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[Reply #15 on:]

I guess that makes sense, since the - output can't really go negative.  It stands to reason that it rides on a bias voltage.  You can probably tell I don't have allot of experience with differential output sensors nor do I use op-amps like this normally.  Thanks for mentioning that. 

[Reply #16 on:]

One of the reasons why I think a simple solution must exist is that I am basing my project off another sip and puff project that failed to raise funding on Kickstarter. You can clearly see on his shield that there is only a sensor, one resistor and an 8-pin DIP package of some kind: http://www.tmientp.com/ipoddock.htm

Problem is, he never released the source files for his project So I'm left guessing at what exactly he did.

[Reply #17 on:]

I guess that makes sense, since the - output can't really go negative.  It stands to reason that it rides on a bias voltage.  You can probably tell I don't have allot of experience with differential output sensors nor do I use op-amps like this normally.  Thanks for mentioning that. 

That firms also sells the same pressure sensors with active electronics already installed inside and it outputs as I recall a 0-5vdc measurement range and it's temperature compensated to boot. Cost more but now you can probably appreciate why that is so.

Lefty

[Reply #18 on:]

True, but they are pretty cost prohibitive for my application. I found one that costs $16.09: http://www.digikey.com/product-detail/en/MPX5010GP/MPX5010GP-ND/464055

But I'm still stumped about why that sip and puff iPod dock example (http://www.tmientp.com/ipoddock.htm) only has a single resistor and a 8-pin DIP IC. I e-mailed the creator about the sensor he used, and he pointed me to the uncompensated model. Curious about how to interface with this thing cheaply.

[Reply #19 on:]

True, but they are pretty cost prohibitive for my application. I found one that costs $16.09: http://www.digikey.com/product-detail/en/MPX5010GP/MPX5010GP-ND/464055

But I'm still stumped about why that sip and puff iPod dock example (http://www.tmientp.com/ipoddock.htm) only has a single resistor and a 8-pin DIP IC. I e-mailed the creator about the sensor he used, and he pointed me to the uncompensated model. Curious about how to interface with this thing cheaply.

Well perhaps he isn't trying to measure the actual pressure value analog voltage over it's complete range, but just detecting the difference between a sip and a puff. That could be done with two simple comparator op-amp sections (one 8pin dual chip) just looking for any positive differential above a certain value and any negative differential below a certain value, thus generating two digital output signals, one saying a puff is active the other saving a sip is active and both off if neither state is active.

Lefty  

[Reply #20 on:]

Well I believe I've found a single-supply instrumentation amplifier that could be used: http://www.digikey.com/product-detail/en/INA332AIDGKT/296-12204-1-ND/413491

If I've understood it correctly, I would connect both the + and - outputs of the sensor to the chip, then add a resistor to adjust the gain to be about 82x. I should get a smooth voltage output from it, which I can read with the Arduino core. And the whole thing would only cost about $2.00 extra.

[Reply #21 on:]

Just to add some closure to the topic, for future people who might stumble on this thread, I hooked up the sensor to an instrumentation amplifier and had awesome results!

I used the INA122 instrumentation amp (planning to use the AD8223 SMT chip in the actual PCB), hooked up the sensor's + and - outputs to it and added a gain resistor. I was able to read the entire range of sipping and puffing through a 10ft long tube connected to the sensor. Here is a complete write-up of what I did: http://jason-webb.info/2013/03/working-breadboard-prototype-of-an-open-source-sip-and-puff-interface/