I would try a low pass filter with 3-10Hz ** and than use a threshold detector to measure the time between two breaths. Or track the time of the last n breaths and do some more math....
The peak detector also is worth a try, if you define the peakwidth big enough your noise shouldn't trigger a false peak
**) Ups, I assume that DC offsets where already eliminated. if not define a band pass of 0.01 to 3Hz ....
For such low frequence signals, it can help to implement a simple RC alaising filter and choose a not to high (>1kHz) sampling rate. On the other hand: If you noise is spiky a median filter can help a lot.
Message Edited by Henrik Volkers on 10-02-2007 02:31 PM
The reason I suggested amplifying your signal is because I had the same proiblem when implementing an EKG using a DAQ as well. Of course the noise would also be amplified, that's why you'd require an instrumentation amplifier, not just an opamp. Plus a Low Pass filter, as someone else has already suggested you. There's a useful article in Scientific American that describes it more thoroughly. I scanned the article for my project (if you're interested, I can send them to you). The reference is " Home is Where the ECG Is" by Shawn Carlson. Scientific American Magazine, June 2000.
As for the filters, the book "Power Electronics" by Rashid is what I would recommend. I have some more info, but I'm afraid it's in Spanish. Might that be useful to you?
Message Edited by Ali84 on 10-02-2007 09:00 AM