Optical Train Detection - 1
Build a detector by interconnecting
pre-assembled modules ...
– By Terry Terrance
Model photos by the author
OPTICAL TRAIN DETECTION
Build a detector for $10
B
eing able to detect trains and rolling stock is necessary
to implement automated control systems and prototypi-
cal signaling on your model railroad.
The cost and complexity of train detection deters many model
railroaders from having a functional block signaling system,
which should be a part of most mainline railroads. The trade-
offs involved in choosing and designing a signaling system are
often treated as a form of ”black art”, reserved for the adepts
or the advanced modelers; but it need not be so.
Types of detectors
When I was designing my model railroad I spent a considerable
amount of time determining how I would eventually signal the
railroad. One key differentiating feature between signaling sys-
tems is how the trains are detected.
Detectors fall into two broad categories – electrical and optical.
Electrical detectors come in many different configurations but all
of them rely on detecting the flow of current between the rails.
Powered locomotives and lighted cars are readily detected
because the motors have windings and the bulbs have fila-
ments that allow current to flow across the rails. But electri-
cal detectors cannot detect common rolling stock unless these
cars are modified with special wheelsets that allow a small
amount of current to flow between the rails.
Some of the electrical detector designs require that the current
to the train pass through an electrical component in the detector
circuit, typically a diode. That diode by its very nature causes a
voltage drop in the block where the detector is installed.
Additionally, the diode must be sized to carry the full current
available on the rails which, if you are in the larger scales or
have a 10 amp DCC system, requires a pair of large diodes in
MRH-May 2014
