Update

Happy Independence Day weekend to my US readers. Well its been several months where time for railroading has been limited. Top of the list, our nine-year-old beagle developed serious back/neck problems a few months ago that ultimately required surgery.

Lewis the beagle; 2 weeks after back surgery.

Lewis the beagle; 2 weeks after back surgery.

Little fellow is recovering and doing well with his rehabilitation program–exercises given to us by the specialty vet. Things like stairs are going to be a problem for him for a while, so he won’t be joining me in the layout room, as he usually does, in the near future.

Even so, I have some L&NC construction news along with updates on on-going projects.

 

ACS712 – How Low is Low Current

ACS712 Board

ACS712 Board

I’ve been exploring the limits of low current sensing with ACS712 sensors. Sparkfun markets a sensor board combining an ACS712 with an OP Amp that they characterize as a low current sensor. I had no luck calibrating and using that sensor; if someone reading this has used it successfully, please leave a comment explaining how you calibrate and use it.  Additional experiments with Op Amps, including creating a fairly stable instrumentation amplifier, have not been encouraging though I haven’t entirely given up and have a few possible circuit designs yet to try.

The fundamental constraint with ACS712 sensors is that their output noise inhibits current sensing below 20 mA (officially, Allegro says that you can’t detect below 20 mA without a specially designed version of the chip; which they will happily work on for a paying customer). At higher currents, the sensor works pretty much as advertised and can be used with or without filtering. At and below 20 mA it is difficult to tell the signal from the noise without some sort of mathematical filter; the RMS method I use can detect down to around 15 mA with an UNO’s 10 bit ADC.  With a Mayhew Labs 14 bit ADC, I seem to get a little more sensitivity, around 12 mA, using the RMS algorithm.

Below 12 mA noise takes over, at least so far as short term measurements go. So I’ve been looking at data collected over longer time periods and have a preliminary method that can discern a low current signal down to around 8 mA.  In essence, a low current change causes the long term output noise range to shift up or down depending on whether the current is increasing or decreasing. Capturing this range shift should improve practical detection sensitivity considerably. Its promising, but I still have kinks to work out of the system.

Is 1 mA sensitivity—the holy grail for detecting a single 10k resistor wheelset—possible with an off-the-shelf ACS712 sensor?  Probably not, and that may matter to people who want to use off-the-shelf 10k resistor wheelsets.

That does not make these inexpensive sensors useless. If you don’t mind making your own resistor wheelsets, then a single 1.5k resistor (assuming one resistor wheelset per car; two wheelsets at 3k—in parallel producing a net 1.5k resistance—one at each end of the car, would be a better idea for a bunch of reasons) will be detectable while still drawing under 1/10 watt (at 11.5 volts DCC n-scale standard; higher voltages allow for more resistance). 100 wheelsets would draw less than one Amp total current. More on that when I get the system operational.

Nevertheless, I’m curious whether a 16 bit ADC will help further resolve the low current signal. I’ll be giving that a try this summer so we will see.

Construction Progress

Laying Track-Bed at the Red Bluff Yard.

Laying Track-Bed at the Red Bluff Yard.

I finished drawing in the track plan, including some additional yard space, on the lower level modules and started in on laying roadbed and track.

Roughed In Turntable

Roughed In Turntable

Roughing in the turntable was fairly straight forward. After preliminary rough siting, I installed the roundhouse base and drew in the track center-lines. That located the turntable center (the intersection of the track center lines), enabling me to cut out an appropriately sized circle to accommodate a pit I’ve already created using a plywood base and styrene strips.

Building the Turntable Pit with Styrene Strips glued to a plywood base.

Building the Turntable Pit with Styrene Strips glued to a plywood base.

The turntable project is a major layout item in its own right and will be the subject of multiple future posts.

Laying Track for the Yard

Laying Track for the Yard

It took a few sessions, but I laid all the lower level track except for the turntable/roundhouse area (which I will do when I install the turntable) and joined it with the helix.

Tracklaying on Lower Level Done.

Track-laying on Lower Level Done.

Track Laid and Ready for Painting and Ballast.

Track Laid and Ready for Painting and Ballast.

For those interested in the nitty-gritty of track laying techniques, I used adhesives rather than mechanical fasteners for all roadbed and track. I use Liquid Nails for projects (water based) to adhere foam to wood, and anything else to foam. For the track I used DAP ALEX clear latex silicone caulk, deploying T-pins to hold the track in position while the caulk cures (just a few hours). What I like most about this method is that once cured the adhesive is completely invisible creating a nice base upon which to detail the track. Now that I’ve used clear caulk for track laying, I can’t imagine using a grey (and definitely not white) product unless the color closely matches the intended ballast color.

I don’t know if you can tell from the photograph of the whole lower level above, but the lighting system using Addressable LEDS that I’ve devised is working rather well (granted the wiring needs  tidying up; I’ll get to that eventually).


 

Look Mom, No Nails!

I decided to try a track laying method that has been shown a few times in Model Railroader Magazine and elsewhere: adhering track to the roadbed with caulk. I hate nailing track. At HO its doable, but at N I find it very difficult. So I really wanted the caulk to work.

ALEXI evaluated several products at the store before settling on DAP ALEX Painters Acrylic Latex Caulk. ALEX fills the bill because it is designed to adhere to a range of building materials, including impermeable plastics and vinyl. Easy cleanup is a nice bonus.

To my delight it works well as a track adhesive. Initial ‘grab’ is great, but you can move things around a bit while it is fresh; using thin layers, all that is necessary to solidly adhere track to roadbed, you get about 5 minutes working time.  I used about 20% of a tube for all the track on the test loop.

I’m finding quilter’s T-Pins incredibly useful for laying both cork roadbed and track. I bought two sizes, 1 1/4″ and 1 3/4″, at Michael’s. (Woodland Scenics markets a version of these as 2″ Foam Nails). The 1 1/4’s are perfect for holding N scale track down which the caulk is setting–they fit through the nail holes in the Atlas N sectional track I’m using for the test loop. Push them down until the head rests on the tie and the track will be secure. Remove them after the caulk sets. The bigger ones worked by inserting them between ties and using the head to hold the track down. Either way, T-Pins are a great help; the smaller ones at Michael’s seem just the right size for working in N scale.

While the test loop is mostly old Atlas sectional track, I did use a piece of Peco code 80 flextrack from my ample supply to do the spur. This turned out to be a little more challenging because the Peco flextrack I have (bought quite a while ago) does not have nail holes in the ties. The key to putting the 90 deg, 9 3/4″ radius bend into the track was in the deployment of pins: mostly along the outside rail to hold the bend, with a few pins between ties along the inside rail to hold the track down, while the caulk set.

In my arid climate the caulk sets sufficiently within 1 hour to allow removal of the pins.

While you want to keep the caulk layer thin to avoid goo getting above the ties, there is such a thing as too thin–adhesion will fail as it dries if there was not enough caulk. If you can see through the caulk layer, it’s too thin.

This caulk layer is too thin.

This caulk layer is too thin.

I had to go back and correct a couple of spots where there was insufficient adhesive. When you get the amount right it works perfectly.

Just right.

Just right.

Without really thinking about it I selected white caulk at the store. There is some possibility white caulk would show through ballast; another color might be a better idea. The product is paintable, so a thin wash could be used to disguise the caulk after it has dried.

I kept the caulk well away from the moving parts of the turnout — an area it might show through anyway if ballast coverage is thin for operational reasons. As to the test loop, I’m not going to ballast it for the time being.

Test Loop ready to go for DC operation.

Test Loop ready to go for DC operation.

I created 4 electrical blocks, using some ugly orange Atlas insulating joiners where needed. Fine for the test loop, but not the way I’ll do it on the layout. I soldered all the track at the metal joiners along with one feeder set for each block. One wouldn’t normally do multiple blocks on something like this, but it’s a test bed, right?

Mikado first run on the test loop.

Union Pacific USRA Heavy Mikado

First test with a real loco was a success with no serious problem spots!

I have to say the test loop is some of the smoothest track I’ve ever laid. I won’t call it perfect, but the improvement from prior efforts is quite apparent. I  have to give credit to the materials, and particularly to the caulk adhesive track laying method. I’m pretty sure now that reliance on nails to hold track down in the past was the root of a lot of track problems.  With the caulk method it is easy to avoid the subtle distortions of track from the laying process that are hard to find and fix.

test loop servo 1 in place

Tower Pro Micro Servo 9g, with a brass rod attached to the turnout.

An off-the-shelf micro servo (about $5) controls the Peco short turnout.  How does that work?  That is the subject for another post.