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[on:]

Ok, it's actually a pressure sensor that has two ports.

http://www.freescale.com/files/sensors/doc/data_sheet/MPX12.pdf

It is the same one used in the bellows of a MIDI accordion.

I however need to use it as a breath sensor.  I figured that since this sensor is definitely the one that they use in the accordions, (I checked), that just blowing into it ought to put out at least something.

There is a very slight change when I use the sensor without and fancy resistors or op amps.  

I believe I need to amplify the signal and so I purchased a few LM324 op amps to play around with, but I'm not getting any change in the value at the data pin.

Of course, I may have the op amp hooked up wrong, but basically, I supplied it with 5+V and ground, and the other three pins are v1 in, v2 in, and v3 out.

Out must be the amplified signal, so that's going to my data pin.

V1 in is getting 5 Volts, and V2 In is getting the v out pin from the breath sensor.

Nothing seems to be happening.

CD

[Reply #1 on:]

Hi, have a search thru the midi projects of Tom Scarff.  You are going to have to configure the op amp as an instrumentation amplifier with a DC offset of +2.5 volts so that a differential pressure sensor goes up or down from this mid-point on blow/suck.  I am having to go there with a project, so I will let you know of successful solution when I get one.

TTFN

[Reply #2 on:]

There always has to be a positive pressure wrt to the "vacuum side" of the diaphragm, the output is between 20 to 75 mV, always positive, see figure 2 of the datasheet. So you need just a simple amplification setup for a gain of 60.

(non-inverting, connect a 10k from in- to ground and a 470k from output to in-, or whatever you have...)

As the ADC can measure 5 mV/count, you should have a reading even when connecting it directly, though there is of course no sensitivity at all.

Hint: Carefully read the datasheet.

[Reply #3 on:]

If you detail your project a little more I can probably give you some options. How much resolution do you need on your breaths? Is on/off enough? In what apparatus are you trying to detect breaths. Do you need to sense inhale as well as exhale?