The eyes consist of a LED ( light emitting diode ) and a photodetector, typically a phototransistor or photodiode, inside the neck or tube of the hopper, to detect the presence of a ball.
12.
These touchscreens had a crossed array of 16 by 16 infrared position sensors, each composed of an LED on one edge of the screen and a matched phototransistor on the other edge, all mounted in front of a monochrome plasma display panel.
13.
Shive is best known for inventing the phototransistor in 1948, a device that combines the sensitivity to light of a photodiode and the current gain of a transistor, and for the Shive wave machine in 1959, an educational apparatus used to illustrate wave motion.
14.
:: * But the real key to making a sufficiently-sensitive detector that doesn't care about slight changes in light level and slight electrical changes in the circuit is probably to use a pulsing light source and to amplify those "'variations "'in light seen by the phototransistor and maybe even LED as your light source and drive it from an oscillator that makes it flash at a certain frequency.
