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'[PICLIST] [OT] sensitive inhalation or pressure se'
2002\04\24@182405 by Brian Taylor

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I have an oxygen system for my sailplane that provides a short burst of
oxygen at the start of each inspiration cycle.  The oxygen burst width
varies with altitude and by firing the oxygen in at the start of the breath
intake, the oxygen gets right through the lungs.  The over all system is
extremely frugal with its oxygen use.  The system uses no mask, it has two
tiny cannula that poke 10 mm into the nostrils.

The pressure sensor is a capacitor fabricated on the PCB as a 50 mm
diameter disk of vacuum aluminised plastic (looks like wine cask
material).  This is vented to atmosphere one side.  When you inhale the
small pressure drop in the cannula moves the capacitor plate and a '555
(yep - it should be a PIC) changes frequency. This is sensed and the oxygen
valve is opened for the required milliseconds.

It all runs on one 9v PP9 battery and works to over 25,000 feet.

Hope this helps.

Cheers
Brian

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2002\04\24@184452 by John Dammeyer

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That's cool.  Is this a commercial system or did you build it yourself.

John Dammeyer



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2002\04\24@193943 by A.J. Tufgar

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That's cool!
I'm thinking I might be able to do something like that.  The project is
similair where someone breathes through the cannula, and I need to tell
when they inhale so this would probably work well.  I was thinking of a
rubber membrane that would have a wire or piece of metal attached and
when you breathe in it moves and close a switch.  But I was thinking
there may not be enough movement.  However using capacitance I probably
wouldn't need to move very far to see a big change in freq.  Any
thoughts on this anyone?  Whould a switch or capacitance work better?

Thanks Brian even if this doesn't work it was cool to learn about,
Aaron

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2002\04\24@195740 by Dave King

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At 08:28 AM 4/25/02 +1000, you wrote:
>I have an oxygen system for my sailplane that provides a short burst of
>oxygen at the start of each inspiration cycle.
>Cheers
>Brian

Brian

Is this a commercial system ?

We''re going to take a run at one of the FAI C1 records this
summer and anything I can find weighs more than the airframe.

Dave

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2002\04\25@033937 by Alan B. Pearce

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>But I was thinking there may not be enough movement.
>However using capacitance I probably wouldn't need to
>move very far to see a big change in freq.  Any thoughts
>on this anyone?  Whould a switch or capacitance work better?

Try using a CD4046 PLL as a capacitance change detector. If the varying
capacitance is the frequency determining C in the PLL you should be able to
set the desired sensitivity by determining the max-min frequency swing for
the control voltage range. You could then measure the control voltage with
an ADC, or if you can get the swing suitable just feed it into a Schmitt
trigger to get a digital signal.

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2002\04\25@145715 by Peter L. Peres

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>The pressure sensor is a capacitor fabricated on the PCB as a 50 mm
>diameter disk of vacuum aluminised plastic (looks like wine cask
>material).  This is vented to atmosphere one side.  When you inhale the
>small pressure drop in the cannula moves the capacitor plate and a '555
>(yep - it should be a PIC) changes frequency. This is sensed and the
>oxygen valve is opened for the required milliseconds.

Imho lose the strange sensor and use a simple capacitive microphone
capsule as is, perhaps in the canula tube on the face using a T joint. It
might work from further away in the tubing. Output voltage (DC coupled)
jumps 0.2V or more under the described conditions.

Peter

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