level of difficulty:
This project describes how to make a strobe tuner "The Tune Trainer". It is quite tricky as it involves an Arduino microprocessor and prototyping board, which can be programmed via your computer. The link below lists the other electronic bits and pieces needed and gives detailed instructions.
The tuner has two tuning modes
"The musician sets the tuner for a string and strums said string while aiming the strobe at it. The strobe causes the apparent speed of motion of the string to decrease as the user approaches the correct frequency. as the the string gets closer to tuned, the string's vibration will happen in visible pulses of speed and apparent lethargy, which slow as the tuning improves. These variations in speed should coincide with the frequency of audible "beats" when beat tuning."
"The musician sets the tuner for a string and strums said string while listening carefully. If the string is very far from true, he'll hear discord. once he is close enough that the discord disappears, he will begin to hear "beats" in the combined sound of the two tones, caused by constructive and destructive interference between the waves. The beats should decrease in tempo and amplitude as you approach a perfect tuning. (in theory)"