Signals and relays

This web site describes UK signals both semaphore and colour light and is animated rather nicely.

When I was of school age I loved to join my father in the Railway Room typically on a Sunday afternoon. My mother was, I think, jealous of the time we spent there, and maybe of the money spent, so my father would disappear clandestinely. I would monitor his movements and thus figure where he had gone and join him sooner or later. And there we would stay until my mother called up the stairs "tea time". I think she visited the Room maybe once or twice, as did my sisters, but never with any particular any interest. In fact it was very rare to find anyone with similar interests.

Dad would habitually control the up line and I the down line. Neither of us were too fussed with the visuals although each of us had made a few half-hearted attempts at scenery. This is so different from the typical model railway display where visual authenticity rules ahead of technicality.  For us time and motion and the feeling of actually driving the train were more important. But this is all described in the previous post and all I want to add here is a more in depth description of the automatic signalling I added to the down line circuit.

Relay operated block signalling

I kept my original hand-drawn wiring schematic for many years but sadly no longer have it. Our community experience in the North was a great reducer of personal possessions. The schematic above (click to enlarge) is drawn from memory and may not be quite as I had it. I do remember I had to add electrolytic capacitors to slow down some of the relays, I suppose to avoid some sort of glitch happening, but I cannot remember where they were placed. I must have been that much wiser back then. But I think it will be sufficient for any other model railway signal-lover to base their design on, in the off-chance that such a one might read this. The scheme is of course based on the absolute block system which I had ample time to study on my daily journeys to and from school, in which only one train may occupy a given block or section of railway and the entrance to each block is protected by a signal and, if single track, by the occupying train driver having in possession a unique "token".

The 4-pole changeover relays were from an old telephone exchange. I may in fact have used 6-pole relays as one extra pole would save the latching effect relying on a resistor to keep just enough current to hold the relay closed. And the 6th pole could be used for four aspect signalling or added complexities such a junctions.

The 2-pole changeover relays I probably purchased. The single pole changeover "current sense" relays were hand made. At the time I was not aware that such a thing existed so I had to invent it.  A model train typically consumes between a quarter and one amp, so the idea was to wind sufficient turns on the coil to operate the armature whilst not reducing the voltage delivered to the train appreciably. The system worked superbly.

Later in life, after I had learnt about TTL logic, I tried reproducing the functionality with integrated circuits. This was unsuccessful for various reasons. I suppose now-a-days one would use a microprocessor. No - now-a-days most folk would not even bother and instead just get the "app".

It's strange reading Sci-Fi stories that date from the 1950's.  Sure, they have space ships and robots, but they are all powered by relays that click incessantly. And the people are smoking incessantly, even in space ships in outer space.  Having said that, relays are very much still with us being almost irreplaceable for many functions.  They are rugged - no problems with damage by ESD or finite on-resistance! Available with many poles. And capable of switching large currents and voltages.

The operative part of my scratch-built colour light signals were 12V grain-of-wheat bulbs which I purchased, as pocket money permitted, in the model shop in Eastleigh. This shop was tiny - a bit like the electronics shop in Winchester only the sales person was a woman if I remember correctly. Not that there is anything wrong with that: she knew her business well enough. In later years I made one using those new-fangled LED's but by then I was away at college so it was never used.

The second innovation was the automatic selection of one of two speed controllers (the "inertial" design described in my previous post). In this way it was possible to control two trains independently. The choice is controlled by the wire labelled "SELECT" which is 3-state: open-circuit leaves the next 2-pole relay in its existing state, connecting to +12V turns it on and connecting to 0V turns it off. A red signal opens the circuit thus preventing any new choice (set by the switch) to ripple through to the block beyond.

The schematic shows only 2 blocks - in fact there were 4 blocks in the overall circuit, not counting the station block itself which was controlled manually. Along the way there was a passing loop and two merging tracks which were also controlled automatically. The above schematic shows none of this detail.

Once I had designed the system I had to cut the track to create the dead section (to force the train to stop before a danger signal) and the sense section (to operate the sense relay) in each block, and run numerous extra wires under the base-board.  Cutting the rails was easy enough using a junior hacksaw, but I did it in much trepidation because I did not first ask my father's permission. This latter omission some might question but, in fact, it was done intentionally - it is a principle of life that the adventurous must "boldly go where no man has gone before".  I find it is the same now with my running hobby - I don't tell people in advance what I am about, I hardly dare even mention it to myself - for fear of disapproval. The principle only works, of course, if it works. I knew my father well enough to figure he would not be overly displeased provided I proved myself. Witness the episode when, much younger, I carved up the lawn. Any parent who stifles their kid's creativity needs their head testing. I guess there are limits, though.

Signal symbols

I invented these symbols to represent signals for track diagrams - the top row are colour light and the lower row the equivalent semaphore.

Signal box at Alresford

If I ever travel by rail my head is often out of the window (modern trains don't facilitate) - and in particular I am looking for the signals. Why signals? Doubtless because of their colour, and figuring out the necessary underlying control systems. Which of course implies my fascination with the signal box with its token system at Alresford where I boarded the train to school every day. My father introduced me to Mr Norris the signalman on one occasion when I got to see the inside of the box.

I found the pictures below on the internet - they date back to my school days.  I caught the 08:15 train every morning and often arrived (panting) at the station after the two trains had pulled in, to run along the platform and over the level crossing to the down line on the left. The picture brings back so many memories, mostly fond.

Up and down trains passing at Alresford

The route passed through Itchen Abbas station (this station was unattended and its passing loop had been removed many years before my time) and thence to Winchester Junction where it joined the main line from Waterloo.  Here the signal box was equipped with a platform where the signalman would wait to collect the token from the driver (or give it him on the return journey).

Passing the token at Winchester junction