Thursday, September 17, 2015

Working Signals by David Tozer

Allingham Signals
By David Tozer

The British Model Railway Club of Montreal’s current layout (Allingham) is approximately 14 by 26 feet in size in a tail chaser configuration with a: two road mainline, branch line, goods yard, and an engine shed area with another attached good yard. In the station runarounds can be performed. With the layout having up to four operators there is the potential for conflicts in operations when access to the yards are made by crossing a facing mainline or an occupied mainline. To manage these conflicts a working semaphore signalling system was installed.
Main line home and branch line junction/home
Main line outer home and Main line distant
Main line starter

Branch distant junction
Main line and branch starter
Figure 1: Signalling Schematic for Allingham
All together there are 16 working signals on ten posts and one fixed distant on the layout. Great Western lower quadrant style home, starter and fixed distant signals are on the branch line. The other eight posts are upper quadrant on the mainline. The distant signals on the mainline are activated by the same pulse that is used to activate the corresponding home signal. The sequence of the signals on the layout is consistent with absolute block working. However, the set up is quite compressed in places.
Shunting across main line
Branch line starter
Main line Home and branch line junction/home
Branch line HomeThe signals arms, balance levers and many other components are made from Model Signal Engineering etched brass parts. Square brass tubing is used for the mainline posts and dolls while round tubing is used for the branch line. The choice of brass tubing was made for robustness since the layout is portable and the signals are removed for transportation. The posts were soldered to brass tubing, which are then inserted into the baseboard of the layout. The posts with motors extend to well below track level.
One challenge in making the signals was to make them as compact as possible and at the same time simple. Solenoids with magnet pistons were chosen as the motor since they are very simple, reliable and compact. The down side of using solenoids is the snap action they tend to have.
The other major challenge was controlling the current in the solenoids. This was eventually done using two pulse circuits of opposite polarity that feed an H bridge that in turn supplies current to the solenoids.
The solenoids were wound on 1/8 inch outer diameter hollow polystyrene formers that are 1 cm long with a flange glued to each end. The wire is 26 AWG enamel coated copper. Four to six layers of closely wound wire are placed on the former. The resistance of the completed coils is around 3 to 4 ohms.
Locking Magnet
Polystyrene tube
Steel Wire
Figure 2: The Solenoid Motor for the Signals
The rare earth magnets (1/2 inch by 1/16 inch) move freely inside the solenoid. On one end of the magnets a 1.5 cm length of polystyrene tube is press fitted to about an eighth of an inch. A small hole is drilled in the tube 4 or 5 mm from the end opposite to the magnet. The hole is used to connect the piston assembly to the signal arm with a thin steel wire that is then connected to the balance lever, which in turn is connected to the signal.
Passing a pulse through the solenoid drives the piston in one direction. Reversing the polarity of the pulse reverses the direction of motion of the piston. To lock the signal in the off (clear) position a small rare earth magnet is attached to the bottom end of the solenoid. This magnet holds the large magnet down when the signal is off (clear) and does not affect the large magnet when the signal is on (danger).
Square Brass Tubing
The solenoids are placed in a bracket and glued to the part of the posts that are below the surface of the layout. In the case where there are three signals on a post and doll set there are three solenoids below track level. The maximum cross-sectional dimension of the posts and solenoids is less than 5/8 inch.
The solenoids are wired to standard telephone or Ethernet jacks. The jacks are connected to the cables that run to the signal control box. A female to female connector is used to make the final connection to the signal from the cable. The wiring also supports the aspect backlighting light emitting diodes.
Power to drive the solenoid comes from a 6 volt battery. The current in the pulse is then around 2 amperes. The 110 millisecond pulse is generated with an electronic circuit that includes an H bridge and a mono-stable flip-flop. The controlling set of electronics was developed to reduce mechanical loads on the signals to an acceptable level by limiting the current passing through the solenoids. With the rare earth magnets large currents in the solenoid are not necessary.
The electronic circuits are placed in two boxes (one for the mainline and one for the branch line). Ttriple throw double pole switches to control the signals. Changing the poles reverses the current in the solenoid. On the bottom of the boxes are various jacks. These jacks are standard telephone or Ethernet jacks. Standard telephone cables and Ethernet cables are used to connect to the signals to the control box. With this method of connecting the signals all the electronic power functions are in the box. All operators have to do for set up is to join the jacks to the cables and the cables to the signals.
The aspects are coloured clear blue, yellow or red as appropriate and yellow light emitting diodes are used to backlight the aspects.
Given the complexity of the system it has worked very well. We will continue to improve the working of the signals to increase reliability. The whole signalling system is in prototype format and is at the proof of concept phase. Perhaps the next signalling system will be more like a productions model and use 3D printing for many of the parts. With a 3D printer all the wiring could be buried in the post, bearings incorporated and lamp brackets added. The motor and wiring block could be built as a set of snap together parts.
Over the coming months the club will develop operating procedures that will use the signals to control what is going on around the track.
Figure 2: A Completed Signal
Figure 3: Home and distant detail showing the arms, balance levers, aspects and back lighting brackets for the aspects.
Figure 4: Outer home and home distant signals in action.
Figure 5: A Terrier starts on the branch line with a 4 COR on the main line with the appropriate signals at off at Allingham Station.
Figure 6: A Spinner cleared to go from Allingham station.

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