If I understand correctly, you want an analog signal to set a digital trigger? How about looking for the max value from a pressure transducer which is basically a (not-so-clean) sine wave. You could actually feed it into a ADC and compare the values and use it to trigger itself. You could store earlier values into a buffer. You don;t need a big buffer if you are measuring someone's breath.
Question: What are you trying to measure in the breathing pattern? Frequency? Intensity? Both? Positive & negative pressures (inhale / exhale)?
What type of signal does the instrument take as an input? If analog, you could have an amplifier circuit feeding the signal from a pressure transducer directly to the instrument. If digital, then an ADC card would be useful
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You can also be creative and use a digital IO board to send out the digital output (5V or 3.3V).
There are many options, but I do not have enough details to steer you into any direction.
-enjoy-
JLV
