Saturday, March 14, 2020

Signal bell boards for the ETE HOm Swiss Narrow Gauge modular layout

Test Setup
Every now and then I'm at shows running trains with the HOm swiss narrow gauge group of the ETE San Francisco Bay chapter.

The operating scheme is straight-forward: The layout is set up with a single track main line, a couple intermediate stations, and staging at each end of the layout. The typical setup consists of staging with a return loop at one end, two intermediate stations, and a simple staging yard at the other end. Operators are positioned at the stations or staging yards and verbally hand-off trains from station to station. A typical conversation between two neighboring stations goes like this:
- "Can I send you another train?"
- "Yes, go ahead. Once that's here, I'll send you a train back."
- "Ok." 
This works quite well. However, problems arise at shows with lots of visitors since the noise level makes communication, or even getting the attention of the other operator, difficult. Especially when the layout is arranged in a way so that they sit with the back to you.

Before the invention of the telephone, railroads in Europe had similar problems. How to tell the following station and all operators at intermediate railroad crossings that a train is on the way? How to do that in a way that works reliably day and night, regardless of weather in rain, sunshine, and fog? Initially, humans were positioned along the tracks and blew a horn to notify the next post that a train is coming. That obviously didn't work very well and became completely inadequate as traffic density increased.

In the mid-1800's bell signals ("Läutewerke") were invented to address this. The Wikipedia article describes the basics. Hans Waegli's book "Hebel, Riegel, und Signale" has a good historical summary as far as use in Switzerland is concerned. The common arrangements were built with two bells tuned to different tones. Each bell was associated with a direction, so that crossing operators can tell when to expect the train and lower the gates early enough based on which bell is ringing. Bell signals were used widely in Germany, and became very common in Switzerland were they lasted well into the second half of the twentieth century, especially on secondary and narrow gauge lines. Some are still in operation today, but only to alert waiting passengers that a train as left the previous station.

I figured we could use a similar approach when operating the HOm layout at shows.

Signal bell boards schematic
Nice two-tone bells are rather expensive, so I settled for a cheap DC door bell. Each station has a button per direction that rings the bell in the next station. Because I only have a single bell, the direction of train announcements needs to be encoded in the ringer scheme. A simple ringer scheme to offer and accept trains between stations could look like this:
2 rings by originating station: Request train move from left to right
3 rings by originating station: Request train move from right to left
1 ring by destination station: Train may come.
If the destination station doesn't want to accept the train, they simply don't acknowledge, or offer a train in turn.

Since this is a modular layout, the cabling length between stations might vary from show to show. RJ45 plugs and sockets make using cables of different lengths easy. For the test setup I used Ethernet cables. Since only 4 wires are needed for the inter-station wiring in the schematic, I mirrored the pin assignments, so that any Ethernet-style cable can be used, and one doesn't need to pay attention to orientation. I tested this today with a 50ft Ethernet cable, which worked just fine.

Intermediate signal bell board
The real railroads used fairly elaborate schemes with two or even only one wire between stations, but those required independent power (mechanical, batteries, or house power) at each station. I wanted to power the whole system only from a single power supply, so had to include a power bus in the schematic.

Signal bell board with power supply connection
The signal boards are clamped to the modules so they can be mounted wherever it is convenient. The left button is connected to the bell of the station to the left, the right button to the bell at the station to the right, so using the bells is very intuitive for station operators.

Signal bell board clamped to the Welztalbahn
The signal bell boards come in two versions (yellow and green in the schematic above), that need to be installed in alternating sequence. So far I built two boards: a yellow intermediate board, and a green board with power supply connection. I have enough supplies to build two more boards which should be sufficient for the HOm layout.

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