Abstract: A portable modular apparatus that simply and unobtrusively mounts on the top of a piano or keyboard, and detects key movement. The rectangular back side of the modular apparatus serves to effectively mount the apparatus. The self-mounting portable apparatus has associated with each key an optical coupler. The self-mounting portable apparatus has associated with each black key a thin, reflective, flexible insert that positions itself between its respective key and optical coupler when the portable apparatus is mounted. The reflective insert then moves in relation to the movement of its respective key.