.58 for 5 of them.But then, if you don't have a strong, clear signal, then you may need more of that "unneeded complication" of a selective and sensitive receiver.

author: @rt
author_rank: Super Contributor
timestamp: October 04, 2015, 03:47:06 am
content: Hi Richard, not in this thread. I’m modulating a slayer exciter, and asked about it in my Tesla coil thread:https://www.eevblog.com/forum/projects/tesla-coil-q's/but I did correctly figure that it would draw attention (on forums) only when I spoke of the carrier containing intelligence.It’s not a beautiful sinusoid, but audio is demodulated with a commercial radio very clearly.Haha my old friend the ZN414. I would have tried it if commonly available (Jaycar) Maybe I will go running to eBay.Did you know the original Dick Smith AM radio kit containing the device had an error preventing it working,and was never corrected? None of them could have worked. I put one together after DSE had been sold.It’s not the AM radio using the same device in the Funway 2 kit series. That is transistor amplified as well.

author: TSL
author_rank: Regular Contributor
timestamp: October 04, 2015, 04:15:49 am
content: Quote from: @rt on October 04, 2015, 01:18:22 amSeriously ? a HAM ticket is not that hard, even for an advanced ticket!Quote from: @rt on October 04, 2015, 01:18:22 amAnd that would put you in the no license required, LIPD or ISM segments where you could play to your hearts content.Quote from: @rt on October 04, 2015, 01:18:22 amAnd those kits invariable transmit in the LIPD or ISM spectrum thus not requiring any specific license. If you ensure your output is in the appropriate band the gubmint is not going to bother you either.Quote from: @rt on October 04, 2015, 01:18:22 amI can assure you that any signal radiating from _ANY_ apparatus that causes interference with licensed services will be swiftly dealt with. Especially if any of those interfere with any emergency frequency. Interference to those services could get you the ACMA + Feds + local police knocking on your door simultaneously.So... just make sure whatever your signal is, make sure you're in the LIPD or ISM bands to avoid any undue attention coming your way.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 04, 2015, 05:34:52 am
content: Well,duhh!-- you can send audio tones on AM!It is hardly breakthrough stuff,MF Broadcast Stations test their transmitters with fixed tones from 50Hz or so,up to 10kHz.Even with the lousy IF bandwidth common in currently available domestic AM receivers,there is plenty of bandwidth for DTMF.I will add my warning to those above.1700kHz & adjacent frequencies arenotfree of Stations you could interfere with.This is the realm of low power MF Narrowcasting---see Attachment 2 on this link:http://www.acma.gov.au/theACMA/narrowband-area-service-licensingThe people on this Band paid money for their licences,& will not be at all happy if you come up on top of themwith DTMF tones.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 04, 2015, 06:38:17 am
content: Art,it appears.from comments on other forums that you have let your "F" call lapse.Is this so?I'm not sure if the "F" qualification is permanent----if it is,you can,& should,renew it,& do your Standard licence,instead of farting around with "Slayer exciters" & such .By the way,why do people give simple circuits such dumb names these days?---Slayer exciter,Joulethief,etc.In the old days we had sensible names like "flip- flops"!!Actually,"flip flop" is fairly descriptive of what the circuit does,& maybe,drawing a very long bow,so is"Joule thief",but what does a "Slayer exciter" slay ---unwary Engineers?

author: Howardlong
author_rank: Super Contributor
timestamp: October 04, 2015, 12:57:40 pm
content: I got my ham ticket when I was 16 in the eighties. These days access has never been easier, 12 year olds regularly pass the test. I read recently of a 5 yo passing.Blasé attitudes to spectrum are exactly the reason why regulatory authorities propose draconian legislation, for example the FCC proposed lockdown of routers, just because of a few folks who thought that spectrum regulation didn't apply to them.If you're not interested in using the spectrum legally then so be it, but equally don't expect too much help to do so.As others have said, there is nothing sacred about using DTMF on LF, HF, VHF or anywhere else, nor in any analogue mode that supports a standard 3kHz voice channel, AM, FM, PM etc. However it's not likely to be used on an SSB channel, you might want to think why that might be. The only reason it became ubiquitous on VHF and UHF narrowband FM was through increasingly common usage.

author: @rt
author_rank: Super Contributor
timestamp: October 04, 2015, 02:19:09 pm
content: Quote from: vk6zgo on October 04, 2015, 05:34:52 amWell duh! it works over a dirty old phone lineI guess I just had it in my head because I’ve never seen a HF rig with stock DTMF, it couldn’t be expected to be reliable.Quote from: vk6zgo on October 04, 2015, 06:38:17 amA joule thief is getting there if you pretend the joules weren’t yours in the first place.A Slayer exciter, I wouldn’t know what to call it. The fellow that came up with thecircuit was selling kits, so a name like Half-Ass Tesla Driver was out“I'm not sure if the "F" qualification is permanent----if it is,you can,& should,renew it,& do your Standard licence,instead of farting around with "Slayer exciters" & such .”I think that it is permanent. Just the same I’d still be farting about withslayer exciters and such, then the Tesla coil, and then the next whatever it is.Quote from: Howardlong on October 04, 2015, 12:57:40 pmI am interested in using it legally. Have I expressed otherwise?More the reason why I may not be.. which appears so far that I just have to move the frequency.But that doesn’t explain AM/FM kits that broadcast right of top of AM/FM commercial broadcast bands.I’m not trying to express any poor attitude, I just don’t do vague well.DTMF SSB modulated? I imagine the bandwidth would be there. SSTV is expected to workat least locally.Hum.. to think about it… For any real HF work it would be unreliable,Both clocks would have to be clocked fairly accurately. Doppler shift,ionospheric movement, and just about any other natural effect like tunnelling would be undesirable.You don’t have too many HF repeaters or such equipment wanting to be controlled bydomestic amateur radio.It might also be an issue that a multi frequency wave doesn’t have vertical symmetry.

author: Howardlong
author_rank: Super Contributor
timestamp: October 04, 2015, 04:02:28 pm
content: QuoteYes, you've stated you're transmitting on 1700kHz, do you have a license to do so?Regarding the AM/FM kits, it depends on the local regulations, in the UK it's quite legal to have such devices but not to operate them for example. When I was a kid, one of my buddies built one. Sure enough, he had his collar felt soon enough.The only unlicensed devices that I'm aware that are commonly allowed globally on the FM broadcast band are the MP3 car radio adapters, where there is often specific local regulation allowing them, but with very limited power.Typically, any spectrum allocation won't just be for a chunk of spectrum: it will frequently specify modes, whether it is broadcast, radiolocation or point to point, whether it is for space or terrestrial use, for data or voice, or any number of specifications.

author: @rt
author_rank: Super Contributor
timestamp: October 05, 2015, 02:47:10 pm
content: That is more about ignorance (and concern). I did ask how a Tesla coil can be legal to use straight up probably within a day of powering anything.I would be surprised if they could be effective in any LIPD frequency, or emit millions or billionths of Watts.I am very interested in what the ACMA’s position on this would be:http://tesladownunder.comThere is no Faraday cage there, and the site does not seem to acknowledge the ACMA at all.Something like that has also been an act on Australia’s Got Talent.I might be wrong here, but I don’t think it’s resonant frequency can be very high.At least the presence of another resonant LC circuit keeps it centred well, but they are also very wide,and the sin is not circular but condensed, with narrower peaks like that of a hand cranked dynamo designed to make distance.and also might mean more harmonics with tighter slopes.They are also better radiators than I think radio people would give them credit for (though a rather unfortunate filter).I’m somewhat convinced some of that is about the carrier being produced with high potential.This is something I’ve given thought to from the beginning.My goal has been to make a kick ass Tesla coil, and a slayer exciter, the safe and responsible way to enter the project.I have actually ordered my first neon transformer, so I am certainly interested.Radio interference, when I talked about it on several forums, It occurred to me, was more on my mind than anyone else’s.Until the signal was modulated. People are doing that with large Tesla coils too.Now if I Google “ACMA Tesla Coil” The first result is me, of course asking about it on another forum,and the second is a funny one:http://www.wansarc.org.au/downloads/WANSARC_NEWS/2013/WANSARC_Vol_44_Issue_03_2013_low_res.pdf

author: vk6zgo
author_rank: Super Contributor
timestamp: October 06, 2015, 03:57:33 am
content: QuoteIn the old days,such things were strictly forbidden,but various people pointed out that such prohibitions didn't apply to Signal Generators,or Oscillators that someone might build up for some other purpose.They argued that very low power from tiny AM or FM transmitters would not cause any significant interferenceto Broadcasting,& besides,it was permitted in the USA,so it must be a great idea!After all,that approach worked well with getting CB legalised!They also followed CB's lead,in "just doing it,anyway".Various Electronics mags started publishing "RF bugs",& the whole silly idea took off.From all I've heard,they are still not really legal,but "slip through the cracks"---if nobody complains,ACMA doesn't do anything about it.QuoteThere is no critical clock with DTMF--the decoder at the other end recognises the tone frequencies & does whatever they mean.HF radios quite commonly use Selcall which is similar,Hams use RTTY,as did OTC & the old PMG on SSB.Quote10 metre repeaters often are controlled by CTCSS,which uses low frequency tones,so is a little different.Why do you say a multi frequency wave doesn't have vertical symmetry?They are just audio tones.Not that it matters ------TV video certainly lacks vertical symmetry,& is transmitted perfectly well on VSB.Tesla coils & such are pretty much regarded as "unintentional radiators".This,plus the fact that they are in intermittent use lets them "slip through the cracks" lust like the other low power devices.Once you modulate them,the radiation is no longer unintentional,so you can get in strife.Think of it from the point of view of a Narrowcast station:There they are,with all sorts of restrictions on how much power they can use,restricted bandwidth,plus the same stabilility & spurious specs as the big guys on the "real" Broadcast Band.Suddenly up you come,drifting back & forth across their allocation,pumping out DTMF tones at a similar power level to them.They well may become quite hostile after a few listener complaints.ACMA is called in,you are told to "cease & desist" & maybe fined.Worse,you have now "drawn the crabs" & in the worst case,there will be a general crackdown on "Tesla coils." & similar devices.Ironically,if you had been down in the AM Broadcast Band proper,your signal would probably have been comfortably overridden by the 10kW to 50 KW stations.

author: @rt
author_rank: Super Contributor
timestamp: October 06, 2015, 06:15:48 am
content: I mean the transmitter and receiver master clocks would have to be clocked accurately (for SSB), not the DTMF clock.An analogue VFO might be a pain. Otherwise the audio freq is shifted.I think they handle SSTV by shifting the whole colour shifted file back afterward (which I suppose you’d know about).I can get down into commercial AM Broadcast easily, but suspected that would be more noticeable.Either way I’m very turned off. However, I was successful in my original intent in this thread.Yes the signal is transmitted from the same room as the receiver. A ZN4141 device fails quickly.When I arced from the coil to a finger the ZN4141 lasted a couple of seconds of screeching.When you think about it the most useful thing in this schematic is the AGC outside the deviceDTMF is also decoded with the Motorola decoder IC with a microphone, when the DTMF audio is produced with plasma.To get a reliable radio I used a TDA2822M audio amp chip after hearing the DTMF through an audio amp when I touched the centre pin.It only has to be loosely coupled, needs no diode, no LC circuit, to work in the same room. Further away it needs an LC circuit,and still no diode. Maybe the carrier is just filtered because the amp is too slow for the carrier freq,or maybe it is still present and inaudible. That receiver would still need an AGC to work close to the coil.I would try more aimless experimentation if it were harmless, but am way turned off a larger coil with neon transformer.I’ve ordered the test neon tube as well so I think I’ll just light Commodore Amiga retro desk 2000 with the neon and find something else to do.

author: ivaylo
author_rank: Frequent Contributor
timestamp: October 06, 2015, 06:45:00 am
content: Ha, "Dual Tone Multiple Frequency" was the only meaning of DTMF I didn't know -http://www.urbandictionary.com/define.php?term=DTMF