Best Small, Cheap Sensor to Detect Breathing for Interactive LED Art Installation

Hi everybody!
I'm a visual artist and currently working on a project for an exhibition. It includes a cube with LEDs installed inside and a peephole. When the audience look into the peephole to see inside the box, a sensor would sense their breathing, so it could change the brightness of the LEDs according to it—breathing out would equal brighter light, breathing in would decrease the light's intensity. 
So my question is: is there any sensor that would enable me to do this? I'd prefer a cheap one - and I need it to be small. 
Any tips would be HIGHLY appreciated,
Anna

[User comment deleted]

Hi Anahita-I'm thinking generally along the same lines as Mike, but with a slight twist.  Since the person will be looking through a peephole, you have a pretty good idea where their nose and mouth are.  I would take several small low-resistance thermistors and feed a constant current into each one to produce a slight amount of heating. As the person breathes, the moving air will carry some of the heat away form the thermistor.  The reason I suggest several thermistors is that we don't know exactly where their nose or mouth is, and they could be breathing through either their nose, mouth, or both. Use an Arduino , Raspberry Pi, or similar board to look at the voltage across the thermistors and find the one that is rising and falling, which would track the person's breathing.  Some trial and error is definitely called for here- but I'll bet you can make it work.-Rick