This shows the output from the turbine sensor driving the stepper motor across the array of optical fibers.  From US Patent #3,680,378, August 1, 1972.  The Spirostat was an ingeniously odd blend of digital, analog, mechanical and optical technology.  The sensor was an in-stream turbine and was one of the first disposable sensors manufactured.  The turbine had a hole that a light could shine through and there were two light sources and two photosensors.  This allowed for both the direction the turbine was rotating and how fast it was rotating to be measured. This signal was amplified and sent to a stepper motor that that moved a single optical fiber across an array of optical fibers which caused a light beam to draw a tracing on Polaroid film.  After a test, the developed Polaroid photo was placed against a plastic template to measure FEV1 and FVC.  The turbine was actually quite small (approx 1/2″ in diameter) and it had a lot resistance.