This optical beam sensor (also known as an optical interrupt switch) is made up of two separate electronic units combined in one housing. The sensor is in the shape of a horse shoe, an infrared LED is mounted in one half and a phototransistor is mounted in the other. The LED is always powered and emitting infrared light (invisible to the human eye). There are two distinct states that this type of device can detect. When there is no obstruction to the beam the phototransistor conducts allowing the circuit connected to it to sense that the beam is not broken. When the beam of infrared light is obstructed the phototransistor no longer conducts and the circuit will sense this also. This device works very well for the Hard Drive Clock since an index hole drilled in the lower platter is all that is needed to interface this device to the platter assembly. With this installed the PIC chip can detect the exact location of the rotating platters once per revolution.

              (c) 2005 - 2006 Alan Parekh