Detect breath


I want to create a project in which arduino would detect when a human (or anything else) would breath into a sensor. I want it to be even a light breath. It has to be breath (CO2) no other gas. I want it to be accurate therefore no microphone. I found some really expensive co2 sensors on ebay for (50-60) USD. I belive there must be something cheaper to do this simple job. For example, do the alcohol detectors for few bucks detect sober humans breath? If not, what else could I use? I want it to be maximum abou 15 dollars. Thank you.

CO2 detectors are relative expensive, 50-60$ are in fact cheaper ones

Breath also has a lot of water vapour so you could make a detector with a small mirror, a led and a sensor to measure the reflection.

Another option might be using a DHT11 (~10$) to detect humidity and temperature.
Air inhaled has different properties than air exhaled.
The DHT11 is not very good in humidity but good enough to see the difference

there is this article from mit ,

A harmonica makes a noise when it detects breath....

Humidity is probably a good way, it doesn't normally jump by 10's of percent in seconds so a breath is
going to be a pretty obvious signal I expect.

Of course temperature will also show a very strong signature unless its already ~37C.

The combination of temperature and humidity is probably even better.

To reply your other question: breathalisers won't work, as they have sensors that detect ethanol, not carbon dioxide. Very different molecules.