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