Gijs wrote in to tell us about his Acid Machine, a synthesizer instrument with an interesting method for visual display -
The circle with the lines that you see on the top of the machine, rotates and displays the note you are playing.
When you play a C the lines in the middle circle will be standing still.. from the C it will go outward, displaying all notes on a keyboard in 12 steps.
It works by making a LED blink in the frequency of the sound, and rotating the image at a set speed. For the distance between the lines i used this table, it displays musical frequencys in centimeters.