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Data communication through sea water. — Parallax Forums

Data communication through sea water.

HollyMinkowskiHollyMinkowski Posts: 1,398
edited 2010-09-10 10:36 in General Discussion
Has anyone here ever sent data through sea
water from a uC powered device? I need to
enable communications between devices on
the surface and devices located at a depth
of 20 to 300 feet or so. I think sound is the best
way but since salt water is very conductive I
wonder if signals could be handled by sending
and receiving current pulses of some kind. An
engineer told me that very low frequency rf might
also be made to work but I'm not sure about that
as it just seems unlikely that it could penetrate through
water very far, at least using the weak power levels
I would have to use to conserve battery power.

This is a pretty odd sort of question but I thought I'd ask
just in case someone here knows anything that
might be of help. googling for data did not turn
up much of value.

Comments

  • electrosyselectrosys Posts: 212
    edited 2010-09-07 01:37
    I'll try with the acoustic sounds first, convert data to sounds and send the sounds through
    water (with a underwater speaker) and a hyderaphone to catch the sounds and some kind
    of filter/modem to convert the acoustic sounds to data signal. That would be a lot of
    works and reserche, but a great project to do!

    The other option as you mentioned, would be the very low frequency or VLF, that can
    penetrate seawater to a depth of approximately 20 meters, which is not quite enough for
    your project. Serch/Google the "communication with submarines" it may be helpful.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2010-09-07 01:52
    Working as a marine carpenter made me very wary of saltwater and electricity. Everything had to stay away from the saltwater, or fail.

    For example, if you had a 20 foot extension cord and there was the smallest of nicks in the insulation, it would burn up in about 15 seconds in sea water due to the grounding and the conductivity of the sea water.

    Even if you don't have such destructive forces in play, corrosion is ever present.

    I've thought quite a bit about sonar - possibly going into the ultrasonic range. The trick here is to create a 'transducer'. I suspect that if you can find a good way to seal the rim of a polypropylene speaker so the unit can be immersed in salt water at the depths you want (that might mean pressurizing the backside of the speaker equal to the depth), it can be used both as speaker and a microphone.

    So it isn't really the electronics that are a challenge, but the hardware build. The only other option is buy rather expensive transducers.

    In terms of size and geometry, I suspect the smaller the speaker the better as the pressure barrier is smaller in size. So little tweeter speakers may be a good starting point. Then finding a case that will handle pressure. And finally setting up a scuba rig so that the regulator will provide equalized pressure within the enclosure. Sure, you could try to just pressurize the unit for one depth prior to immersion, but I suspect you would rupture the speaker membrane.

    Another alternative is to create a fully immersible tweeter and having only wires pass the pressure barrier.

    A third would be to have the coil remain inside the enclosure, and have a rod go to the outside that drives an immersed speaker cone.

    The choice is up to you.
  • localrogerlocalroger Posts: 3,451
    edited 2010-09-07 05:27
    When I was a kid I had a handed down issue of Popular Electronics magazine from about 1970 which had a project (implemented with discrete transistors) for doing this kind of communication by regarding the water as a sheet resistance. Basically you plonked two electrodes down in the surf say 20 feet apart, and out in the bay the boat is parked parallel to the shore with electrodes fore and stern; the fact that the aggregate resistance of the water is slightly more on the diagonal than it is straight was enough to pass a recoverable signal.
  • LeonLeon Posts: 7,620
    edited 2010-09-07 06:34
    The problem with VLF radio is that very long antennas are needed for the transmitter; the US Navy's antennas are several miles long. Something a lot shorter could presumably be used for short range underwater communications, but it would be difficult to design. Communications would only be one way, because transmitting from the submerged unit isn't feasible. Maximum depth is only 20m or so.
  • bill190bill190 Posts: 769
    edited 2010-09-07 06:52
    I would look to mother nature. Whale and dolphin communications underwater has been in development, testing, and refinement for thousands (millions?) of years!

    Genetics and survival of the fittest are guaranteed to only allow systems which work to survive.

    I searched google.com for the words...
    whale communication frequency

    ...and found this which gives frequencies and db...
    http://en.wikipedia.org/wiki/Whale_song

    Then here is a google.com search for the words...
    dolphin communication frequency

    http://www.google.com/#hl=en&safe=off&q=dolphin+communication+frequency&aq=f&aqi=&aql=&oq=dolphin+communication+frequency&gs_rfai=&pbx=1&fp=db0d63dc956f954a

    Then next is IR communication like would be used with a TV IR remote control. Here is the Sony SIRC protocol...
    (Perhaps instead of 38/40 kHz, use the whale sound frequency as a carrier. And use sound instead of light.)
    http://www.sbprojects.com/knowledge/ir/sirc.htm

    And old fashioned computer modems used sound over telephone lines to send data. There were all sorts of error detection and correction schemes developed for this.
  • kwinnkwinn Posts: 8,697
    edited 2010-09-07 20:56
    Holly, there was a discussion on this topic on the old forum and the conclusion was that using a fish finder transducer to transmit data was the most practical way to go. Current pulses in salt water are basically shorted out at the source.

    If anyone has more info on that thread please pass it on to Holly.
  • HollyMinkowskiHollyMinkowski Posts: 1,398
    edited 2010-09-08 04:23
    I have built 2 simple devices. Each are contained in a short length
    of plastic tube.
    One contains a board with a mic and circuitry for tone detection using
    an NE567 tone decoder chip. A bright green LED mounted on top indicates
    a tone detection.

    The other has an uC I/O line connected to a small plastic coned speaker.
    The uC sends pulses of 5khz to the speaker once a second for a few ms.
    (beep beep beep).

    Both plastic cylinders are completely filled with mineral oil to allow a
    better connection to the external water so the sound can travel
    freely.

    I'm going over to the port of Haifa to test these at 5pm and if they
    work the next step is to create the software to transfer some actual
    data between them.
  • LeonLeon Posts: 7,620
    edited 2010-09-08 04:33
    I'd try it in the bath first - it'd be a lot easier. By reducing the intensity you should be able to get a feel for the range you will get.
  • John R.John R. Posts: 1,376
    edited 2010-09-08 05:48
    I did some very rough research and testing on this for an ROV design that never got very far. Sound is the way to go, but there can be problems, especially in shallow water, with echos...

    John R.
  • skylightskylight Posts: 1,915
    edited 2010-09-08 18:04
    would a laser work after adjusting for refraction?
  • HollyMinkowskiHollyMinkowski Posts: 1,398
    edited 2010-09-08 19:27
    The units did seem to be able to communicate
    after a small adjustment of the 567 circuit for
    frequency. But the range was limited to about
    20ft. I guess I need more audio to the spkr or
    better mic sensitivity.
  • K2K2 Posts: 693
    edited 2010-09-08 20:22
    Lead zirconate titanate (PZT) piezoelectric disks and power MOSFETS.
    Work like a charm! Much better, in fact.
  • bill190bill190 Posts: 769
    edited 2010-09-08 20:40
    One of the whales from the link above makes sounds at 155–186 db.

    The following link has a chart as to how loud that is (jet plane, gun shot, fireworks)...
    http://www.dangerousdecibels.org/hearingloss.cfm

    This says the blue whales sounds can travel many miles and also has sound db info...
    http://www.enchantedlearning.com/subjects/whales/species/bluewhale/Loudest.shtml

    As to making a sound that loud, many high school age kids sure seem to know how to make the windows in my car vibrate with their windows and my windows up! :lol:
  • pharseidpharseid Posts: 192
    edited 2010-09-10 09:20
    In the 70's, an astronomer friend of mine had access to a tract of land that the Navy owned which had a radio telescope on it. I went cross country skiing there one time and it struck me as odd that the Navy would own this land which ran for miles with only the telescope and associated buildings on it. This being landlocked and in the middle of Michigan. It was later suggested to me that it might have been an ELF antenna site. The Navy had attempted to build an antenna on a much larger piece of land here, but it was stopped by environmentalists and stories about it mentioned that they had built a smaller one somewhere in the state, so I suppose that was possible. Also, a couple of later stories opened the possibility that the Navy keeps subs in the Great Lakes.

    Not particularly helpful to Holly's project, but I'm curious if anyone here has run across any information on this.

    -phar
  • bill190bill190 Posts: 769
    edited 2010-09-10 10:36
    Here is a bit on ELF. Might follow the links and maybe you can find some info...
    http://en.wikipedia.org/wiki/Extremely_low_frequency
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    An interesting old Parallax thread.
  • koehlerkoehler Posts: 598
    Someone asked for whales with frickin propellers on them?

    Standard sound might be worth experimenting with, unless you have funds for HF.

    Waterproof lexan case, speaker, and rechargeable batter with a wireless/inductive charger circuit for the deep end. Similar for top end, but tethered would be better since you probably want to keep track of it anyways, just might be a bit more difficult to waterproof, however its also at a lesser depth.

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