Rail Books
modules, and pre-made off-the-shelf
systems. Computer-controlled
Rail Books Review
DIY Advanced Model Railroad Signaling Electronics: Sensors, Interactivity, Track Control
by J. Richard Kerr, PhD published by
CreateSpace Independent Publishing Platform
Without a signaling system, rail-
roads could not function, yet sig- naling is rarely incorporated in model railroads. Functional model signal systems have been avoided in the past because they were com- plex and fiddly. Given the trend in model railroading, however, to- ward more prototypical operation, an operating signal system com- pletes the realism. This book illustrates how to
design and build a prototypical signaling system without an ad- vanced degree in electronics. With some basic electrical/electronics knowledge, the reader can create a realistic system. Keeping the PC off the layout is the author’s stated goal because most people are not proficient in writing software. For some background infor- mation and comparison to other types of systems, I did a search in Railscan Index and found five pages of articles. This excludes the relay-only systems as proposed by W. K. Walthers, Linn Westcott, and others. Signaling systems di- vide into three categories: com- puter-controlled, discrete electric
32 RAILROAD MODEL CRAFTSMAN
systems have the advantage of flex- ibility but require programming. Divided into two subsections, dis- crete systems are electronic ex- clusively and those that contain a mixture of electronics and elec- trical components such as relays. The latter is the scheme the author uses with a significant advantage. If power is removed or lost, the re- lays remember the last state and usually don’t require an initializa- tion routine at power up. The au- thor’s inclusion of three electronic circuits significantly enhances the functionality and reliability of the relay-only designs. Finally, off-the- shelf systems, such as those by Circuitron, Dallee, Atlas, and Aza- trax — to name a few — are mod- ules that plug together. For more information on the different signal systems, refer to the references listed below. Independent of track power,
the author’s system is compatible with conventional DC and DCC systems. Depending on the user’s preference, circuits are shown to use either low level (0 volts) or high level (12 volts) activation sig- nals. Later in the book, the author describes track control circuits for collision avoidance and so forth, which he clearly indicates are not DCC compatible. Including approach mode and tumbledown operation, the system is based on Absolute Permissive Block (APB). The primary purpose of an APB system is to protect op- posing train movements on single track or single track between sid- ings. As per the prototype, ap- proach mode keeps signals extin- guished until a train approaches, saving power and lamp bulbs. Un- like model railroads, prototype railroads have multiple blocks be- tween passing sidings. Positioned at the entrance to a single-track block — exiting a block with a passing siding — the head block signal controls the aspect of all signals on the single track, includ-
ing the next head block signal. The aspect of the head block signal “tumbles down” through the sin- gle-track signals. Based on three basic modules,
the author’s system architecture includes a sensor, a master circuit, and an OR-NOR circuit for signal control. Correspondence with the author indicates, “every circuit was built and refined several times un- til it worked without any quirks.” (While tracing the logic circuits, sometimes I cheated and wrote the logic equations.) Additional- ly, component part numbers and prices are listed throughout the book, simplifying the construction process. I checked the part num- bers and prices; they appear to be up to date. While not absolute- ly necessary for the design of the system, but to size the power re- quirements, current requirements should be considered by the user. Description of signaling circuits
begins with an overview of sensor circuits and the author’s rationale for selecting optical sensors. In ad- dition to a “home-brew” version, various commercially available sensors are described. Optical sen- sors allow the system to detect di- rection. More information on sen- sors is included in the Appendix. Next, the Master Control module
circuit and functionality are de- scribed. Based on a 555-type tim- er to prevent false triggering, it is a simple, straightforward circuit. Each block has a Master Control module. Providing three functions, the MC accepts the track sensor inputs, remembers where the train is in the block, and finally dis- tributes this information to other blocks to set their signals. To implement the rules for signal activation, the third module is an OR-NOR circuit. With four inputs (condition A or B and neither C nor D are true), the circuit will ac- tivate the block signal, permitting adjacent track block conditions to influence the signal control. Also shown is an adaption for the Atlas signals. Armed with these three mod-
HEAD END
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