Signal Systems -		  

Types of Signals and the circuits to drive them.
Signals | LED's | Tri-LED's | Relay | Electronic | Passage | Controllers | Computer |
Signaling has been a necessity for railway operations since they built the second locomotive. In the beginning it was only necessary to indicate whether a driver should stop, start or continue his passage.
A lantern was first used as a signaling device, but after the first mis-reading of this sign a fixed post with a moving arm was erected. This was operated by a signalman to permit or stop passage.
But like all good things, use of the railways grew until today a simple stop - go is not sufficient. The information needed by the driver to get his passengers or goods safely to a destination and allow maximum use of the rails has grown several hundredfold. 
The modern train running on congested track systems has to be controlled
precisely. Speed has to be correct, weather cannot interfere with control, communications are vital and 'in cab' signal indicators are being used by more and more rail systems. 
So what do you do on the model.
Adding a signal system to your layout may seem like a good idea when contemplated from a distance but when you get to the layout you have second thoughts. Look at those wires, there isn't room for more is there. Then the other problems start to surface...where to put the signal, what sort of signal would go there etc. And if those things are sorted out then, 'where do I buy it, can I understand how it works', not to mention, can I afford it. 
Do you make it prototypical, functional or just to look pretty. How you tackle signaling depends on your layout, the way you operate it and your skills at designing and wiring.
If you have a large layout where you have 'nights' and several engineers working, you could install a functional system that would advised the engineers about conditions ahead of them; such as turnouts, switches, occupancy, CAB settings in the next block etc. 
If you have modeled a prototype situation you might consider installing working signals in the correct locations as in real life. 		 Some things to consider.
The additions to the layout wiring and your records.
o There will be more wire but it will be light weight.
o You will need to keep GOOD notes/diagrams - I have found a computer drawing program can help.
o You will need an auxiliary power supply that is separate or isolated from the track CAB power transformer.
o Adding to an existing layout is harder as track may have to be cut, power wiring and other services can get disturbed.
Cost has to be considered
§ Ready made Signals can be bought in some scale sizes. You may be able to make them from parts or scratch build to save some dollars.
§ Electronics are available either as complete units, as kit sets or as PCB's that you can solder your parts onto.
§ The type of signal (bulbs, LED's, semaphores) will have a bearing on the electronic circuits you will be able to use.
§ In the handout I list some links to signal systems on the Internet
There are several ways you can do it. These I will discuss in more
detail later.
§ Perhaps the simplest method of setting up signals is to use block detector relays and the switches on the turnout motors. The extra wiring needed is electrical and the result realistic when used for main line signaling. You can even incorporate power control. This is best used on mainline blocks with dual track running.  
§ The use of simple logic elements, integrated circuits and transistors, might represent the next stage of complexity. Really the electronics only control which signal aspects light and the priority of them.
You still need to wire in the switches, detectors etc. It is more usual to use this with something like a single main line with two way running to emulate APB signaling.
§ A controller or sequencer will give you the option of more aspects, sequencing and interlocking with turnouts and loops as the train proceeds along the line. Sometimes CAB power control will be included.
Such systems are made by Marklin, IRdot (UK), SignalReasearch (Railmaster) in the U.S.A.
§ There are several forms of computer control available. Ranging  from those that use dedicated boxes to software that makes use of the family PC or an old one that is too small to run Windows.
Names to look at are - CTI (Tbrain), Marklin, Digitrax.
What you decide on will to some extent depend on your skills and those of your club mates or helpers. It is no use deciding on 'Computer Control' for your layout if you have no computer experience.
The signal devices

It would be best to first decide what type of signal device you are going to use, as this will influence the electronics.
Bulbs - The 12volt incandescent grain of wheat lamp is used in many commercial signals. One lead is a common and the others go to individual bulbs. Usually 12volts at 35 - 60 Milliamps. There is no lead polarity but a bulb needs more power than a LED. It is usual to use these with relay operated signals.
Semaphores - You may need a semaphore arm and this can be treated as a 2 aspect signal but the driver used would have to be able to supply more current either to drive a solenoid or shape wire (memory wire - 130 ma).
Kerosene lamps - For the traditional pre-christian modeler.
Light Emitting Diodes - or LED's, are the preferred source nowadays as the life expectancy is much greater. They come in different types though and a driver circuit must be selected for the type that you
will be using. All must be connected the right way round and need a current limiting resistor in the circuit.
1. Single LED's - When multiple single LED's are used in a signal, they will have a common connection between the leads. I prefer the positive (anode) to be common, that lets you switch the other side to 0 volts to light the LED. A current limiting resistor is needed in the circuit to allow a 10 - 15ma maximum flow. This can be in the common lead as normally only one LED is on at a time. If there will be 2 on at a time then each needs a resistor which will allow for different voltage drops in the LED's (technical stuff) 

The circuit (right) is for a ground turnout indicator. The switch (S2) is actuated by the throw bar on the turnout and allows the twelve volt supply, through the limiting resistor to flow through the Green (Normal) or Yellow (Reversed) LED.

 Using Single LED's
2. Dual LED's - these have a red and green light in the one device with two shared leads. Current one way lights the red element and the other gets green. This is good for two aspect but for three aspects the red and green have to be pulsed to simulate an orange/yellow light. Not very good as the red and green still show unless you are some distance away.

The driver circuit allows the current in the LED's to be reversed. With a Ground on the 'In" the transistor N1 does not conduct and the positive through R1 and R3 will turn on the transistor N2. This will put a ground at the base of R2 and the LED's and allow current to flow through the Red LED. When N1 is switched on (+ve at 'In') the Green LED will be put on.
3. Tri LED's - also have separate red and green elements but with three leads, one being a common lead. A better yellow can be achieved by lighting both red and green together. Some changes to the resistor values may be required to balance the colours as the red is usually brighter and the eye actually is more sensitive to red. This adjustment is needed on all dual LED's. The Common (middle lead) is normally the low or negative voltage cathode lead. This is the better type to use for Searchlight heads in HO scale and one with a diffusing lens, best.

The switch allows the voltage through the resistor to power the appropriate LED. When in the  middle position the voltage divides through the isolation diodes to power both LED's at once thus giving an Orange/Yellow light.
Here is a circuit where the switch is replaced by the contacts of a relay (driven by the Block Occupancy detector.
Again the Red or Green LED's are driven depending on the relay.  However there is an input (Yellow In) from the next signal so that when it is at Red the twelve volts also powers this Red LED and with the Green on this will produce the Yellow Approach colour by driving both the green (this block clear) and the Red LED's.

 

The Various signal methods
Lets look more closely at what the options are. The layout will have been divided into blocks of track, 3 to 4 train lengths long, between stations, junctions or yards. Each block will have a detector
of some sort and this will give an output when a train is occupying the block. Block Detectors are another subject again. Some study would be needed to get the detector most suited to your layout.
Relays - Relays were the first device used to provide model signalling.
There is nothing much in this if 2 aspect is your system. When the train is in the Block controlled by the signal then a red is shown, when it goes the green comes up. Or at a turnout, you could set up
a 2 light ground signal, activated by a switch under the turnout. For the more ambitious a three aspect signal can be used. With this you need an interlock connection to the signal ahead so that a yellow/caution can be shown in the block behind the red signal. This system suits
double track mainline operations and can be used to signal diverging routes at junctions and yards.
Relay power control - With a suitably isolated and connected rail ahead of the signal the train can be stopped in front of a red signal. When the next block is occupied a relay can be picked to isolate the
power on the track in front of the signal.
Simple logic - Electronics can be used to supply the current to the bulbs or LED's in the signal heads and give the signal aspects logic precedence. This means that Red has priority and stops yellow or green
aspects.  There are many types of circuits available to do this and several are shown on the last page for those able to understand electronics. All these different circuits can be shown as a box wired to a signal.
You supply it with the connections to select Red or Yellow aspects.
I will go through some typical diagrams that represent the actual wiring you will install.
Here is a diagram of the wiring needed for the Model Railroader signal system (see March 2001) The aspects are simple, with the turnout set to the junction track there is a yellow aspect when facing. Opposing trains see the red aspect at the start of block 3.

 

Passage Signalling - My Module Signals circuit has a 3 aspect head driver and a timer. It does not need block detectors but can still be interlocked to switches, other signals etc. It detects when the
train passes the signal and then set it to red. After a time delay the signal returns to yellow then to green. If the track power is
reversed, the signal goes to red. This sort of signal looks very realistic and is most useful for a smallish layout or on a module where there is not enough track length to have multiple blocks and a detector
system. 
In the diagram above, a simple passage detector controls the adjacent signal. The detector is a small 1 centimetre section of isolated track and when the loco wheels pass over it the signal action is started.
The electronics are on a single board and all you need to do is wire the three track connections and the signal. There are also logic inputs to the LED driver, labelled "Yellow" and "Red". When these
are switched to ground or the negative supply voltage they hold the aspect and over-ride the time out. This makes the signals more useful in loops, junctions and yards. The turnout position, red on the next
signal ahead or other interlocking conditions can hold the signal aspect. 
The diagram illustrates Approach and Home signals with interlocking from the points setting. As the train passes the right hand 'Approach' signal the colour goes from green to red and the logic starts the time out. The next (Home) signal will be at green if the route is set to the main line, otherwise will be yellow if the loop is to be taken. As the train passes Home, the signal goes to red and through the 'red out' a ground is wired back to the 'Approach' signal to hold it at yellow. The distance between the signals needs to be longer than the longest train and the delay time to be about twice the time the train normally takes to pass the signal position. Reversing power on the track will automatically change both signals to red and the
timers will be disabled.
Controller - A controller is a collection of logic that will sense the switches, detectors etc. and send the red/yellow logic levels to the signal head. The LED drivers might be included on the logic
board. A controller is usually needed where you are driving more than 3 aspects to the signals and usually will included, as part of the general system, detectors, signals etc. It simplifies some things
but because it is usually all in one box it does mean there is more wire going to signals and turnouts around the layout.
NZR signalling which uses multiple aspects with auxilliary speed indicators is a good example of a controller situation where each signal has 2 heads and there are 6 or 7 possible aspects. The signals around a station and yard with interconnections might be best done this way. At least 6 signals or 12 heads plus any in the yard could then be controlled. The logic in the controller can be programmed into a PIC. This is called a Programmable Interface Controller IC and saves the use of a lot of other components.

Ref. Marklin for controller boxes to set the route and signals. 
Heathcote Electronics (UK) for detectors, signal controllers, station shuttles and points controllers. This system uses Infra Red Position detectors for train location, and allows speed control for station stops etc.
SRI - Signal Research Roadmaster have a complete control system with block control modules, turnout modules and signals. Each train is located as it travels around the layout and its speed controlled.
Signals are part of the system although they do not control the trains. This is not DCC and requires no locomotive modification. You are limited to one train per block though.
Computer - Computer controlled layouts are a practical possibility nowdays. They could range in complexity from those that just control the setting of the signals to complete control of the route selection
and train speed, setting the signal aspects and moving multiple trains around the layout.
The programs or software to do these functions are available. Another option is to write your own. In this way you are making it proprietary to your layout and therefore more efficient. The computer does not
need to be the latest as railroad things happen slowly in computer time. There are many older PC's lying around unwanted now and these will do the job well.
Extra hardware will be required to enable the computer to interface to the layout. This is available from several sources. This is needed to report the status of things that are happening on the layout to the computer, every turnouts position, block status etc. has to be known. The computer then sends control bits back to the layout to change turnouts or set signals. As it knows what has been set and what the status of the layout is it has complete control. You would tell it the route that you wanted to follow and from the train position it would set the turnouts and signals as you drive around. It could even stop track power if you were getting into danger. Locomotive control - Of course a most important part of running the layout is controlling the loco's. You could do this while observing the signals and driving the train around, but wouldn't it be humbling when as you run a red light or go through a reversed turnout, the
computer lets out a squawk of anger!! The computer then, needs to be able to set speed and direction of the trains to give it total control. If I were programming I would allow for a human controlled loco to run at random, observing signals etc, so that the computer had some challenges while running a standard route with several other trains.
DCC programming
I have not mentioned Digital Command Control yet but everyone has it - haven't they? 
Well expensive as it is, that is the way to go especially if you want to use computer control. To some extent DCC really needs a computer or really good power control signalling system as there is much more possibility of rear end collisions since trains can be in the same block at different speeds and if you are not watching....there will be a shunt and usually a derailment.
The signal systems described here will all work with DCC track power and the Block Detectors for DCC are available as kits.
Digitrax sells an interface to allow the PC to see the 'LocoNet' communications and be one of the contibutors. Software is available to enable you to control from the computer through the accessory decoders, so that turnouts and signals can be set. The throttle to loco signals can be monitored and speed and direction found for each loco. With transponders installed, each Loco's position can also be known. On the screen will be a diagram of the layout (inserted by you at startup) and the loco information is shown on it, updating as the train moves around.

Some Internet sources of systems and hardware:
"http://www.mcs.net/~weyand/home.html - TrakTronics - detectors, signalling,
ABS and CTC (American) systems
"http://www.alltel.net/~nw90840/ - signals, block system using magnetic
detectors and PIC controller.
"http://www.signalresearch.com - a layout CAB controller that includes
signals - Roadmaster
"http://www.cti-electronics.com - Software and hardware modules (American)
"http:// homepages.paradise.net.nz/~rdmurg - for time delayed signals
and for CAB detecting signal systems
"http://users.cybercity.dk/~ccc29506/index.html - North American Signalling
Documents
"http://www.railtronics.com - components etc. Canada
"http://www.mergrpc.freeserve.co.uk - Railway electronics, DCC, block
controllers etc.
The I.C. driver uses logic to perform the priority. If there is a ground or 0 volt on a red input then this will be converted to a high voltage to drive the LED and prevent yellow or green from getting drive voltage. Similarly yellow at ground will stop green from showing.
If both red and yellow are high then green will get the drive voltage.
The 'SEL' resistor is selected to adjust the red brightness if required.
This circuit uses LED's wired for common cathode and a resistor to
ground.


The other example uses 2-input amplifiers and resistor dividers to
determine the priority. Any grounded input will go to the minus input
of an amplifier it is inhibiting and the positive input of the amplifier
for the LED that will be put on. The LED's are common Anode and go through a resistor to the positive supply.
Explanation - A Red in (0 volts) from the detector will prevent the
yellow and green showing by switching on the transistors which short
out the LED's. Yellow in at 0 volts prevents green and when neither
red or yellow are called for then green will show. This only requires
2 wires or logic conditions - make it red (such as when the next block
is occupied) or make it yellow. If you try to make both yellow and
red then only the red shows. The conditions needed to force red or
yellow come from switches or relays connected to turnouts and block
detectors or from the signal ahead. In the diagram there is an option
for approach lighting or constant green (Green is connected to ground)