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The small 2mm marker led boards posted earlier in this category can also be easily used for red-green directional leds on any can motor equipped engine; that change color with engine direction change. All you need besides the boards is a stash of 2 or 3 mm bi-color 2 pin diodes.

The picture and video shown below are using 5mm leds, which is all I have on hand at the moment, but 3mm bi-color are easy to find and more realistically sized. The direction change is done by attaching the two wires to the power connections of the can motors in each end of the engine, one alternately from the other. The idea of course is to get each end leds green when moving in that direction, and red when moving the other direction. You could also set them all up to go green when moving forward and red in reverse.

The original 2mm Marker led topic is;

A Handy Little PCB for Adding Marker LEDs to a Hood Diesel

It has all the info and gerber files for ordering some boards. Note that its the first thru-hole DC R1.0 gerbers you want; not the later smd AC version. I tested these using 12vdc and 470R load resistors in the boards as you see. They work great.


2mm bi-color 2 pin leds are not very commonly available it seems. I found one German source who wanted 9 Euros for 10 leds, and 30 Euros to ship them! I gracefully declined. Couldn't find any on ebay or AliExpress, but they may be out there somewhere. We can only hope that they become more readily available.



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Videos (1)
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Yes Steve, they work quite well. And the green tends to be a much richer shade of green than many of the wimpy green leds available out there. I really like them. The video clip does not do them justice.

For the testing what I used is a small DPDT toggle switch to alternate the polarity quickly to both sets of leds. Installed in an engine, when in neutral neither set of leds would be lit at all if they are powered by the +/- supplied to the motor of course. Using 470R load resistors I found the leds are weakly lit at 5.5 volts, and nice and bright by about 7 volts. There is not much noticeable change in intensity from about 8 volts up to 12 or so.

A better way to smooth out the variable intensity would maybe be to replace the load resistor with a CL2N3 controller soldered into the same pads. Theoretically then the leds would be at 20ma current as soon as the voltage exceeds the total Vf. May have to give that a try and see what happens. Clearly the CL2N3 would be the most expensive component on the board!

@Magicland posted:

Sounds like a fun project, but why would you want green lights on the front of an engine moving forward? I understand red marker lights if the engine is on the rear of a train.

Colored lights did have a purpose...

White. Indicated an “extra” train not shown in the timetable. For much of railroad history, train-movement authority was granted by timetables. If a train was listed in the timetable, it had the authority to operate according to its printed schedule. Deviations from the timetable, such as a train running late, were handled with train orders from the dispatcher. Under this “timetable-and-train-order” system, it was important that trains kept as close to schedule as possible, and that any special trains not shown in the timetable be clearly identified as such with a white light. Many freight trains operated as extras, and thus carried a white classification signal.

Green. Indicated that, while the train displaying the lights was a regularly scheduled one, a second section was following behind it. This was done, for example, when ridership demand exceeded the capacity of a single passenger train. If there were too many passengers for a single section of, say, New York Central’s 20th Century Limited, a second section was operated, and, if needed, a third, fourth, fifth, and even sixth. The engine of each section except the last would display green lights. While each section was a separate entity, the timetable’s “train 25” would not be considered to have passed a given point until the last section of the train had gone by. For operational convenience, special trains that otherwise might have carried white “extra” signals were sometimes operated as advance or second sections of regular, but unrelated, trains.

Red. Indicated the end of a train. A train, be it a single engine, a group of engines, or an engine(s) with cars, must have a marker on the rear end. In the (relatively rare) situations when the last element in a train would be a locomotive, the red lights would be lit.

Classification lights phased out

The timetable-and-train-order system has been replaced by other forms of movement authority, and classification lights are no longer used, although older locomotives still have them.

I decided to play with the CL2N3 drivers to see if there is any advantage to using them. Here's the circuit:

Leds with CL2N3 Drivers

The 1N4007 diodes are added to block the reverse polarity. So one set of CL2N3 plus 1N4007 drives the red, and the other side drives the green. I initially had the leds in series but the required voltage to make them bright was too high, so parallel leds got the nod. The minimum required voltages to reach steady intensity are 6.5 and 7.5 for red/green respectively using this setup. Higher voltages right up to 24vdc don't affect brightness. I experimented removing one 1N4007 diode, and poof! Splash two leds, which I kind of expected. The CL2N3's both survived though.

The real question is; for a typical two can motor diesel, what is the needed voltage for them to start moving?? Anyone know? If its much less than the voltages above, the leds are still going to vary in brightness, which might not be good.



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  • Leds with CL2N3 Drivers
@BOB WALKER posted:

When I check out an engine before installing a control system, I test the can motors to determine start voltage and direction polarity. In most cases a DC voltage of 3 to 5 volts will start the motors rotating.

Great thanks Bob, good to know. So I am thinking that with engine on track and a bit of a load, starting voltages should be 5V+. So the led starting voltages with the CL2N3 might not look too bad. A bit of experimenting once we get home should tell the tale.

Here's a simple test with a bi-color LED and a 470K resistor.  I cracked the throttle until the motor just started moving at first, note this is a bare motor with nothing holding it back.  Then I cranked it up a bit and finally reversed it and repeated the steps.  Yes, it gets brighter with speed, but not as much as you might think.


Videos (1)
Bi-Color Marker Test
Last edited by gunrunnerjohn

First off, the CL2 want's filtered DC, so that will be one problem with the hash that comes from the motor.

Have you tried the bi-color 2-pin LED directly from the motor with a 470 ohm resistor?  You might be surprised at how well that works.

I have not tried that yet, looks like it may work just fine from the video clip in your later post. Thanks!

One benefit of not having any other devices in series with the LED is you get a lower voltage startup.  The nice thing about the two-pin bi-color LED's is the have internal reverse polarity protection based on the two LED chips.  Sometimes the K.I.S.S principle works out best.

Yes we are on the same page John. The idea of trying CL2N3's was to see if they offer any great advantage over the KISS approach. The advantage is that from about 7.5 volts up there is no change in LED intensity because the drivers are controlling the current. The downside is the two blocker diodes add to the required start up voltage making it around 6.5 volts. Plus more complexity and cost. Not good. And I proved the blocker diodes are essential.

In the end I will likely go with a resistor and two LEDs in parallel. I modded one of my 2mm class marker led boards to put the leds in parallel, just for testing. Playing with that a bit I found a startup voltage of about 5.4 gives pretty good intensity. So now I want to test that with a can motor and see if we are in the ballpark.  I figure the typical engine can motor voltage range would likely be from about 5vdc (as Bob said) up to about 10vdc. So that's the sweet spot. Research continues.

So after some testing with 2 parallel bi-color leds with a 470R resister and an unloaded 12vdc can motor, here are some results. (The test motor is about 1-3/8" diam x 2" long and quite heavy. Its resistance is about 5.2 ohms. It's manufacturer is not known but it's not a Mabuchi or any other common train motor.):

2.2vdc: Red leds steady but dim; motor just turning over. Green leds behaved same but needed 2.8vdc.

4.0vdc: Both colors fairly bright, motor running faster.

6.0vdc: Both colors comfortably nice and bright.

8.0 and 10.0vdc: No change in brightness to speak of.

When the voltage is reduced both colors dim noticeably below 5vdc.

This is all highly subjective of course, but in conclusion it looks like voltage levels above 6 or 7vdc will produce acceptable brightness, which agrees pretty well with grj's assessment above. Next step will be some actual engine testing. Since this board is exactly the same in size etc as the class led board put in my Soo Line MP15 switcher a year or so back, that will be the easy to mod test bed! More to follow.......

So I received the R1.21 directional led boards, and they work just fine, Here is the board layout and a test picture:

R1.21 Board snipR1.21 Board

Note this pcb has provision to use either a fixed 1/4W resistor, or a 3362P style trim pot. With the pot you can dial in the led intensity that you like best. A 500R or 1K pot would be good choices. As with the earlier testing above, reversing the polarity to the board switches the red leds to green. It's important to note that you must be cautious turning up the juice to the leds otherwise they will go poof if you get too aggressive. An ammeter inline as you make adjustments would be a good idea. The gerbers are attached below for any who wish to order their own. I also have a few spare that I can sell; shoot me an email if interested.



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