System design document: https://docs.google.com/document/d/1FCr_1DzBJ3AlNbu3_3PSzmd7HQe7VcK2Rn7YlFluygg/edit?pli=1
There's really not much more to say than is contained in the description and the aforementioned document. The whole thing is an ARM core sampling the amplified output of a microphone, running an FFT, matching the dominant pitch to a note value, then lighting an LED to indicate whether the pitch is in tune. Not terribly complicated.
The fancy part of the project is the form factor- the goal is to make it look really nice, hence the CNC-milled aluminum enclosure.