In the author’s opinion the secondary light source “solution” is
more trouble than it’s worth.
However, if you don’t have night operations, and never want to
have night operations, the ambient light systemmay be the per-
fect choice. Ambient light sensor detectors will also work in hidden
staging or anywhere else that the lights are kept on constantly.
Reflective:
The third type of optical train detector is generally
referred to as a “reflective” detector. In the reflective detector
a light source is mounted between the ties aimed upward. Next
to it, and also looking upward, is a light detector.
Light is emitted from the source and, when no train is pres-
ent, the detector sees nothing. When a train is present, the
light is reflected off of the bottom of rolling stock and back into
the detector, thereby signaling the presence of a train. This
arrangement is shown in [3].
Like the ambient light system, the reflective system has no sen-
sor mounting or aiming issues and nestles between the ties
and does not interfere with scenery. Unfortunately, like the
ambient light sensor, the reflective sensor has the inter-car
dropout; which can be addressed in the same manner with a
time delay.
Optical Train Detection - 5
Each reflective sensor has its own light source, so night opera-
tions are not an issue. Since the light source is pointed straight
upwards and readily visible, a reflective system is rarely done in
visible light. Infrared is almost always used; which also means
that the detector is either a photodiode or phototransistor for
maximum sensitivity.
The reflective sensor is not perfect, however. The detection
range is usually limited (ergo the need for maximum sensitivity)
and it is sometimes difficult to prevent the detector from “see-
ing” the light source right next to it and generating false triggers.
Installing optical detectors
By now it probably comes as no surprise that I decided to go with
optical train detectors for my own railroad. I chose optical to pre-
serve the wiring advantages of a DCC-controlled railroad and, as
an O scaler, the immunity to high currents was another plus.
What may not be quite so obvious is that I chose the reflective
sensors for their compatibility with night operations and easy
concealability.
Having made the decision for optical detectors and having the
sensors in hand are two different things. I had obtained a sam-
ple of at least one commercially available reflective train detec-
tor when I stumbled across something interesting.
The robot experimenter’s community has a number of mod-
ules available to them that allow the robot’s microprocessor
to sense and detect the outside world. These modules are self-
contained, easy to interface and generally inexpensive. Among
these modules are reflective optical sensors.
I found a reflective optical sensor offered by a company called
Good Luck Buy
and it goes by the awk-
ward title “Infrared IR Reflectance Sensor Module for Smart Car
LM393” and it’s SKU 74447; cost is $4.59.
“Ambient light sensor detectors
will work in hidden staging or
anywhere else that the lights
are kept on constantly.”
MRH-May 2014
