Large Scale Central

LED help


I kinda sorta understand LEDs I think. I have played with them a little but mostly single LED’s with pre-wired resistors. I did run a small string of them in series with a single resisto but don’t remember enough about what I did to make it work.

So here is what I want to do. I have discovered “pico” LEDs Now for modeling these little suckers are the cat’s meow. At 1mm they are tiny and the original place I bought them from had them pre-wired with a resistor and they work over a nice range of voltages. But I want to run a series of them. I won’t bore you with the details of what I am building except to say I want them in series and want to use only a single resistor. Think Christmas lights. I know that when playing with LEDS voltage is a concern but more so is miliamps. As long as the voltage is within the usable range of the LED, say 12v, then I have to know the mA output of my power supply and add up the number of LEDs and their mA rating and then provide a resistor to protect from the extra amperage, right?

If the LED runs on 12v and is rated at 20 mA and I have a 12v supply that produces 100mA I can run 5 LEDs in series with no resistor. Any more and I run the risk of not powering them up and any less and I run the risk of burning them up without the use of a resistor. Resistor sizing I know is based on the total mA supplied and the total mA draw with all LEDs combined, right? So if I have this correct, then I need to know how many LEDs I want to run, how many mA they draw and either provide enough power to run them or add a resistor to protect them, correct?

And to further complicate things, I want to run a single source, likely 24v, on a single set of wires around my entire indoor layout with feeders to each of the locations. I would want this sized so that it will run a whole host of buildings, cars, and other details. So can I knock down each feeder to the correct voltage and then add a resistor to give me the needed V/mA at that particular location? Greg and I talked about doing this when I was doing 1:24 and running the wire on poles and having “transformers” at each location. This is out now that I have gone to On30, the wires would be way to big to be anywhere near adequate to work reasonably. So all of this will be under the bench work or in the building itself. So is it best to run a “big” power supply and knock down each feeder and make each feeder individually designed to provide the output I need for that particular area then running all the LEDs in series?


You’re on the right track (yup pun intended), but what is the forward voltage of each LED? If they’re 2.4 volts then yes what you say works. It would still be good practice to have a resistor included, even if a 1 ohm 1/4 watt resistor.

If the LED forward voltage is less than 2.4 volts than a higher rated ohm resistor is needed.

If the LED forward voltage is greater than 2.4 volts than all 5 won’t be able to be in series. You could still try it with 5, they may not be as bright.

LEDs are fun, all I use now for in engines, rolling stock, buildings, etc.

Devon, I find that LEDs tend to be on or off, and as long as you give them some power through a resistor they light up. In other words, I can’t tell when an LED is dim or bright - they always look bright to me.

My last attempt was the headlights on the Delton/Hartland woody, and I wired them with a resistor each. Then I connected them in series as that simplified the wiring - and they still look bright even though I have 2 resistors.

If you LEDs have a resistor in the package, why not wire them in parallel? That way all you care about is total number of LEDs and whether the power suppy has enough oomph to drive them all.

Forgot to add this website.

I trust my math but I still run this too, especially when I’ve wired those little 0402 LEDs that are time consuming.

Eric Warhol said:


You’re on the right track (yup pun intended), but what is the forward voltage of each LED? If they’re 2.4 volts then yes what you say works. It would still be good practice to have a resistor included, even if a 1 ohm 1/4 watt resistor.

If the LED forward voltage is less than 2.4 volts than a higher rated ohm resistor is needed.

If the LED forward voltage is greater than 2.4 volts than all 5 won’t be able to be in series. You could still try it with 5, they may not be as bright.

LEDs are fun, all I use now for in engines, rolling stock, buildings, etc.

Okay I think I understand where you are going with this. I have to add up forward voltage and make sure I am providing enough voltage. So as we are discussing 5 LEDs at 2.4 volt would be added up to 12v. 6 would require 14.4v and I wouldn’t have enough power at 12v; conversely 4 would only require 9.6v and therefore would need a resistor to deal with the extra voltage? If I am understanding that correctly then it still makes sense to me to run a 24v supply and then figure out my demand at each feeder and knock it down at each location with a resistor. By demand I mean the number of LEDs in series. Yes?

Pete Thornton said:

If you LEDs have a resistor in the package, why not wire them in parallel? That way all you care about is total number of LEDs and whether the power suppy has enough oomph to drive them all.

The only reason I don’t want to do them in parallel is for a couple ideas where I want to use a string of LEDS without a bunch of wire. One idea I have it a string of warehouse lights where the wiring would be seen so I was ging touse the wire as “conduit” and just string one to another like a real world application. This idea is the least of my concerns as I a know most all of it (especially in O) will be hidden and I could run individual LEDS with their own resistor.

The other more radical idea is a string of Christmas lights going around a tree out in the yard. A bunch of wire would be seen and having them run in a series would even be prototypical and have a single wire run around the tree.

I wasn’t going to go into to much detail but why not. I have been collecting train related Christmas decorations and each Christmas will decorate my hobby room. Christmas is a big deal at our house and my wife decorates every room in the house with a different theme. I want to “help” by decorating my room in a train theme. On the layout I think it would be fun to have the houses and some trees decorated for the holiday. And strings of Christmas lights would be fun. And in true redneck fashion my citizens will leave their Christmas lights up all year. (

The site I am looking at isn’t terribly forth coming on what their voltage requirements are. They set them up in 3 varieties that cover 3v, 5-12v, and 7-19v. The 5-12 and 7-19 show pictures with resistors. The 3v they show no resistor and say that you just wire them up. So I am assuming that I can safely say they are a 3v LED.

Running them parallel with a common ground would be my suggestion.


Yup, you’re getting it. You comfortable calculating OHM’s Law by hand? It’s pretty easy, fun to know the science behind it. Otherwise that LED calculator website makes it easy as well.

I noticed that too on the website about the LEDs at 3V not having a resistor. Some of my LEDs are rated 2.2-2.4 volt, some at 3.0-3.2v. 3v is a “general” or “ball park” number for LEDs. 20mA is also a “general” for current. I have my LEDS seperated by size and color into small packages. I label the packages with the LED size, color, forward voltage, and mA.

Here’s a link to a What’s Neat video from Model Railroad Hobbyist. I have the video link set to start at the 17:15 mark. He installed Christmas Lights into an HO train just like you’re kind of talking about.

I do agree with Craig that parallel would be best. With series I believe if one LED burns out they all will stop working, like Christmas lights that don’t have the Stay Lite feature. I also think whether in series or parallel you’ll have lots of wires. Series you’ll end up with lots of bunches of “strings” of LEDs along with lots of power sources to power all the “strings” of LEDs. With parallel lots of wires for all of the individual LEDs, but possible one power source.

Devon, add up the voltage drop across the LEDs in the string, at 2.4 volts each. Subtract that from the supply voltage. Divide the answer by the current the LEDs need (.020 amps) and that is the value of resistor you need to keep the current at 20 miliamps.

The only problem I have with running them parallel is the amount of wire it takes to accomplish the job. For each LED you want you have two wires. If I put 15 LEDS in a tree or wrap 20 around a house to simulate Christmas lights then I end up with a heck of a bundle of wires. While easier to wire, it would be a mess, especially in O scale. the idea of running them in series is you have “one” continuous “wire” starting at the + side of things and then ending at the - side of things. With the bulbs all being on that one line. As you wrap them around you would only have basically one wire; the Negative of one bulb is the positive of the next and so on.

Now with that said, I have talked to two different suppliers now and neither want to actually answer my question on how to accomplish this. Maybe I am asking to much. But each has suggested running them in parallel and using the already attached resistor. So either they don’t want to answer, don’t know how to answer, or it can’t be done, and won’t say as much in any case.

Now as for stringing Christmas light around the eve of the house that could be done in parallel no problem. Wired from inside with the LEDs poking out where I need them to and the resistors and wire all being hidden in the building. This would be easy, for parallel you just have a common + and a common - and attach each lead of the LED to the appropriate feed with its built in resistor. So I can manage that one. Its the damn Christmas trees that will be the issue.

No I have an idea on that as well if it is not possible to run them in series. Which I am suspecting it not. if each LED drops 2.4 v then 24v will only run 10 bulbs in series right? Where as running them in Parallel I maintain 24v (or whatever) through out the entire string as long as my power supply can handle the current load. So if i want to put 150 (yes I am being sarcastic) LEDs in a tree I would have to run 480v to it (yes I am being sarcastic again). This is of course I am understanding things which I am not sure yet that I have it. So here is an idea, one of the ways I made a tree for my snow shed build was twisting wire and then using liquid nails painted on to make the trunk and heavier limbs. I am thinking is I want to get real creative I could kill two birds with one stone and run all the LEDs in parallel and use the wire as my armature for the tree along with some stiffer wire for support. then each of the little bulbs will double as a branch.

This is why my wife is convinced I have a brain condition. I obsess over the minutia of this stuff. But its challenges like this that are very rewarding once you get it done.

I watched that vid, and I think for sure I will run the buildings this way. It would be way easier to make the electronics work as I can bring in the 24v into the building, drop the voltage to say 12v or whatever works with these LEDS and then run them all in parallel. All the stuff can either be in the building itself with plugs to just plug it in. So I will work on that principle.

That leaves the outdoor Christmas trees.

On the way home from work, there was a blinding flash of light and I was knocked out. When I came too, all of a sudden I knew everything. I could even predict earthquakes, if you need me too I can provide this in Portuguese. (a special prize to the first one to name the movie reference). I am dropping the whole “in series” and going on the “in parallel” idea. Where I was getting hung up was thinking all the wires needed to head back to the power supply or feeder. But then it dawned on me that I could run a + and - together as my feeder and wrap that around my tree then each individual LED would be wired to each leg. If the voltage on the feed was kept to say 3 volts then no need for a resistor on each LED. According to the website I can run 30 LEDs on a single 3v AAA. That would be way more than enough LEDs for my tree. Now I wouldn’t use a battery but I could make a “transformer” that would have an output of 3v and then run some thin wire twisted together and solder each bulb onto it. Heck add some flocking and the wire would be garland. Worst case scenario and I could run one, two, three strings on each tree. That way the current load isn’t exceeded.

Does that sound better?

Yes, yes.

A cheap power supply is a USB converter and USB cord. I see them at the dollar store for (wait for it…) a dollar. USB devices put out 5 volts at 1 ma.

Well don’t I feel like the fool. lol

I finally got one of the people to work with me and I am trying to reinvent the wheel. The place I am working with is Evans Designs/Model Trains Software and they specialize in LEDs and trains and already have EXACTLY what I am looking for.

I wasn’t even considering flashing. But this string of flashing nanos is exactly what I want.

Over-designing something. Yup, I know that I have done that before.

why not just go to your local Wal Mart and use the real small strings of lights they have that can be either battery powered or hooked to your power suppply? Maybe it would help with your brain flashes? Wish I knew more about how LED’s worked I need a electrical lesson, and not the touch this see if we have power kind of lesson

Pete, don’t give him an easy way out (’re supposed to make it harder for him, that’s what friends are for (, I could tell him to go get an arduino and some nano pixels and program them himself so he could have varying colors and themes but I wouldn’t do that to him … or would I (

Pete, I’ve been messing with them for about 2 years now, it took me a while to get my brain off voltage and on to current draw but I have finally gotten to a point to where I can make them work. Got mine from eBay and all different sizes, and even though different color operate a different voltages pretty much they will all work on 3 volts. use a lot of solar pathway lights that I either just cut a hole in a building roof and leave it at that or take the lights apart and customize there location. Funny thing I found as although there is a rechargeable 1.2vdc batter in them that powers the LED at night the LED it’s self is 3 volt, seems as though there is a mini transformer call a converter or jewel thief that boosts the voltage from 1.2 to about 2.4. I’ve added voltage reducers to some of my engines and then added resistors to the lowered voltage, the reduced voltage is set at 5vdc for a smoke unit and by adding a 100 ohm resistor it drops the output to 3 vdc which I then power my engine lights and have added several flickering LEDs to for a firebox look in the cab. My passenger cars are LGB 3080 style and I added a 2 unit AAA battery holder with external switch then a couple of 10mm soft white LED in the roof and hide the battery pack in the clearstory, works real well and the batteries last for ever. You can see a lot of it on my Pinterest site at, and if you have any ? you can reach me at [email protected], Bill