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Signalling a layout

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Dhu Varren
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Watching “The Derry Road’s” Interesting thread on signalling ‘Rachelstown & St Stephens Green’ reminded me of a signalling project I undertook twenty five years ago, and which is still working well.

The layout is a large terminus station and loco depot, which are approximately 35 feet long, before the tracks disappear onto double track running behind the scenes. This length gives the opportunity for some 4 aspect colour light signalling.

The signals are a mixture of Eckon signals, and scratchbuilt signals to look like Eckon signals. They were fitted with LEDs at a when Eckon were still using bulbs. The signals were also built before the advent of white LEDs, so, as a compromise, yellow LEDs were used where white lights were required, such as feathers and shunting signals.

All signals are controlled by 3 position switches. The centre position is the signal ON, showing the stop aspect (red). The down position is the signal OFF, showing a proceed aspect (amber, double amber or green), depending what aspect the next signal is showing. The up position switches on the appropriate calling on signal, if fitted. The passage of trains cancels each signal as the train passes (returns to red) by means of a treadle in the track. Magnets and reed switches were considered, but decided against, as a treadle will work with any vehicle. All signals, except 4 aspects will stay red until the switch is returned to the centre position, moved back to the down (OFF) position, and the train has passed two more signals. 4 aspect signals reset themselves automatically after the train has passed two more signals, showing first a single amber, then a double amber, and finally a green.

Signals are interlocked with the points on the route the signals control, so that if any point is not set for the route, the signal cannot be pulled OFF. As this layout was originally an exhibition layout, it was decided that points should not be interlocked with signals. This was to ensure that should there be a signal failure, the layout could still operate, albeit without signals, until the fault was sorted.

The first set of pictures show from left, Platform 5 starter with calling on unlit. Platform 4 starter showing green with feather, and a calling on unlit. The ground signal is showing stop. There seems to be a bit of red light leakage into the top led, never noticed that before. Platforms 3 & 2 showing reds and calling ons unlit, and then Platform 2 with the calling on signal OFF.

These signals are all scratchbuilt. Platform 1, not shown, has a simple 3 aspect starter with calling on.

 

Signals 3.jpg

 

The second set of pictures shows the full length of the layout, with the starter signals nearest, the advance starter signal in the middle, and the first 4 aspect section signal in the top centre. Apart from the starter signals, all the others are standard Eckon signals with LEDs, plus calling on signals where necessary. The pictures show the sequence of events as a train departs, and once the signalman has pulled off the signals, everything is done automatically, with no further input from him.

From left, all signals are ON. Next, Platform 4 showing amber plus feather. Then advance starter showing amber, and P4 starter showing green with feather. The section signal has gone straight to green, which allows the advance starter go to green. The train leaves P4, the starter changes to red and the feather goes out. As the train passes the advance starter, it also changes to red. Lastly, the train has passed the section signal which has gone to red, and then the next signal, not shown, has been passed which has released the advance starter, which now shows amber. With a bit of cunning circuitry, the section signal will work it's way back to green, via a single amber which will let the advance starter go to green, then a double amber, and then the green.

 

Signals 1.jpg

 

Signals 2.jpg

 

The third set of pictures show, from left, the home signals all ON. Then there is the outer home signal, which is scratchbuilt, showing an amber as the following home signal is red, and the feather for the second crossover. Next are the home signals, with that for Platform 3 now showing an amber. There are no green aspects on the home signals, as the next signal is permanently red, the buffer stops. Back to the outer home signal which now shows a green, as the Platform 3 home signal is now showing an amber aspect.

 

Signals 4.jpg

 

I hope this shows what can be done with a signalling system, and a bit later on I will be putting together a piece on how the system works electrically, which will really frazzle your minds.

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Second part.

 

The signalling system uses TTL technology based on circuits that were in a book on electronics for railway modellers. TTL is transistor logic where the output from a circuit is either + or -, Hi or Low, 1 or 0. The terminology depends on how you think. They all mean the same. TTL circuits work on 5v DC, and an output is classed as Hi if it is greater than 3v. Conversely, an output is Low if it is less than 2v. These figures are not the exact values, but just give an idea. TTL outputs are controlled by ‘gates’ of different types. There are ‘AND’ gates, ‘OR’ gates, ‘NAND’ gates, ‘NOR’ gates, and ‘INVERTER’ gates. Each gate can be considered as an electronic switch, the output of which depends on the input(s). The INVERTER is the exception as it has only one input, and is used to invert the output to the opposite of the input. So a Hi input will come out of an INVERTER gate as a Low output, and vice verce.

When initially reading the aforementioned book, I just could not get my head round what it was talking about with ‘AND’ & ‘OR’ gates. It was only when my daughter came to me in desperation with her homework as she had to come up with everyday scenarios where AND & OR situations arose. This had nothing to do with electronics but it made sense when applied to TTL gates.

An everyday situation is when either the driver’s door of a car is opened, OR the passenger door is opened, the interior light comes on. Likewise, when the ignition switch is turned on, AND the wiper switch is operated, the wipers work. The list is endless. Applying that to signalling, an AND gate with an input from a switch on the panel for a signal, and an input from a switch on a point will give an output to a proceed aspect LED when both inputs are Hi. Should either of these inputs be low, then the LED will not light up.

TTL gates are like building with Lego, only you have to draw out your plan on paper for the circuit. Once that is done, it has to be translated into a physical plan for the installation of the ‘chips’ and how they will be wired/connected together on a circuit board. Most TTL ‘chips’ have four gates on each, although the ‘inverter’ has six. There are also ‘chips with gates with more than two inputs, the number of gates on one of these ‘chips’ depends on the number of inputs to each gate. For instance, three inputs will only give three gates, and so on, as there are only twelve pins available on each ‘chip’ for gates. In addition, there are two pins for the 5v power supply. Surprisingly, NAND and NOR gates are the most useful. They function just like AND and OR gates, only the output is inverted. They can also be used as INVERTER gates simply by connecting an input to both input connections. TTL chips are very robust, and can take a lot of punishment when connecting up, particularly where a short circuit situation arises. A very useful tool, particularly for fault finding, is a Logic Probe, which will indicate which type of input or output, Hi or Low, is present at any connection.

Each signal on the layout has a plug in circuit board. These boards are standardised, and can be used for either 2 or 3 aspect signals. The 4 aspect boards are also standardised, but can only be used for 4 aspect signals. Each signal board has inputs from the panel switch, from the points (via a points circuit board), from the track treadle and from the next signal red aspect, and in the case of 4 aspect signals, from the single amber of the next signal. There is also an input from the track treadle of the signal after the next signal, which releases the first signal for resetting.

To show how simple the circuitry can be, the circuit below is for a basic 2 aspect signal with a panel switch, and an input from a point switch. Both the panel switch, and the point switch must be closed for the signal to show a green aspect, if either one is open, then the signal will remain red. If there were more than one point protected by the signal, then a multi gate NOR chip could be used.

The switch inputs are held Hi by means of a 10K resistor. When a switch is closed, it connects the input to 0V giving a Low input. Being a NOR gate, if either input is Hi then it gives a Low output, which will not light the green LED. However, the second NOR gate, wired as an inverter, will invert the Low output from the first gate, to a Hi output, which will light the red LED. Conversely, when both switches are closed, both inputs are Low, giving a Hi output which will light the green LED, but the second NOR gate will invert that Hi output to Low, which will not light the red LED. I hope that all makes sense.

 

Simple circuit.jpg

 

The next pictures show the gate plan for a 2 or 3 aspect signal board, and the physical layout of the chips on the board with wiring. There are spare unused gates on this board, which could be utilised for controlling a feather attached to the signal, or even a ‘calling on’ light. Note the use of NAND and NOR gates as INVERTERS which does away with the need for an INVERTER chip on the circuit board.

 

Standard Signal Plan.jpg

 

Standard signal layout.jpg

 

Picture of a completed circuit board, front and rear. The right hand edge of the board is designed to slot into an edge connector, vary useful for fault finding in that identical boards can be swapped round to determine if the fault is with a board, or with wiring elsewhere.

 

signal board both sides.jpg

 

There is a TTL relay included in the circuit, which is what the track treadle activates when the signal is passed. The relay locks on and holds the signal at red until it is reset either by the panel switch or the treadle for the next but one signal, depending on whether the signal controlled by this board is an automatic one or not. Flip-Flop switches were originally used for this purpose, but it was found that locomotives produced too much electrical ‘noise’ which affected the Flip-Flops, which then gave spurious outputs. The TTL relays were substituted, and have worked perfectly ever since. There is an extra component on the signal board, not shown on the plan, namely a diode across the coil of the relay, to dissipate any Back EMF generated when the relay unlatches.

If the power supply to the layout is switched off for any reason, any signals that are still showing red due to being cancelled via treadles will return to a proceed aspect when the power is restored. This is due to there no longer being any power to hold the relay closed, so it opens and the signal is released.

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