Traffic Lights Part II - Traffic Light Controller

By Ty’s Model Railroad - 8/08/2012 10:39:00 PM

A note about this post: I do not take any credit for the design of this circuit. I instead turn the spotlight on Rob Paisley and his website, who has without a doubt spent an unimaginable amount of time, effort, and talent developing dozens of model railroad circuits that ultimately make our layouts achieve things we never thought possible. The intent of this post is to only describe my experience of applying and constructing Rob’s designs on my own layout. Please visit Rob’s website, which has a wealth of wiring information and useful model railroad circuitry. 


20 output custom scratch built traffic light controller circuit on a blank PC board

If there is one thing I've learned from building my layout thus far, it’s that the most time consuming, frustrating, problematic, and complex projects usually are the ones that are taken for granted. Often, these fade into the background and go mostly unnoticed but play such a huge role in making your model something extra-special. My traffic lights were no exception to this. Now, I’m not exactly saying my traffic lights go unnoticed; they are definitely a nice feature. It's what controls those tiny little LED lights that I'm referring to. I myself took for granted the electrical complexity behind making all the lights synchronized, especially as I was building the entire system completely from scratch. I know commercial controllers are available from $50.00 to several hundred dollars, but doing it myself was a challenge I was up for.
Wiring diagram for a 20 output traffic light controller circuit

Last year while searching for methods to synchronize model traffic lights, I came across Rob Paisley’s 20 output sequencing circuit. Rob had designed a circuit that essentially created 20 separate outputs that progressed one at a time from 1-20 in a continuous loop. When combined with a novel lighting circuit, synchronization of an entire intersection in both directions is accomplished within the 20 output steps, then repeats. I am going to refrain myself from even trying to go into detail on the particulars of this circuit, as Rob Paisley has already explained it in depth on his website. Rob’s site includes detailed diagrams and parts lists, as well as in-depth explanations of how the circuits work. Rob also offers commercially built circuit boards and kits for this and other circuits he’s designed, many for model railroad use. Nevertheless, I was determined to build it myself.

Traffic light controller circuit

After placing my parts order with Mouser Electronics (using the parts list off of Rob’s website), I went to work putting together my own wiring schematic for this circuit. I made my own schematic for 3 reasons. First was because I wanted a plan of the entire circuit, from the controller right to the lights on the layout. The second reason was to plan the actual layout of the circuit board. I drew the schematic in a way that I could literally plan exactly where every connection and lead would go when building the actual circuit, ensuring that everything was spaced correctly so I didn’t run out of room on my PC board. The third and final reason was to better understand how the circuit itself worked. Even though Rob explains the function and how this circuit works in detail, if you do not have a good understanding of how it work, then it will be a lot more difficult to build and even harder to troubleshoot. I am no electronics expert myself, so I spent a lot of time researching each component and IC, finding and reading the datasheet for each one as Rob recommends on his website.

20 output custom scratch built traffic light controller circuit on a blank PC board

Once all of my components arrived from Mouser, I purchased a 2200-hole PC board from an electronics store and eagerly went to work putting it together. I used my schematic almost exclusively to construct the circuit on the PC board, marking off each completed component and section of the circuit with a highlighter to ensure I didn’t get lost or miss a connection, or worse, make a wrong connection. Double, triple, and quadruple checking my work against my schematic as well as Rob’s original wiring schematic almost guaranteed no major errors were made.

Soldered connections on the back of a 20 output custom scratch built traffic light controller circuit

I used my soldering iron to make all the connections on the back of the PC board, and used bare steel wire for the leads between connections. Special consideration needs to be made for spacing of connections that pass over each other, which requires the steel lead to pass up through the PC board, over the existing connections below, and then back down through the PC board to its intended location. Care must also be taken when soldering connections that are right beside each other, as it is easy to unintentionally solder two separate connections together. There were also several spots on the PC board where the soldered connections were shorting out on each other because they were so close. To resolve this, I carefully used a razor blade to chisel out a space in between each connection, ensuring the connections were no longer touching.

Soldered connections on the back of a 20 output custom scratch built traffic light controller circuit

You will probably notice when comparing my schematic to Rob’s schematics that the LED traffic light portion of my circuit utilizes PNP transistors to control the LED lights, whereas Rob’s examples show the LEDs connected directly to the 20 outputs. The reason for this is because the 20 outputs of the circuit are LOW (negative). To control the red, yellow, and green LEDs of the traffic lights, the LEDs need to be supplied with a common positive (+) current, connected to the anode of each LED. A separate negative (-) ‘controller’ lead needs to be connected to the cathode of each LED, and then to each output of the controller. However, I built my traffic lights the opposite way, with a common negative and separate positive controlling wires. Thus, my lights could only be controlled by applying separate POSITVE current connections to each LED.

Soldered connections on the back of a 20 output custom scratch built traffic light controller circuit

To get around this issue, I applied Rob’s wiring schematic where he explains how to use the 20 output circuit to control high-current bulbs by utilizing PNP transistors as switches. In other words, instead of directly controlling the LED lights by having them directly connected to the 20 outputs of the controller, I used the 20 outputs to control the PNP transistors. The transistors act like switches, either allowing or stopping the separate positive, high-current 12V flowing through to the bulbs. The transistor’s switching capability is controlled by its base terminal, which is connected to the 20 outputs of the controller. I then simply replaced the high current bulbs with resistors and my LED traffic lights.

20 output traffic light controller and 2 terminal strips awaiting installation on a hardboard base

Terminal strips and brass support bolts fastened to a hardboard base

Once the circuit was completed, I needed to make the connections from the controller to 2 – 10 position terminal strips, which I would then later connect each traffic light to. To do this, I first cut a piece of tempered hardboard, to which I attached the 2 terminal strips along the bottom edge. I used ¾” brass flat-head machine bots applied from the back of the hardboard to fasten the terminal strips, allowing the hardboard base to lay flat. I also installed an additional 4 bolts through the hardboard base to support the circuit board, allowing it to be secured without having its bottom circuitry come in contact with the hardboard base.

20 output scratch built traffic light controller connected to two terminal strips and mounted on a hardboard base

20 output scratch built traffic light controller connected to two terminal strips and mounted on a hardboard base

The final step was to connect each positive output on the circuit board to the screws on the terminal strips. I used high-quality phone cable to do this, which is convenient because most telephone cable contains a red, green, yellow, and black wire, making it easy to colour-code each terminal based on what colour LED will be connected to it. I have a total of 6 traffic lights on my layout, so a total of 18 separate LEDs, 6 of each colour, and one common negative, so a total of 19 connections. The first 9 connections will control 3 complete traffic lights in a north-south direction, and the following 9 will control the other 3 complete traffic lights in an east-west direction.

20 output scratch built traffic light controller mounded below model railroad benchwork

After I had attached everything to the hardboard base, I installed the entire module unit under my layout. The circuit requires a 12 volt DC power source, so I connected it to a 12 volt terminal on a previously installed power terminal strip. I had already installed my actual traffic lights on my layout’s intersections, running the wires to the underside of my layout through 1/8” pilot holes. I utilized telephone cord again here to connect the traffic lights to the terminals on the controller module. After connecting each traffic light to its corresponding power terminal on the control module, all that was left was to test it. And just like that, I now had working traffic lights on my layout!

Scratch built traffic lights being operated by a scratch built 20 output traffic light controller circuit

Well, it wasn’t really “just like that.” I spent days and countless hours pulling my hair out for over a week, testing and trying to locate small short circuits that arose over and over again on both the control module and in the traffic light wiring itself. With so many small connections so close to one another, it’s pretty much impossible to get it right 100% the first time. There were nights were I literally had to walk away from the entire project in frustration, but after sleeping it off, I always tackled it the next day with a fresh and positive attitude. In the end, the final result was a working, fully automated, synchronized traffic light system, which I will definitely never take for granted. And to be honest, if others don’t notice it, it’s only because it’s working how it should.

Scratch built traffic lights at an intersection being operated by a scratch built traffic light controller

Check out my YouTube video showing my hand-built traffic lights and controller based on Rob Paisley’s 20 Output Sequencing Circuit in action!

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  1. Nice job! This is an example of a really nice prototype build. Did you breadboard the circuit first before soldering it up or did you go "whole hog" and solder it in per the schematic?

    I'm an electrical engineer by trade and someday, when I have the room, I'd like to build up a layout. I was surfing about model trains and found your blog.

    Please forgive me if, deep down, you are an electronics expert, but have you heard about the Arduino? It's an easy to use microcontroller platform that would allow you to control a circuit such as this with a custom program downloaded into it. It runs about $30 for the board and it is very easy for newcomers to electronics/microcontrollers to program and use.

    However, your sequencer circuit is great. For as much as I like to see all of the newcomers who are jumping right into microcontrollers, it is heartening to see someone like yourself doing it "the old fashioned way" with discrete chips.

    1. Hey Bill, thanks for the comment. For the circuit, I built the entire thing "whole hog". I've explored options of pre-made and/or programmable devices, however built this circuit just to see if I could actually do it!

    2. I used an arduino to develop my traffic lights and it's pretty cheap, The computer board costs less than $10.00 and the program is easy to develop. The computer is totally underutilized but works very well.

      I also used an Arduino to run my turntable using a Walthers manual 90ft with a NEMA 17 stepper motor powering it. This way I can select each track position and angle with a simple step number (1 - 3500) and am not limited to the Atlas 11 degree increments.

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