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Author Topic: detect water and turn on!!!  (Read 780 times)
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hi guys

so i am building this www.diveduino.com

right now it has a couple of features such as logging your dives on an SD card, with time, date and depth.

many dive computers have some other features, like scrolling through menus by tapping it, buttons, etc...

one function that dive computers  have is that they turn on automatically when they are 3 feet underwater... and start working..

I guess that work by detecting water somehow, why? because almost ALL computers  have two metal contacts exposed like this(see in the middle between the two buttons)



another way i may think is that the phisically have a pressure switch that makes phisical contact to switch power to the main processor aftee 3 feet of pressure ( but i am not convinced about thsi since as a manufacturer you may want to avoid using mechanical parts inside this)


so i wonder how is that with two metal plates they detect water? is there any specific IC that does that?
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Since they can log the depth they must have some kind of pressure sensor. They simply check that value once a second, turn on if more than 1 meter under water or go to sleep again for one second.

The metal contact are probably to detect salt water or sweet water because the two have a different resistance. It's possible that they use the same sensor for turning on but then it would turn on as soon as you have contact with water and not wait till your 1m deep.
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The metal contact are probably to detect salt water or sweet water because the two have a different resistance.

Is the density of fresh/salt water significantly different? If so it would make sense that the logger needs to know what type of water it is in, to do the pressure/depth calculation. A resistance sensor could enable it to detect the difference.

On the other hand, I suppose there might be some cost to monitoring the pressure, and the designer might have designed it to only do that when it immersed. I suppose the resistance measurement would tell it that, too.

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On the other hand, I suppose there might be some cost to monitoring the pressure, and the designer might have designed it to only do that when it immersed. I suppose the resistance measurement would tell it that, too.

If nothing else it might be an energy saving feature.  The OP mentioned that dive computers turn on automatically below a depth of 3 ft (and presumably they turn off when above), so this type of automation also could be eliminating one thing a SCUBA diver will have to have on their metaphorical checklist before a dive.
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Quote
(and presumably they turn off when above),

Yes, but only after a while - 10 minutes?
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Quote
(and presumably they turn off when above),

Yes, but only after a while - 10 minutes?

Hmm, makes sense thanks for the information.
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i can turn on my computer on the boat(which most of us do, or just check it at my hotel room) then when i go in the water below 1 meter it will turn on.  After you get out it will turn off automatically after 10 min...


the computer does not detect salt water or fresh water...

but still  isnt it a waste of battery to check the pressure sensor every minute? i can descend 10 meter in less than a minute, so i dont think it checks the pressure everyminute..

mmmmm...

i once saw an application with a water container that had two plates when the water reached tthose plates.. the valve will turn off to stop filling the container with water.. thats why i guess it has something to do with it...

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@buton:
As stated by @pylon above, given that you are using an SPI-based pressure sensor, the only way using the pressure sensor itself to make the inference about that threshold would be to take measurements once every X milliseconds/seconds, and go to sleep, unless threshold crossed. This is a reliable way to do it -- and draws hardly any power (actually negligible compared to the self-discharge of a Li-ion); remember that a microcontroller like on an Arduino processes the measurement (a few instruction cycles) in much less than a millisecond, and then goes to sleep, where the current draw is very close to zero.

Also, a sidenote: just took a look at your webpage parts list for this project. You could REALLY cut down on the parts cost (and earn a little extra margin on your Kickstarter) if you make your own board instead of trying to assimilate all those individual modules (but you probably know this and have different reasons?)
« Last Edit: October 24, 2012, 01:05:56 pm by giantsfan3 » Logged

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update ::

 i took my volt meter and  put it in water and i measured a resistance of 300k to 400k.. i guess salt water will have less resistance...

it make sense because diveshops test computers by putting them in a pressurise chamber but they have to be in a small container with water so they will turn on...

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@buton:
As stated by @pylon above, given that you are using an SPI-based pressure sensor, the only way using the pressure sensor itself to make the inference about that threshold would be to take measurements once every X milliseconds/seconds, and go to sleep, unless threshold crossed. This is a reliable way to do it -- and draws hardly any power (actually negligible compared to the self-discharge of a Li-ion); remember that a microcontroller like on an Arduino processes the measurement (a few instruction cycles) in much less than a millisecond, and then goes to sleep, where the current draw is very close to zero.

Also, a sidenote: just took a look at your webpage parts list for this project. You could REALLY cut down on the parts cost (and earn a little extra margin on your Kickstarter) if you make your own board instead of trying to assimilate all those individual modules (but you probably know this and have different reasons?)

Actually  that will be my second goal after i finish my first prototype...
i know i can pack all in a small board and buy the components my self instead of getting them all preassembly, the first prototype i just want people who likes diy project be able to do it...
but i will follow your recomendation, as my second goald will be to put everything in one small board to save space and money smiley...

this has been pretty helpfull you have a good point, checking the pressure every second will not take that much power.... smiley
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This is a reliable way to do it -- and draws hardly any power (actually negligible compared to the self-discharge of a Li-ion); remember that a microcontroller like on an Arduino processes the measurement (a few instruction cycles) in much less than a millisecond, and then goes to sleep, where the current draw is very close to zero.

My thought was that the energy savings would primarily come from the LCD being off.  While the microcontroller can go to sleep as you described, while in use the LCD must either be on constantly or cycled on and off fast enough to appear to be constantly on to the human eye.
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@Far-seeker:
Until the threshold is crossed, @buton would keep the LCD completely off.
But indeed, the LCD would probably be the largest power consumer once the diver passes the 3-feet-underwater threshold and the device turns constantly on.
Although I should note that the Nokia 5110 LCD mentioned on @buton's page is extremely low current-draw, under a milliamp without the backlight IIRC.
« Last Edit: October 24, 2012, 01:20:26 pm by giantsfan3 » Logged

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this has been pretty helpfull guys... thank you
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