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[on:]

my next project has to go 50m (150ft) underwater and transmit 64 character string to an other arduino to the surface. my first guess is to go to ultrasounds while sound works ok underwater. should i go on some kind of modulation ( best guess amplitude ) ? and if yes has anyone some circuit diagram to go on ?

[Reply #1 on:]

Fifty metres of CAT5 would be simpler.

[Reply #2 on:]

the problem is that there are no fixed points. so it really has to go wireless .
thanks.
thanasis

[Reply #3 on:]

salt water will completely trash CAT5 after a few weeks as the salt diffuses through the insulation and corrodes the wires.  The connectors stand no chance.  Unless you want it to be a disposable item a proper marine cable would be needed.  Fresh water is less aggressive, but still issues!

For ultrasound first identify affordable transducers (hydrophones) I guess.  Modulation: many things possible, OOK, FSK, GFSK, QAM, you may have to take the Q of the transducers into account when selecting modulation rate.  50m might be pushing the sensitivity limits with low power transducers. Very high power can be dangerous to living tissue, note.

[Reply #4 on:]

my next project has to go 50m (150ft) underwater and transmit 64 character string to an other arduino to the surface

Can't you just log the data?

[Reply #5 on:]

MarkT thanks for your response. I have a pair of 25khz receiver/trancemiter and the problem is my knowledge in electronic engineering is none. i have found some diagrams for ultrasound receivers but i don't know if they will do the job
 here are some of them :

http://1.bp.blogspot.com/_cjfdUPWPfAc/S_LDSDAkcfI/AAAAAAAAAP8/7y1ltyO2ktE/s1600/Ultrasonic+receiver+circuit+Diagram++.jpg

http://4.bp.blogspot.com/_cjfdUPWPfAc/S_LC2QvKljI/AAAAAAAAAP0/cfQqegd0hC8/s1600/ultrasonic+transmitter+circuit+diagram.jpg

https://engineering.purdue.edu/ece477/Webs/F04-Grp08/imgs/tx-rx.jpg

and a 2 pdf files with amplitude modulation

http://www.ece.ucsb.edu/yuegroup/Teaching/ECE2C/Lab/Lab5.pdf
( witch i have a problem with the 2khz sin tone generator ?????????? is it correct ? shouldn't that be 40Khz ?????? )

http://www.ece.ucsb.edu/yuegroup/Teaching/ECE2C/Lab/Lab4.pdf

Are all this going to do the job for me ? should i look for something else ?

thanks in advance
Athanasios

[Reply #6 on:]

florinc i am trying to create an underwater texter, so logging the message won't do the trick. 
thanks any way. I am already over that 2 weeks now and my mind is a mess right now, you never know when a fresh idea is going to come.

Thanks
Athanasios