An acoustic instrument is fitted with a flexible member that is fitted into the instrument and attached to the bridge of the instrument in such a fashion that the member vibrates freely in sync with the instrument. A compact optical pickup unit is attached to the flexible member in an arrangement comprised of cooperating optoelectronic devices which are mechanically isolated from the vibrations of the instrument and are positioned on the member. The member interferes with, and when stationary entirely obstructs, the path of a light beam generated by a light emitting device (LED) which is directed towards a light receiving device, the frequency and intensity of the vibrations of the member modulating the frequency and intensity of the light from the LED impinging on the receiving device. The result is an electronic output signal that corresponds to both the frequency of the note (or notes) played and to the tonal quality of the acoustic instrument on which the pickup is mounted. Circuitry associated with the receiving optoelectronic device is biased so that the output produces a signal which can be connected to any industry standard instrument amplifier or sound mixing board.