Diagnostics
A picture of the position sensors on a "guitar" built for testing out various ideas. The two LED/photo detectors are mounted on microscope X-Y positioners.
Time histories of the string position are measured in two directions, normal (y) and parallel (x) to the "top plate". An approximately circular motion collapses to dominantly vertical.
Sometimes you just want to measure something. We wanted to measure a guitar string motion, mostly to see if we could, but also as part of understanding how a guitar works. Others have done it, but...
Whatever diagnostic used had to be inexpensive, accurate, and amenable to a garage ambience. Initially thinking of and inspired by a solar eclipse by the moon, perhaps a LED and lens could be used to produce a beam of light. A string passing through this beam would occlude the beam: The transitted light beyond the string would depend on the string position in the beam.
Next we realized that this is an exact description of a photo-interrupter. These consist of an LED and miniature lens that produce a cylindrical beam of e.g. infra-red light with a diameter ~ 1mm. After a gap of ~ 1 cm there is a second lens focusing followed by a phototransistor. Anything blocking the beam switches off the output voltage of the detector. We bought four Optek OPB815WZ for $6 (including postage) from E-bay. Then we needed a data acquisition system: We found and bought the “Dataq Instruments DI-1100 4-Channel USB Data Acquisition Starter Kit with 12-bit ADC” for $60. Later we added a couple of microscope X-Y positioners, to help adjust the sensor positions more easily and accurately. Add another $25 or so for both.