Photocells were used to turn on and off the sound sources.
A small hole in the cylinder allowed the photocell to scan an image on a brightly illuminated glass slide.
A strobe light, photocell, and oscilloscope were positioned at one end of the corridor, and a mirror at the other.
An example would be to turn on the parking lot lights when a photocell indicates it is dark outside.
This is because the wiring from each photocell leaves the cell from the side that is nearest the light such that the wiring interferes with the path of light.
Each time one of the cylinder slats passed the light path a reflexion occurred which was recorded by the photocell.
The evolutionary reason for this peculiar design appears to be a historical accident in which the earliest photocells randomly oriented in this "backwards" fashion.
The drawn waveforms pass over photocells, illuminated from above by a steady stream of light, to the right of the flat surface.
