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Thread: have an idea for a project. Anyone have experience in underwater signal transmission?

  1. #1
    Senior Member SoulFetish's Avatar
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    have an idea for a project. Anyone have experience in underwater signal transmission?

    So, I can't get into the specific application, but here is what I need to do. I need to be able to transmit a wireless signal over a (lets say) 25 meter horizontal range and down to depths of probably not more than 35ft. I need to remotely trigger a switch. It's a simple function, so I don't need to do any complicated signal processing.
    Obviously RF is out of the question. There is no way the device would come close to being able to receive any low frequency RF. This leaves me with sonar as my best option. Does anyone have any experience designing/using small size transducers suitable for this application? Even though the device and I may not be stationary, I don't I have to worry to much about Doppler effects. Essentially, its a on-off/either-or condition. However, I want to prevent false tripping, so I would imagine I need to create a band pass filter. Anyway, I'm really at the "will this even work" investigating part of the idea. I have heard that the transducers suitable for this can be expensive?
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  2. #2
    rjb
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    Quote Originally Posted by SoulFetish View Post
    So, I can't get into the specific application, but here is what I need to do. I need to be able to transmit a wireless signal over a (lets say) 25 meter horizontal range and down to depths of probably not more than 35ft. I need to remotely trigger a switch.
    Calling Homeland Security, calling Homeland Security, come in please....
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  3. #3
    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by SoulFetish View Post
    transmit a wireless signal over a (lets say) 25 meter horizontal range and down to depths of probably not more than 35ft. I need to remotely trigger a switch. It's a simple function, so I don't need to do any complicated signal processing ...... This leaves me with sonar as my best option. ....... Essentially, its a on-off/either-or condition. However, I want to prevent false tripping, so I would imagine I need to create a band pass filter. Anyway, I'm really at the "will this even work" investigating part of the idea. I have heard that the transducers suitable for this can be expensive?
    Yes, ultrasonics is the way to go.

    Experiment with cheap readily vailable componnts to see how far can you go, starting inside, say, a bathtub, or garden pool or cows drinking tank or nearby lake/river, whatever´s available.

    Start with a couple piezo disks which can be bought at Radio Shack or equivalent, then progress to piezo tweeter disks which can handle quite a lot of power.

    Drive one with, say, 10kHz to 25 kHz signals , and check how far can the other detect that.
    Start with the raw unfiltered signal and transducers a meter away, then as you separate them increase gain on receiver end and you´ll have to add bandpass filtering to avoid being triggered by unrelated sounds.

    Eventually you will also need to introduce some kind of signal coding and decoding, so receiver reacts *only* to your special signal and not to another sound which my appear at same frequency, anything from bats or birds to insects to a squeaky ungreased wheel which chirps at that frequency.

    But of course it looks like it can be done.

    We are talking some 40 meters straight line which is not a small distance by any means, but not impossible either (for us) such as, say, 200 meters or more.

    In the glorious old days (think the late 60´s , early 70´s) when people did actually build all kinds of stuff at home, Heathkit used to sell an Ultrasonic Depth (or fish) finder kit , obviously they supplied the transducers.
    Popular Electronics published an ultrasonic alarm.
    Search for those projects for ideas.

    **Maybe** somebody is still selling surplus ultrasonic tranducers.

    Just searched:
    https://www.google.com.ar/search?num...k1._Kh90NrD-jw

    and
    http://chinaultrasound.com/
    Last edited by J M Fahey; 08-30-2017 at 05:16 PM.
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    Juan Manuel Fahey

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    Supporting Member eschertron's Avatar
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    What's the environment like? The best SNR will be in a part of the spectrum that has little noise content. You may have to establish that experimentally. Sound travels farther and faster underwater (due to density of water v air) so distance is not too much of a problem. It's the background noise in a living environment that will be a problem.

    Quote Originally Posted by rjb View Post
    Calling Homeland Security, calling Homeland Security, come in please....
    If you want to set off a 'percussive device' under water, set off a smaller, similar, device at the surface. Trigger on the strength of the wavefront. This is for fishing, right?

    edit: USN does use ELF radio transmission to talk to subs. So it is possible.
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    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

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    Supporting Member Jazz P Bass's Avatar
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    The US Navy uses very low frequencies to communicate with submarines.

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    Senior Member Enzo's Avatar
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    But that is over global distances. Locally they use much higher freqs for sonar and sonic location.
    Education is what you're left with after you have forgotten what you have learned.

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    Better Tone thru Mathematics bob p's Avatar
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    congrats to juan, escherton, jazz and enzo -- you are now on SF's list of known cohorts.
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    "Stand back, I'm holding a calculator." - chinrest

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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by rjb View Post
    Calling Homeland Security, calling Homeland Security, come in please....
    Ha!
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    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by eschertron View Post
    This is for fishing, right?
    maybe

    Quote Originally Posted by eschertron View Post
    edit: USN does use ELF radio transmission to talk to subs. So it is possible.

    Quote Originally Posted by Jazz P Bass View Post
    The US Navy uses very low frequencies to communicate with submarines.
    yes but at those frequencies, they are limited to 1 way communication as the size of the antenna needed for receiving is ridiculous. Subs have to rise to periscope depth for radio transmition
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by bob p View Post
    congrats to juan, escherton, jazz and enzo -- you are now on SF's list of known cohorts.
    ...suckers
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    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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    Not an easy thing. This is a territory that was covered by (aside for already mentioned) quite demanding applications such as underwater robots, bathyscaphe communication or underwater oil exploration devices. Either expensive, well kept secrets or both.

    On a bright side water is a good transmission medium for audio range frequencies, so if you could use any of the well established protocols, such as ITU-T V23 modem you can couple with standard piezo/electret/dynamic transducers and succeed.

  12. #12
    "Thermionic Apocalypse" -JT nickb's Avatar
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    I've worked on the design end of sonar a couple of times for a few years, ranging from 40KHz up to 1.6Mhz. The attenuation increases rapidly with frequency so, if you don't need precise timing, low frequency is the way to go. The second component is transmitted power, the more the merrier.
    Experience is something you get, just after you really needed it.

  13. #13
    Supporting Member The Dude's Avatar
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    Perhaps why whales use low frequency?
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    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by nickb View Post
    I've worked on the design end of sonar a couple of times for a few years, ranging from 40KHz up to 1.6Mhz. The attenuation increases rapidly with frequency so, if you don't need precise timing, low frequency is the way to go. The second component is transmitted power, the more the merrier.
    Well, I have much more flexibility in designing the transmitter. Limitations in the power supply or physical size are not huge concerns on that end. My constraints are in the receiver – in both size and power.



    Quote Originally Posted by nickb View Post
    Not an easy thing. This is a territory that was covered by (aside for already mentioned) quite demanding applications such as underwater robots, bathyscaphe communication or underwater oil exploration devices. Either expensive, well kept secrets or both.

    On a bright side water is a good transmission medium for audio range frequencies, so if you could use any of the well established protocols, such as ITU-T V23 modem you can couple with standard piezo/electret/dynamic transducers and succeed.
    Since the device will need to be triggered (poor choice of words, rjb?) manually, speed in relation to transmission frequency isn't a concern. Audible frequencies travel much faster in water anyway. Unless I'm not considering some aspect, I don't think I'll need anything approaching 40KHz. My concern is it needs to be reliable and be able to penetrate through underwater cover. As long as it is faster that the time it takes me to yell "fuck" if it fails, I should be fine.
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  15. #15
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Look at these as an example New 25mm 40khz Waterproof Ultrasonic Sensor Transducer Receiver Transmitter R+T | eBay

    Diameter: 25 mm
    Center Rated Frequency: 40 KHz
    Sound Pressure Level (at 10V): ≥ 115 dB
    Receiver Sensitivity (at 40KHz): ≥ -62 dB
    Capacitance at1KHz,<1V (PF):1800±30%
    Maximum Input Voltage: 140Vp-p

    Woefully incomplete specs making it impossible to estimate the receiver voltage but think in the mV range. Buy a couple and measure the output. I see you can drive up to 140Vpp. Note this will not be continuous but pulsed operation at a fairly low repetition rate. Again, not specified. They will overheat if driven with too much power. You drive it with a square wave.

    It sounds like you need a simple trigger so just send a say 100ms pulse, have a bandpass filter in the receiver to keep the noise down and measure the length of the received pulse. If it between (say) 95 and 105mS then accept it as valid. Multipath reflections may smear out the received pulse.
    Experience is something you get, just after you really needed it.

  16. #16
    Senior Member SoulFetish's Avatar
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    First things first. I have the power supply

    img_3348.jpg
    Last edited by SoulFetish; 09-03-2017 at 12:06 AM.
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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    rjb
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    Are you sure there's nothing nefarious about this project?

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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by rjb View Post
    Are you sure there's nothing nefarious about this project?
    C'mon, rjb, do you know anyone who DOESNT have a thousand watts SKiiP power supply lying around in their garage? Nothing to see here.
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  19. #19
    rjb
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    Well, OK, as long as it's only a kilowatt....
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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by rjb View Post
    Well, OK, as long as it's only a kilowatt....
    ... think I should get get two?
    Damnit! This whole thing was never gonna' work from the beginning, was it?

    In all seriousness though, I feel I should have been more specific earlier on. This might change how this would be approached, so I apologize. I need the electronic signal to be able to trigger a mechanical action (so I'll be triggering a solenoid in all likelihood). That may even simplify things somewhat, but leaves me with mechanical challenges using materials (springs and bearings) that will perform and hold up over long exposures in marine environments.
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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