Model Railroad

Automatic Block Signal System
A brief overview of the parts, operation and connections employed

(Download this (long) article as a PDF File. Adobe Acrobat Required.)

 

Recently, Metrolina Model Railroaders in Belmont, NC  http://www.metrolinamodelrailroaders.org  implemented an Automatic Block Detection and Signal Light System using technology gleaned from several different public sources but modified to meet their specific requirements.

 

When researched, there was no “complete system” located that satisfied the requirements of the group. They found several suggestions and partial solutions to the task, but nobody offered a complete design from sensor to signal.

 

Metrolina Model Railroaders has a LARGE, modular layout that can be assembled in many forms and configurations. They carry their layout to various model railroad shows and expositions as well as to local activities and celebrations and have to make it fit in the space allocated.

 

The club’s layout has two main tracks with trains traveling in a clockwise direction on the inner track and in a counterclockwise direction on the outer track. They decided to use single-sided, three color block signal lights, positioned at the beginning of each block.

 

The club decided to ignore sidings and their large train yard while getting the main Block Signals to work correctly.

 

The club also decided to run their system using the “Approach Lighting” technique (a method used by some railroads to conserve signal batteries) which has signals normally dark but are lit when a train enters the preceding block.

 

The Automatic Block System consists of:

· Sensors (quite a few).

· Multi-Sensor Circuit Boards (one for each track on each module).

· Four Event Controllers, with Transistor Amplifiers (one for each module).

· Signal Light Controller (one for each track on each module).

· Single-sided signal lights.

 

                

                              

 

 

The core component for the system is the Signal Light Controller. All blocks have a Signal Light Controller and they interconnect in “daisy chain” fashion. When a train is in a block and ANY ONE of the Sensors goes into darkness, it causes the event controller to “notify” the corresponding Signal Light Controller that the block is occupied.

 

At this point, the Signal Light Controller:

• Sets the CURRENT block signal to RED
• Sets the NEXT block’s signal to GREEN
• Sets the PRIOR block’s signal to YELLOW

 

The Signal Light Controller operates with a constant, +12V connection (Common Anode). We switch the –12V (ground) on or off to activate the Signal Light Controller.

 

The Signal Light Controller design is taken from an article by Jeff Scherb in Model Railroader’s March 2001 issue.

 

We changed the Circuit Board Layout to include connectors we had available and felt would be easier to maintain in a modular type environment where there is no “fixed” or “permanent” layout.

 

After a bit of research and prototyping, it was found that the “Across Track Infrared Detection” circuits, available on Rob Paisley’s web site (http://home.cogeco.ca/~rpaisley4/CircuitIndex.html), were close but not exactly what was necessary to activate the Signal Light Controllers. His switch designs use a Voltage Comparing Op-Amp using a LM339 chip.

 

The power capability of the LM339 was less than what was required by the Signal Light Controller. Also, because each block (module) was at least four feet long, we needed many sensors in each block to accurately know that a block was occupied.

 

The power problem was solved by making the LM339 a PNP device, instead of the NPN as suggested by Mr. Paisley. This was accomplished by setting the comparison voltage on the - inputs instead of the + inputs and creating a +12V output instead of -12V. This output was then channeled into a NPN Switching Transistor (2N3904), which created a strong ground when triggered by the LM339. Pull-up and current limiting resistors were added to finish the circuit.

 

Mr. Paisley suggests a way to attach several sensors to the same switch channel so we expanded on that suggestion and created a separate, MultiSense circuit board that allows up to eight sensors per board. The MultiSense boards were designed to be combined to allow even more sensors per block. If less than the eight sensors are required, we used “Berg” type connectors so empty connections could be easily shunted to allow the other sensors to do their job.

 

Because the Signal Light Controller already handles the voltage drop required to protect the Signal’s LEDs, they don’t need to be included in the event controller module. Also, because we were going to use multiple sensors, it wasn't necessary to include the balancing and voltage drop resistors for the many sensors in the event controller design.

 

The Boards:

 
MultiSense Board

 

Four Event Controller

 

Signal Light Controller

The basic operation is quite simple:

 

 

Each track on each module of the layout (the “city” module is an exception) is considered to be one block. The train on the OUTER track runs Counter-Clockwise and the train on the INNER track runs Clockwise.

 

Each block has multiple sensors embedded between the track “ties” and they sense the presence of light. When a Sensor goes dark (due to the train passing over the sensor), the MultiSense board causes the Event Controller to send a GROUND (-12V) to the Current Block Signal Light Controller. The Current Block Signal Light Controller switches it’s signal to RED and sends a ground signal to the NEXT block’s Signal Light Controller causing that controller to signal GREEN. The Current Block Signal Light Controller also sends a ground signal to the PRIOR block’s Signal Light Controller causing that controller to signal YELLOW.

 

Only the adjoining THREE blocks are affected and the rest of the signals remain dark.

 

You can also set ALL block signals to display GREEN if you prefer. The RED and YELLOW functions remain the same. To do this, you will need to add a constant ground (-12V) source at each Signal Light Controller.

 

When a train is long and is in more than one block at the same time, all occupied blocks are signaled RED and the next “clear” block is signaled GREEN while the prior “clear” block is signaled YELLOW.

 

The way the Sensors work is that if ANY of the Sensors are in darkness, the entire block is assumed to be occupied.

 

One snag arose because the Sensors are sensitive enough to see the light allowed through when the coupler area passes over the Sensor. This causes the signals to BLINK. Once a couple of cars are in the block, there is always a Sensor covered, so the blinking stops until the last few cars leave the block and again, the signals BLINK.

 

To correct this, we have installed an extra sensor approximately FOUR inches beyond the first sensor and also approximately FOUR inches before the last sensor in each block. This basically guarantees that there is always a sensor covered in any occupied block.

 

The rest of the sensors are placed approximately SIXTEEN to TWENTY inches apart and have worked well with trains that are at least TWO feet long.

 

As always, there are exceptions to the plan, most notably, the club has a very intricate, fragile TRESTLE Module that doesn’t allow us to place sensors on the actual trestle roadbed. Also, there are no sensors in the MIDDLE (four foot) module for the City Layout since it is all “underground” – basically, tunnels.

 

There are no lights inside the tunnels yet, so the sensors would think a train was always present. We will be adding Infrared Emitters in the tunnels so sensors can be added to allow the detection of shorter trains.

 

How it is all put together: 

 

The Sensors used are Infrared Phototransistors, Part # LTR-4206 from Lite-On Corp. The 3mm devices work well in many light environments like Fluorescent, HP Sodium, Mercury and Incandescent. They are connected to the feed wires using a Wire Wrap tool. This keeps the Sensor safe from solder temperature and is less expensive than using a pin type connector.

 

The Sensor looks like a clear LED (Light Emitting Diode) and has short leg/long leg identification for polarity. They also have a flat side that is adjacent to the Short Leg – also called the Collector, Cathode or Positive connection. The Long Leg is also called the Emitter, Anode, or Negative connection.

 

The standard connection method is to use “cross connect” wire pairs that are Yellow/Blue, Blue/Yellow. We connect the Yellow/Blue to the SHORT leg, trim 3/8” off the bottom of the leg, and use a piece of heat shrink tubing to protect it. We then connect the Blue/Yellow to the LONG leg, without any heat shrink tubing.

 

                       

Sensor Connection

 

 

The Sensor is inserted through the layout from the bottom side and held in place with a spot of hot glue.

 

 

The Sensor Wires are grouped by inner or outer track and brought to the center of the module where they are connected to the respective MultiSense board and then any empty connections are closed with a jumper.

 

 

Small Module Installation

 

                         
MultiSense Connections

 

 

There is a jumper marked JMP1 that adds the resistance required to balance the voltage. Only one resistor is needed if you are combining multiple MultiSense boards, so that is why you remove the jumper for all additional boards on one block.

 

The MultiSense boards are connected to the Event Controller with standard, PC type CD-ROM Audio cables. They are polarity sensitive and by the use of an empty / missing pin, it is easy to connect the cable correctly.

 

The Event Controller has input connectors the same type as the MultiSense board. A NICE feature of the event controller is it has the capability to control up to FOUR circuits. This allows for redundancy and/or allows us to have two more items controlled on each module. An example would be a set of grade crossing lights and/or a set of grade crossing gates.

 

The Event Controller gets powered by two unused black wires running in the DCC cable bundle and the event controller provides the power for the other boards in each block.

 

The Event Controller sends the occupancy signal (Ground) to the Signal Light Controller, so there is a cable from the Event Controller to each Signal Light Controller. The cable POLARITY is very important, so attention to these connections is required. 

 

There is a bridge rectifier (four diodes) on each Event Controller so polarity is not an issue and you could even connect to A/C power if necessary.

 

 

      
Event Controller Connections

 

 

The bottom, (white) connection is for power in.

 

The next FOUR pairs of pins (plugs) are for the output to the Signal Light Controller. The plugs are marked O1, O2, O3, and O4. (Please see note below) The outside pin of each of those four connectors is the constant POSITIVE, the inside pin is the switched NEGATIVE (ground).

 

You need to make sure that the positive connection of the Event Controller goes to the positive connection of the Signal Light Controller.

 

Our standard has the WHITE wire of the connector cable as the POSITIVE connection. I will show the other end when I describe the Signal Light Controller.

 

The input from the MultiSense board is connected to the chosen plug in the upper (Input) group. The plugs are marked I1, I2, I3, I4 and we are using Input 1 and Input 3.

 

**** NOTE: There is a small mistake in the design of the Event Controller in that the bottom four connectors are marked O1, O2, O3, and O4 designating the four channel outputs. Due to a layout mistake, the correct order is really O2, O1, O3, and O4. That is why you see the output cables on plugs 2 and 3 and the input cables on plugs 1 and 3.

 

The Final Component: 

 

The last component of the system is the Signal Light Controller. This controller is easily connected AND it is compatible with light units from companies like New Jersey International and others. We are researching the Atlas units and hope to confirm connectivity with them too.

 

The Signal Light Controller has several connections:

• Power from Event Controller to the Signal Light Controller.
• Cable to NEXT Block.
• Jack for cable from PRIOR Block.
• Signal output socket.  

 

The Signal output socket we use is a standard Telephone Jack that connects via cable to a bulkhead connector passing through the layout and which has the Block Signal plugged into the upper socket. The lower socket has a short telephone cable that goes to the Event Controller.

 

The Jack for the PRIOR Block cable is a standard DC Power Jack.

 

We use this Jack so we can unplug the modules from each other, rearrange them as desired, and reconnect the Block Signals with no concern for other elements.

 

The cable to the NEXT block has a matching DC Power Plug attached and has an easily gripped shoulder to help make insertion easier.  

 

IT IS IMPERATIVE THAT THE SIGNAL LIGHT CONTROLLER IS HELD FIRMLY 
WHILE THE CABLE/PLUG IS ATTACHED.

 

The Power from the Event Controller to the Signal Light Controller is passed through a short, red jacketed, two-wire cable coming from the Event Controller. The cable doesn’t reach all the way to the Signal Light Controller on the modules, so we use a short length of telephone wire to bridge the gap.  

 

The Signal Light Controller’s power connection is a SCREW CLAMP type block. The POSITIVE side is the side adjoining the YELLOW PRIOR Block Jack. We connect using either a yellow/blue, blue/yellow or an orange/red, red/orange pair of wires to the Signal Light Controller. The other end of the wire pair is slipped into the plug on the end of the red jacketed, two-wire cable coming from the Event Controller.

 

 

                     
Different Connections on the original Signal Light Controller

 

 

             
Power Connections for the original Signal Light Controller

 

 

The Revised Signal Light Controller:

 

In an effort to make each circuit board as small as reasonable and to standardize the method of interconnecting the circuit boards together, we have redesigned the Signal Light Controller to use “Berg” type connections, which allows us to use standard PC Type cables or to just hard solder cables to each connection point.

 

Small layouts can probably justify using ready made connecting cables but large layouts make the added cost of prefabricated cables a burden. If you don’t have the need to move modules around, hard soldered connections are less likely to have problems and “cross connect” wire is readily available and quite affordable.

 

The revised Signal Light Controller still uses a screw type power connector (like the earlier version), but it also has the spacing for a “Berg” type connector as well.

 

 

 

 

The LEDs shown are for display and you would either use “Berg” pins or direct solder connections to feed your Signal Lights.

 

 

Want to buy some of these boards?

 

All of these circuit boards are available for you to purchase and have been designed to display component locations and identification on each board. They are professionally fabricated, dual layer boards with solder mask on both sides and all holes are plated with lead-free solder.

 

We offer individual, bare circuit boards, DIY kits with the core components (board, resistors, diodes, transistors and berg pins) and fully assembled boards.

 

We don’t include items like Sensors or LEDs as everyone has personal preferences and might want different components.

 

We do offer a list of à la Carte components that are available on a piece-by-piece basis.

 

Multiple board / DIY kit discounts are available and members of recognized Model Railroad clubs and groups are also eligible for a discount.

 

We are currently working on a Grade Crossing, Flashing Light Controller that will work with our current Event Controller and sensor system.

 

We will also be offering a circuit board / DIY kit designed to control the lights in and around a piece of rolling stock (train car).

 

As there is often a need to use several different lights in the same car, this will be a multi-channel power source for multiple LEDs.

 

For additional information, please check out our website: http://www.modeltrainsignals.com/.

 

PayPal is accepted, as are Postal Money Orders and Cashier’s Checks.

 

Some Layout Pictures: