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Guitar tuners are quite common these days. Only problem is that they use displays or LEDs to indicate how the tuning process is going. Sam from J44 Industries made a guitar tuner that a blind person can use for a charity called Remap. His design starts with an off the shelf guitar tuner, only problem with this one is that it used LEDs for visual feedback. He used an arduino to monitor the state of the LEDs and generated audio feedback using an earbud for the user to hear the process. He has some smart audio cues that lets the person know when they are getting close to having the string tuned so that you can adjust the amount of peg turn accordingly.
