The coolest application of an FFT I implemented was for a custom oxygen sensor for a wearable breath measuring device. The sensor would flash a blue light at about 40 kHz, and there was a phosphorescent dye which would flash orange with a phase lag that changed based on temperature and oxygen level.<p>Using an FFT we could get that phase lag at 40 kHz and back out the % oxygen in exhaled breath! Typical oxygen sensors at the time were not anywhere near fast enough to do that.