An Optical Sensor For Detecting String Motion

by Mark French

Originally published in American Lutherie #106, 2011



For luthiers who want to make accurate dynamic measurements on their instruments, it is a challenge to achieve precision for a cost that stays below the pain threshold. Commercial test equipment is readily available, but the prices can be breathtakingly high. There is always eBay, but the selection isn’t always there and the condition of the equipment can be a little sketchy. That leaves the option of making test equipment for yourself. Since the average luthier is constantly making nifty jigs and fixtures, it makes sense to offer a very inexpensive DIY option.

I needed a pickup for some projects that require measuring the first twenty or thirty harmonic frequencies of a vibrating string. I’m worried that the pickup response at high frequencies might drop off. All pickups have a window over which they are sensitive, and that window needs to be very short in order to observe the really high harmonics. Single-coil pickups have a shorter window than humbuckers. This makes them more sensitive to higher frequencies and is at least part of the reason that they can sound “brighter.” Also, the presence of a strong magnetic field from a coil pickup might affect the motion of the string. In order to affect the string as little as possible, I wanted to make an optical sensor.