each detector. Most electrical detectors require that you make
additional connections in the wiring to the rails, each one of
which is a potential source of trouble.
Some electrical detectors will work with AC and some won’t.
Some will work with DCC and some won’t. While we’re talking
about DCC, all electrical detectors require that the railroad be
segmented into blocks. This isn’t so much of a problem if you
are already running DC, but it defeats the otherwise simplified
wiring associated with DCC and requires the same amount of
wiring complexity as DC.
Optical detectors, on the other hand, have certain advantages.
Optical detectors will detect all rolling stock – powered or
unpowered – without modification to the models.
Because optical detectors are independent of the rails, they
will work with AC, DC, DCC, MTH’s Protosound, Lionel’s Legacy
& Trainmaster, 2-rail or 3-rail, even with catenary systems. Also,
since the system does not detect the propulsion current you
could be running a Z scale system at less than an amp or a large
scale railroad at tens of amps – it makes no difference.
Since optical detectors have no connection to the rails, they
cannot introduce any maintenance trouble spots or interfere
with the operation of the trains. Finally, segmenting a DCC rail-
road into electrical blocks is not necessary and the simplicity of
DCC wiring is maintained.
Optical detectors are not perfect however; and to understand their
quirks we need to look at the three types of optical train detectors.
Types of optical detectors
Interrupter:
I’ll start with the type of optical detector that I’ll
call an “Interrupter”, a light source (lamp, visible or Infrared
Optical Train Detection - 2
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
MRH-May 2014
