The Gameboy Pitch and the Pro Sound modification has been covered previously. However several readers asked about a relatively simple feature to select between the LTC Module Pitch control and the Normal Gameboy Speed.
To do this correctly you need the LTC Module or the GameBoy Kit ( now including the switch) from the Shop. The Switch must be high quality with smooth movement switch, otherwise any resistance or noise in the switch will crash the Gameboy during the selection. The rest of the hook up could not be easier, simply cut the output wire in half and insert the switch in series as seen in the above photo on the yellow wire. Black and red wires are standard Power connections to the LTC. Mounting of the switch will be the hardest part of the procedure. Make sure that everything lines up and the Gameboy will close in the end. Good Luck.