This Breath Sensor was built for the project “Breathless“. It’s really very simple. You’ll need a 1k resistor (give or take), a unidirectional condenser mic (my fave is from Radio Shack), and a microcontroller. I used an Arduino/ Atmega 8, but I’m sure this would work with just about any microcontroller that has ADC inputs. This circuit will give a reading of about 150 on a 10bit scale (0-1023) when nothing is happening. This number will vary depending on the voltage and the resistor size. When you blow on the mic you’ll see the numbers rapidly fluctuate from 0-700 (give or take). Take a reading of the peaks to get the intensity of the breath.