LED) is aimed across the rails. On the opposite side of the
tracks from the light source is some form of photo detector
(photovoltaic cell, photo-resistive cell, photodiode or photo
transistor).
When the train comes along it breaks the beam between
the light source and the detector and the train’s presence is
detected. Don’t be put off by the technical terms within the
parenthesis above; the key concepts are: light source, light
detector and breaking the beam.
You can see the principles illustrated in [1]. In [1-A] the light
source shines perpendicularly across the rails and is received at
the light detector. The train comes along and breaks the beam,
which results in the train being detected.
This illustration points out a shortcoming of the interrupter detec-
tor. At each gap between cars, the detector will “drop out” and
give a clear indication. This can be addressed in a couple of ways:
1
1. The “Interrupter” Optical Train Detector.
A
B
