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Looking for a suitable transmitter

Sadlercomfort

Ash
Feb 9, 2013
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Hi Guys,

I would like to find a transmitter capable of transmitting data up to a range of 10-25 meters. The transmitter should be capable of transmitting underwater, fully-submerged at a depth of 1 meter.

Now here's my problem. The transmitter needs to be small enough to fit inside a thick wristband, because the idea is to send data from a heart rate sensor inside the wristband.

I understand that a 2.4GHz frequency is easily absorbed by a water medium, but lowering the frequency means I need a bigger transmitter aerial. The transmitter will only be used underwater, so I thought maybe sonar could work? But am not familiar with this.
 

Harald Kapp

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Water absorbs every wavelength of electromagnetic much stronger than air (that is why e.g. submarines use ELF - extremely low frequency - for communication).
Ultrasound may be an alternative, but I don't know how big and heavy a waterproof transmitter may be.

Have you considered not transmitting the signal as an alternative? Store the data e.g. on an SD card and evaluate it after your dive.
 

Externet

Aug 24, 2009
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For only 15 metre range, I would use any light transmission, omnidirectional LED or laser directional.
Will take minimal power and the emitter can be a square millimetre. Hope the water is clear.
Like a TV remote control, no buttons. Just the wrist heart rate sensor modulating and powering the emitter.
The reception part, well, photodiodes to taste.
 

davenn

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For only 15 metre range, I would use any light transmission, omnidirectional LED or laser directional.
Will take minimal power and the emitter can be a square millimetre. Hope the water is clear.
Like a TV remote control, no buttons. Just the wrist heart rate sensor modulating and powering the emitter.
The reception part, well, photodiodes to taste.

yup, agreed
radio is virtually useless for through water comms
 

Sadlercomfort

Ash
Feb 9, 2013
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I've done a little but more research and it should be possible to use sound as a transmitter, produced by a transducer. However the power consumption and the size of this could be impractical.

I love the idea of light! Its the only electromagnetic wave capable of travelling through water without being absorbed to much. The water will have chlorine impurities, because its a swimming pool. Furthermore, I could use two light emitters on opposite sides to each other to increase the probability of receiving the signal. Unique code could be attached to the beginning of the signal in-case the same transmitted data is received twice.

A signal must be transmitted and recieved, because this is part of my coursework. Was just looking for some more information.
 

Externet

Aug 24, 2009
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For heartbeat rate transmission, and no other data, sound can be used at any audible frequency. Clicks as dolphins do would be very effective.
A single click or tone burst every beat, by using a proper transducer. The attenuation would be very low at 15 m.
Even a plain miniature noisy relay can click a useful signal.
A couple of well placed microphones should pick up the clicks, but could pick pumps as floor noise. Try a microphone to find the noise level you would have to overcome with the emitter.

----> http://www.dosits.org/images/dosits/ainslie-mccolm.500.jpg
----> http://misclab.umeoce.maine.edu/boss/classes/SMS_491_2003/sound/Dusen9_17.jpg

Maaany years ago, I had my scuba plain mechanical waterproof watch. The tic-tac type. At SeaWorld, the dolphins were scattered in a large pool for people to pet and observe.
When placed my arm in the pool, ALL dolphins came to pile up on me and the rest of the public was admired scratching their heads on how did I managed to do so, repeatedly. Had fun 'betting' with others that I would bring dolphins only to my attention, no matter which side of the pool I went. Nobody else could then pet them.

----> http://www.best-orlando-getaways.com/images/seaworld-dolphins-people.jpg
 
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Sadlercomfort

Ash
Feb 9, 2013
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That's interesting lol, I'm using alot of different data. I think light would be a good way to go for this (Y)
 

BGB

Nov 30, 2014
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( non expert, just stating things as far as I understand them. )

water transmitter... dunno, I would guess maybe try to use comparably high power at low frequency (such as VLF or ULF band), and signaling would likely be by turning the carrier on and off.

note that generally, the antenna doesn't need to be the full wavelength, but roughly a power-of-two fraction of the wavelength is usually ok. another trick is basically making the antenna be a coil or spiral, which does reduce its range, but allows it to be more compact. possibly, the antenna could be connected to or submerged in the water.

ADD: apparently people have done ULF transmissions by communicating via pairs of spikes hammered into the ground (rather than using antennas), so possibly pairs of electrodes in the water could be used for a similar purpose with ULF or VLF.


as for light, the issue with light is generally needing to isolate the signal from any background light, which for a swimming pool could imply a fairly bright light source (effectively a strobe light).

IR would be fairly quickly absorbed in water, but could work (a pool is small enough that there may be only modest attenuation). green or blue could be distracting (bright flashing strobe-light wrist band), though it could be used like a flashlight if the pulse rate is sufficiently fast (30 or 60Hz or more).
 
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