Sonoff is an ESP8266 based on/off switch that will let you control electrical devices around your home. Out of the box it can be only managed by the eWeLink app which is running on Chinese servers. If you want to take full control of the device you can flash the firmware and access it inside your local network.
Fast Fourier transform (FFT) is an analysis method that let's you dissect complex waveforms and determine out of which frequencies they were composed. This function is often present on an oscilloscope and in this blog I will show a basic application of it.