underwater communication

Hi all,

I want to build a small communication system with the following requirements:

  1. up to 15m distance between transmitter and receiver
  2. up to 30m depth underwater
  3. maximum of 2000 bits per second
  4. low voltage (~5V)
  5. small size

i was thinking about using ultrasound for this project

is there any references or projects i can start from ? recommendation for sensors or other parts i may need ?

Thanks, Eric.

Sounds like a cool project! Do you have experience with underwater communication?

If I were doing this, I'd get a cheap "fish finder" (~200USD) and experiment with it's parts. If money is no object, then this place has some badass looking equipment.

Or a $35 fish finder! I just bought one of these for reverse engineering, but haven't even had time to try it yet.

You need an underwater transducer, and I've never seen them for less than that.

If the water is clear, using blue or green light as the medium would be a lot easier.

The transducer looks good. There must also be a hydrophone, or something that works like one in the black cylinder part of it. OP should attempt to modulate the amplitude of the 200kHz carrier at 2 Kbaud. After that, the project finishes itself.

edit: I like the light idea!

I don't particularly like this article but it suggests acoustic data rates are around 1kb/s https://www.digitaltrends.com/cool-tech/acoustic-frequency-underwater-communication/

Since the OP is looking for a maximum of 2kb/s this may not be a problem, but it is kind of strange to specify a maximum. If the OP really wants a minimum of 2kb/s then actual data rates and cost of achieving it in commercial systems would definitely be worth exploring.

Maybe the OP will always be in clear water but if that is not certain then communicating by light is a bad idea.

A target distance of 15m sounds achievable. Waterproofing will be a problem, a ball at 30m depth would be crushed to 1/4 of its surface volume.

There must also be a hydrophone, or something that works like one in the black cylinder part of it.

Yes, the “$35 fishfinder” transducer is pretty solidly constructed and heavy. It must be submerged by at least 20 cm, and comes with a foam float and cable stopper to set the distance.

Battery power is 4xAAA and the lifetime is only 4 hours of sonar readings (3 per second or 1 per 3 seconds in battery saver mode), so the unit must output quite a bit of acoustic power.

Will get a chance to try out the “fishfinder” this weekend in ocean water, then it comes apart for reverse engineering.

ChrisTenone - i have experience with Wifi/BT but not underwater communication, so i expect this to be challenging in some ways :slight_smile:

Thanks jremington i will look into this, i don’t want to buy some cheap transducers that will stop working after the first time they hit the water, the water is not clear and i prefer the ultrasound idea better then blue/green light.

Thanks everyone i will keep you posted!

"buy some cheap transducers"?

There are't any that will work in any useful way, underwater. Highly specialized construction is required to match the acoustic impedance of water. That is why the inexpensive fishfinder is interesting!

Various people have worked on "cheap underwater communications" over the last couple of decades, without commercial success. But they generally have had much more demanding requirements than yours. Spend some time with Google to see what I mean.

Or you can use underwater speaker and hydromicrophone

https://www.google.ca/search?rlz=1C1NCHB_en&biw=1440&bih=787&tbm=isch&sa=1&ei=mT73Wti0POeGjwS7kbqIAg&q=arduino+hydromicrophone&oq=arduino+hydromicrophone&gs_l=img.3...11755.18264.0.19116.10.9.1.0.0.0.227.745.8j0j1.9.0....0...1c.1.64.img..0.0.0....0.pV2fBaEt5fI

Here is somebody having a go with a home made hydrophone in a pool. https://www.youtube.com/watch?v=cmsp5zs83vo

So voice was not a great success because of bubbles, but it obviously shows data could get through at least if there are no divers around making bubbles.

I tried out the $35 fishfinder/depth sounder in the ocean this weekend, and it works quite well. 100 meters range as advertised, matching a commercial sounder.

The heavy, durable transducer is connected to the fishfinder by a removable cable with waterproof barrel jack and plug, so I inserted a matching plug/jack pair and hooked up a scope.

The open circuit driving signal is 400Vpp, 200 kHz, 100 microseconds duration. My cap meter reports the transducer capacitance as 2.9 nF at 1 kHz excitation frequency. The transducer is probably resonant at 200 kHz, so that capacitance is not a good guide for the load impedance, but no wonder that battery life is short.

Unfortunately, the snap-together clamshell case of the fishfinder is very well designed and solid, with no screws, and will have to be cut apart.

More later, but my preliminary conclusion is that the purchase is worth the price of the transducer alone, as I haven’t seen even naked transducers for less than $35 on Alibaba.

There has recently been a real breakthrough in underwater acoustic communications.

Researchers as Newcastle University in the U.L, have developed spread spectrum nanomodems: very cheap ($70 instead of $7000), very low power, small and yet long range (km).

https://www.economist.com/news/science-and-technology/21741965-tiny-acoustic-modems-will-do-job-better-way-transmit-messages-underwater

https://www.york.ac.uk/electronic-engineering/research/communication-technologies/projects/smart-dust-underwater-wireless-sensing/

I would hope that commercial versions of these might appear in the next couple of years.

jremington: There has recently been a real breakthrough in underwater acoustic communications.

Researchers as Newcastle University in the U.L, have developed spread spectrum nanomodems: very cheap ($70 instead of $7000), very low power, small and yet long range (km).

https://www.economist.com/news/science-and-technology/21741965-tiny-acoustic-modems-will-do-job-better-way-transmit-messages-underwater

https://www.york.ac.uk/electronic-engineering/research/communication-technologies/projects/smart-dust-underwater-wireless-sensing/

I would hope that commercial versions of these might appear in the next couple of years.

The data rate is only 40bps but that meets the OPs requirement of a maximum of 2000bps. Transmitting power consumption drops from 35W to 1W which has to be a good thing, though it is still high.

I worry a bit though about whether or not they are testing what effect these devices might have on sea creatures. The range is given as 2km and they are broadcasting on several frequencies. If the devices are small, low cost, and low power they could start being used in far greater numbers than current technology.

jremington: There has recently been a real breakthrough in underwater acoustic communications.

Researchers as Newcastle University in the U.L, have developed spread spectrum nanomodems: very cheap ($70 instead of $7000), very low power, small and yet long range (km).

https://www.economist.com/news/science-and-technology/21741965-tiny-acoustic-modems-will-do-job-better-way-transmit-messages-underwater

https://www.york.ac.uk/electronic-engineering/research/communication-technologies/projects/smart-dust-underwater-wireless-sensing/

I would hope that commercial versions of these might appear in the next couple of years.

There's a slide package describing the system at a high level here which in turn references a IEEE paper.

One of the interesting side benefits is that with multiple fixed reference nodes, the system can do platform navigation, though they don't quantify the accuracy.