Traffic Lights Part II

A note about this post: I do not take any credit for the design of this circuit, not even for my own rendering and modifications of the circuit schematic. 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 (HERE) , which has a wealth of wiring information and useful model railroad circuitry.

-Tyler

If there is one thing that I 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 fading into the background or going unnoticed when combined with all other aspects of the model. And my traffic lights were definitely no exception to this. Now, I’m not exactly saying my traffic lights go unnoticed, (they are definitely a nice feature) but I myself somewhat took for granted the electrical complexity behind making all the lights synchronized, especially because I was building the entire system, from lights to controller, completely by hand. I know commercial controllers are available from $40.00 to $100.00 (or higher), but doing it myself just seemed more appealing.

Last year while searching for methods to synchronize model traffic lights, I came across Rob Paisley’s 20 output sequencing circuit (HERE). 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 depthon 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 on his website, but nonetheless, I was determined to build it myself.

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, as shown above. 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 on his website, if you do not have a good understanding of it yourself, then it will be a lot more difficult to construct 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.

Once all of my components arrived from Mouser, I purchased a 2200-hole PC board from The Source, 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.

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 each other, so I carefully used a razor blade to “scrape” the space in between each connection, ensuring the connections were no longer touching.

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, and a separate negative (-) ‘controller’ lead 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, where the 20 output circuit controls the LEDs by separate NEGATIVE current connections to each LED.

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, which 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.

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.

 

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.

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!

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.

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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Traffic Lights Part I

The same day I was putting the final touches on my Northern Light & Power kit, my little box full of green, yellow, and red 2mm LEDs arrived in my mailbox from eBay, thus kicking-off the start of my next project: traffic lights! I have two ‘T’ intersections on my layout’s road system, both of which I wanted to be controlled with traffic lights. I wasn’t satisfied with many of the commercially-available light systems that are available in my area, as most were un-realistic and far out of scale, not to mention expensive. The ones I did like however, wouldn’t fit correctly on my roads as I had built them slightly too narrow.

Faced with the issue of cost and adaptability, I decided to attempt my hand at building my own traffic lights. I had previously built my own trackside signals (see here), so I figured traffic lights would be quite similar, even though I wanted to build these a bit more to a prototypical scale size then the trackside signals. My model street lights are based on a common North American style with a curved light standard which holds the traffic lights horizontally over each controlled lane of traffic.

I started by cutting the face plate for each traffic light from thin styrene sheet, then carefully drilled 3 – 2mm holes in each face plate, spacing each hole 1/8” apart (center to center). For the lens shades, I used a single hole punch to punch out round pieces from very thin styrene. I cut the styrene disk back 2mm from its edge, trimmed each pointy edge, and shaped each piece by gently rolling it between my fingers until it had the correct curve to fit over each LED. I was able to make 2 shades from each styrene disk. I then glued 3 of these directly above each hole on the face plate.

I masked the backside of each face plate, and sprayed the front flat back, figuring that it would probably be easier to paint these prior to installing the lights. I masked the back side to prevent paint from getting on this area, which would need to be removed anyways when it came to gluing the LEDs to the face plate. I then counted out 6 LEDs in each colour (red, yellow, green) and tested each one with a resistor and power supply to ensure each one worked.

For the light standards, I used 3/32” diameter copper tubing, which I cut into 4” sections. I cut the sections at 4” so I had extra length to insert into the pilot holes in my layout when it comes to installing these. I needed to thread 4 wire leads through each, so the wires needed to be quite thin. I found an old computer hard drive ATA cable, which is comprised of several dozen very thin insulated wires, so I cut this up and saved the wire leads. I then threaded 4 wires through each copper tube, one for the green, yellow, red LEDs, and common ground. There were only 3 colours of wire however (red, blue, and white), so I just used a second white, marking it with a black sharpie for use as the common connection.

Once the wire leads were threaded through each copper tube, I bent the top of each tube to an almost 90 degree angle about 1” from the one end. I bent the curve over a thick marker container to keep the curve uniform and round. It’s also very important to make sure that the wire leads are pre-installed in the copper tubes, as the tube needs the internal support when bending. Without the wire leads inside, the copper tube would simply collapse and kink.

By this time, the paint on the front of the face plates had dried, and it was time to install the LEDs. I first cut off the long connectors to each LED with flush cutters, leaving only a small portion protruding from the back of each LED. I then glued each LED with CA to the back of the face plates, making sure the anode (+) connection on each LED was on the top position. You can distinguish what side is the anode connection on most LEDs from the curved profile on the base of the LED itself. The cathode (-) side is flat and squared off, and the actual connection lead itself is usually shorter than the anode.

Now for the fun part; soldering the connections to the LEDs. I used a small cardboard box which I cut a notch into for the front of the faceplate to fit into for support. The common (-) connection was the first one that I made. For this, I used one of the metal leads that I had originally cut off of the LEDs, spanning it across and soldering it to all three cathode connections on the LEDs.

For the controlled (+) connections, I carefully soldered each wire lead from the copper tube supports to the anode on each LED. I soldered the red wire to the red LED, the blank white wire to the yellow, and the blue one to the greed LED for each traffic light. The other white wire, which I previously marked with a black sharpie, was soldered to one end of the common connection. When soldering the wires, I made a point of positioning each wire so it would easily exit off to one side of the traffic light (the side facing the support column). This made the back of each street light look a lot less cluttered and easy to work with.

Once all connections were made, I gently and carefully (and I stress the gentle part), pulled the wire leads at the base of the copper tube supports, slowly bringing each traffic light closer to the support column. After I had positioned each traffic light right up against the copper tube supports, I adjusted each light so it was positioned level to the ground when positioned upright. The wires provided enough rigidity that no glue was required to fasten the lights to the support column. I then painted the back side of each light, including the connections and face plate, with 3-5 coats of black enamel paint, making sure I put on enough coats that no light was visible from the back of the traffic lights when lit.

The final step was to paint the copper support columns an aluminum colour, for which I used Humbrol metallic aluminum enamel. With the traffic lights now complete, they only now need to be installed and connected to a traffic light controller on my layout. I am currently exploring a couple of options for controllers, including building my own. That will all however be in my Traffic Lights – Part II post, which will hopefully be up sometime this summer. For now, I will return back to my current task of building more trees.

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