Weekend Update 10/4/26

[Lemosteam]

Nothing pretty from me - I'm still working on getting all of the track on the club layout working.

To that end, I made a "dead spot detection car" that has only one axle pickup, hooked to an LED.




The design is simple.  Just a Kato caboose truck with one axel replaced with a plastic wheel set.  The "car" floor is cut away to give a direct view of what the pickup wheels are touching, and the body is transparent to avoid shadowing the view.

It works well at home - even found a dirty section.  We will see how it does at the club for individually pinpointing each of the myriad of track problems on the unused sidings.

I assume this is for a DC system only.  What would a DCC version look like?  Would only a bridge rectifier be required to supply voltage to the LED?

[Maletrain]

Lemosteam, This version of the "dead spot detection car" works fine on either DC or DCC.

The only things I needed to do to achieve that were to use a bi-directional LED (red + green in this case), and put the proper value of resistor into the LED circuit to avoid burning out the LED with overvoltage.

The bi-directional (2-wire) LED is really 2 LEDs in the same package, set up to light one or the other when the voltage across the 2 wires is in opposite polarities.  That way, one is always conducting, so there is no excessive backward voltage on the other.

I used a variable resistor to chose the value for a fixed resistor that gave me a good glow, but not maximum brightness, on my DCC system, which is set at 13.8 volts.  That will still give me a lit LED on a DC system with the track power set to a fairly high value.  It also gives me some margin for  other layouts that have DCC voltages up near 15 volts and open-circuit DC systems that somebody might turn up to as much as 20 volts.

The only issue I am expecting to find with using it on a DC system is that it might not light up at all if I try to push it with a locomotive at crawl speed.  But, I tend to use it with my 0-5-0 hand to check track spots.  I do think it will blink at a short dead spot when pulled around a DC layout at modest speed.  But, I will be checking that today at the club.  It does blink when I go over a dead spot on DCC track at modest speed, and then I can go back and use the 0-5-0 to slowly push the car back-and-forth over that spot.

Bidirectional LEDs will show one color or the other on DC track, depending on the polarity of the track.  On DCC track, both colors will be lit.  The combination of colors might produce something like orange, which I see in other (commercial) setups.  But, in this particular case, the DCC result just appears to be green.  Looking carefully at the lit LED, I can see a small red spot, too.  Apparently, the green element is a much more efficient light producer than the red element.  (Or maybe my old eyes are more efficient at seeing green than red, these days?)  Anyway, this LED does look red on DC track with enough voltage to light it and the polarity that activates the red element.

Probably this Friday, I'll post on what my experience was with DC at the club this evening.  (Thursday is medical  )

[Dave V]

@arbomambo , those modules are looking sick (the good kind of sick)! You've been a huge inspiration for me not only to try T-TRAK, but also to try to push the envelope with Unirack. Having been all over the Southwest, I definitely feel like your scenery is spot-on as well.

[Ed Kapuscinski]

Heading into the home stretch on the ‘Arbo Canyon’ redux.
All the juniper bushes have been glued, and just need to add field stone clutter/spread over the slopes…still left to do -
Cut down the skyboards
Add the telephone poles
Repaint the fascia and glue the plaque to the fascia
It’s amazing what you do with a module only 13” deep!

So absolutely true!

Have you thought about painting the rails?

[arbomambo]

The rails and ties ARE painted… you’re seeing the shiny top of the rail right after cleaning with a bright boy to remove any errant scenic glue.

[superchief]

love the module Bruce, you always capture the southwest beautifully. Gordon

[KSTXFLID]

Had a very stacked weekend at MARPM.

https://conrail1285.com/marpm-2026/

Those modules are stunning. Amazed that it's T Track.
How were the backdrops done? Especially Scott's (Ed's too), the low relief and seamless transition is amazing. Especially hard in flatland.

[Ed Kapuscinski]

Those modules are stunning. Amazed that it's T Track.
How were the backdrops done? Especially Scott's (Ed's too), the low relief and seamless transition is amazing. Especially hard in flatland.

Honestly, I think he just bought commercial sky backdrops and lights them well.

And that was our point: T-TRAK isn't just SpongeBob and "my first module". It's a format that is awfully convenient in which to do some really good modeling too.

[arbomambo]

Finishing the Kato foam module redux (added strung power poles) while keeping a close eye on the latest hurricane track (we live on the Alabama Gulf Coast)

[chuck geiger]

Finishing the Kato foam module redux (added strung power poles) while keeping a close eye on the latest hurricane track (we live on the Alabama Gulf Coast)

Bruce the rockwork is the biz on shiz. How please?

[arbomambo]

Bruce the rockwork is the biz on shiz. How please?

These are hydrocal castings I made with some Woodlands Scenics rubber rock molds.
Hydrocaled them into the plaster covered hill, dabbed them with some various WS tjnts, used a series of water-based washes to blend, drybrushed, then added Arizona Rock & Mineral products to blend everything together…
See the build album on Facebook - CATT-Coastal Alabama T-TRAK group under “albums”

[Maletrain]

After testing this behind an analog loco at the club, yesterday, I can say that it is hard to get my version to shine the LED brightly when the loco is moving slowly.  Even using a Atlas Shay, the LED is not bright at crawl speed, although it can be seen to be lit.

My original intent for my version is for DCC, where the LED stays bright no matter what the loco speed is.  And, frankly, I really expect to be pushing it by hand to really pin-down exactly where the conducting wheels are sitting when the LED goes out.

Of course, if you push it by hand on a DC layout, all you need to do is set the throttle at a voltage that gives the desired level of brightness.

But, if I were designing for a DC system, I would just use a lower ohm resistor.  The one I am using is 51Kohms.  That is intended to give me some margin on somebody putting it on a DC track with the throttle wide open.  That might have a track voltage as high as 20 volts.

I had also thought about putting a variable resistor in place of the fixed value resistor, so that the same car could be tuned to the track voltage conditions.  If you keep the track voltage below about 3 volts, you would not need a resistor at all.

But, having something adjustable like that in my club environment doesn't seem to have a very long survival time.  For example, all of the Atlas and Tomix vacuum cars exposed to that environment are dead, already. 

[peteski]

But, if I were designing for a DC system, I would just use a lower ohm resistor.  The one I am using is 51Kohms.  That is intended to give me some margin on somebody putting it on a DC track with the throttle wide open.  That might have a track voltage as high as 20 volts.

That LEDs looks like it will work comfortably at 20mA. After all is is not some tiny 0402 SMD LED rated at 5mA.  The Vf of both standard red/grn LEDs will be around 2V.  So, if you somehow got 20VDC onto the track then for 20mA you will need (20-2)/0.020=900 ohm.  Yes, a 900 ohm resistor would send only 20mA through those LEDs.  If you are super paranoid, use 5100 ohm resistor. I won't show the equation again, but with 5k resistor, at 20VDC the LEDs will receive 3.6mA and even at 50VDC, the LED will only see 9.6mA. As you see the calculation for the LED current is super easy. Not sure where you got 51,000 ohms.    No wonder you can barely see the LED with low DC voltage at the track.  At 3VDC, with 51k resistor the LED sees 0.00002A or 0.02mA. Even at 12V the LED will see 0.2mA. Those are extremely low currents. Are you sure the resistor is 51k?

[Maletrain]

Peteski, regarding the resistor, there is a picture of it in my post from which you should be able to verify its ohm value.  Its package says "51K" and I selected the value by varying a 1-to-100 K resistor and watching the LED intensity (with the LED + resistor connected to my NCE Power Cab output).  When I got the intensity I wanted, I then used a multimeter to see what ohm setting I had ended up with on the variable resistor.  I then took a close enough value fixed resistor and soldered it to one LED lead.

So, I am pretty sure that the resistor on my car is 51K ohms.  And it works great for my particular situation.

As I posted, all I was trying to do was get a nicely lit LED for my DCC track, not maximum brightness.  And I was not thinking at all about brightness on a DC track with variable voltage.  I was just grabbing stuff from my "stash" to make a car that would work for me in my particular situation.

There very well may be a more optimal resistor value that would both protect the LED on a wide-open DC throttle voltage and also provide a nicely lit LED at a DC voltage that makes a DC locomotive crawl slowly.  I just did not try to figure out what that would be for this particular LED.

The optimum value could probably be calculated if we have the specs for voltage and current for the LED (in both directions, considering that it is really 2 LEDs conducting in opposite directions), and we know the wide-open throttle voltage of our DC power pack along with the voltage that will make our intended loco creep at the desired speed.

But, that is probably going to vary significantly, based on what DC power pack and what locomotive we are optimizing for.  For example, my MRC Tech 7 at home puts out a max voltage of only 15.36 VDC, but some of the Tech 2 versions at the club are putting out 19-to-20 VDC, and I have read of 22 and even 25 VDC.  And, of course, there are the Atlas low speed locos as well as the typical 250 smph locos in N scale.

If I were trying to make a DC version, I would probably just calculate a resistor value that would give me a voltage drop at the LED's max current rating that got the voltage that the LED would see down to maybe 90% of the LED's max forward voltage rating, and that would result is the brightest low speed LED output that I would feel safe with ON MY PARTICULAR DC SYSTEM.  Whatever the LED intensity turned out to be at my "best crawling loco" crawl voltage would have to do.

A similar approach could be made using some max DC voltage that is set to the highest anybody has found when surveying DC power packs.  That would just make the crawl voltage somewhat dimmer.  MAYBE that LED brightness would still be satisfactory for  EVERYBODY'S favorite loco.  I just don't know because I have not tried to do that.

I really have 2 modes for using this car:

1.  The main one is to slowly push the car back and forth over an area with a "bad spot" to see EXACTLY where that spot is so  that I can figure out WHY it is bad.  I typically do that by hand, rather than with a loco.  So, the issue of loco speed vs LED brightness for DC power would not come into play, because I would not have a loco on the track and could turn a DC throttle up to whatever makes the LED as bright as I want it to be.

2.  I also do drag the car around behind a loco at moderate to low speed while looking for the LED to blink off.  For DCC, the LED brightness is independent of the loco speed, but for DC, those 2 factors are interconnected. 

A blink is noticeable over a large span of LED brightness, so I would expect that a somewhat lower resistor than my 51K ohm one would provide a car that is useable in DC mode for my second  use mode.

Maybe you could use your knowledge of LED specs and DC power supplies to suggest a one-size-suites-all resistor value that could be used universally for DC and DCC?

[peteski]

Steve, read my post again.  I suggested 5.1K for even 50V DC/DCC, and estimated the current passing thought the LEDs to be in extremely safe range.  I assume since those LEDs are indicators, you do want them readily visible.  Calculating the LED current dropping resistor is not about the voltage but the current. I just don't  understand why you are worried this much. 

Do you have actual specs for that bi-color LED?  what is the typical operating If (forward current) and Vf (forward voltage) for each LED?