[mod edit: updated thread title to include the topic of this thread]
There are probably a thousand ways to detect breathing. We need more details, e.g.:
- How near (or far) from the person will the sensor be located?
- Do you need to distinguish between breathing through the nose or the mouth?
- Will it be possible to attach the sensor to the person to be monitored by a rig?
- ...
Once you've sorted out the requirements I'm sure we'll fins a suitable sensor.