My mechanical abilities are not great, so first I tried to find the closest ready made sensor I could use.
Since it's puffing and sucking we're talking about, I started to look for pressure sensors on the Digikey web site. They don't carry EVERYTHING but they do carry A LOT of parts.
They appear to sell 29,562 kinds of pressure sensors and/or transducers (some may be the same part but packaged differently) but we can quickly filter down to just a few when we select "Active", "In stock" (only 2,120 remaining) and then the two lowest available "bipolar" (+ and -) sensitivities because we're human and don't have pumps for lungs: +/-0.36PSI and +/-0.72PSI.
Only five left and the cheapest is more than $50 so I gave up for now as too expensive but might get back to it if I find nothing else works better.
What's left is to try to make a sensor.
Since it has to be "bipolar" (puffing increases pressure and sucking reduces it) I kept wondering how I could modify a small balloon that would work for puffing, to also work for sucking.
Something springy could hold it into a half-inflated state (is that how Whoopee Cushions work?) and it would grow (closing one set of contacts) when puffed and shrink (closing another set) when sucked.
I couldn't find any way to make this small enough with my limited means, but something could be designed that could be 3D-printed so it's not a dead end.
For now, I had another idea: suspend an elastic flat membrane in a small cavity that has a tube attached to one side and may be somewhat (we'll see later) open on the other side.
When puffed or sucked, the membrane would move one way or another and block the light between an LED and a photo-sensor on either side of the membrane.
Said and done; I'll be back soon, with another
cartoon build log.