Trains.com

Subscriber & Member Login

Login, or register today to interact in our online community, comment on articles, receive our newsletter, manage your account online and more!

Can two LEDs share a resistor?

5750 views
41 replies
1 rating 2 rating 3 rating 4 rating 5 rating
  • Member since
    February 2021
  • 1,110 posts
Posted by crossthedog on Sunday, May 7, 2023 3:55 PM

Henry, that's the exact look I'm aiming for. Wow, that bookstore or library is amazing, complete with a print of The Flying Cloud on the wall!

-Matt

Returning to model railroading after 40 years and taking unconscionable liberties with the SP&S, Northern Pacific and Great Northern roads in the '40s and '50s.

  • Member since
    December 2015
  • From: Shenandoah Valley
  • 9,094 posts
Posted by BigDaddy on Sunday, May 7, 2023 7:01 AM

lots of useful info here  https://electronicsclub.info/leds.htm

I used an orange translucent paint (Tamiya?) to warm up a bright white led.  That might take the green out of the lower floor.  I wouldn't use too much though, or else it will look close to the top floor.

 

 

Henry

COB Potomac & Northern

Shenandoah Valley

  • Member since
    July 2006
  • From: 4610 Metre's North of the Fortyninth on the left coast of Canada
  • 9,352 posts
Posted by BATMAN on Saturday, May 6, 2023 11:52 AM

Brent

"All of the world's problems are the result of the difference between how we think and how the world works."

  • Member since
    December 2004
  • From: Bedford, MA, USA
  • 21,483 posts
Posted by MisterBeasley on Saturday, May 6, 2023 11:34 AM

Be careful if you are using different LEDs in the structure.  They may have significantly different resistance values so using parallel wiring may "steal" the power from one LED to light the other.  I wired a signal bridge using red, yellow and green LEDs, and my parallel wiring didn't work and I had to re-do the circuit for series wiring.

It takes an iron man to play with a toy iron horse. 

  • Member since
    February 2021
  • 1,110 posts
Posted by crossthedog on Saturday, May 6, 2023 9:00 AM

Overmod
That slightly greenish color is how I remember the ceiling in Hopper's Nighthawks.

Overmod, great catch! Nighthawks is one of my favorite paintings of one of my favorite painters. And one of my favorite bands of the '80s/'90s made a video that riffed on it.

I also see that you can buy the whole Nighthawks neighborhood in HO scale.

-Matt

 

Returning to model railroading after 40 years and taking unconscionable liberties with the SP&S, Northern Pacific and Great Northern roads in the '40s and '50s.

  • Member since
    September 2003
  • 21,669 posts
Posted by Overmod on Saturday, May 6, 2023 6:08 AM

That slightly greenish color is how I remember the ceiling in Hopper's Nighthawks.  I thought of it then as being light reflected from pale celadon green interior paint.  I think if you put further resistance in the line to those particular resistors you could re-create that 'look'.

(I went back to check if I remembered this right and there are all sorts of 'corrected' tonal ranges in commercial prints of the painting...)

  • Member since
    July 2006
  • From: Bradford, Ontario
  • 15,797 posts
Posted by hon30critter on Saturday, May 6, 2023 1:24 AM

crossthedog
I have a five or six story hotel to do, and I may wire it up with Brent's copper tape method before I even assemble the walls.

Hi Matt,

I think that learning how to plan ahead for details like lighting and wiring is one of the more challenging aspects of building a structure. In the past I have been a bit like a bull in a china shop. I always wanted to charge ahead with putting the major parts together when I should have spent much more time planning for the interior details. My bad!

Good luck with your lighting project!

Cheers!!

Dave

I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!

  • Member since
    February 2021
  • 1,110 posts
Posted by crossthedog on Friday, May 5, 2023 11:56 PM

gmpullman
I can see the profile of the resistor under that heat shrink. It looks like a snake that swallowed a dumbell.

Speaking of dumbells, color me embarassed. It turns out there ARE resistors in there. Here's a test hookup with no (additional) resistors.

I feel sheepish having run everyone "all around Robinhood's barn" all day trying to figure out how to get resistors into the wiring.

It's actually not that much brighter than the test with resistors, but the fact that the bulbs didn't blow up proves that they are protected by embedded resistors. I think I was sold something I did not order, or else there was "order incompetence" on my part.

You can see that greenish cast to the light in the cafe. It's worse in person. I may paint the inside of the lower floor white -- and the ceiling -- to see if I can make it more like a busy diner in there. I may also put up a false wall in front of those wires coming down the inside. Or paint them, as someone suggested. And I'm hoping that painting the exterior of the building will take care of the light leak that's showing right through the outer wall above the upper windows.

The upper floor light I like a lot, although I may have gone a bit heavy on the red paint on the bulb, because it almost looks like firelight, and I was aiming more for a reading lamp vibe.

Anyhoo, this parTICKular job will be much easier than I'd anticipated. But thank you all. I will save this thread and refer to it because I'll only be doing more and more structures in the months to come. I have a five or six story hotel to do, and I may wire it up with Brent's copper tape method before I even assemble the walls.

-Matt

Returning to model railroading after 40 years and taking unconscionable liberties with the SP&S, Northern Pacific and Great Northern roads in the '40s and '50s.

  • Member since
    August 2003
  • From: Collinwood, Ohio, USA
  • 16,367 posts
Posted by gmpullman on Friday, May 5, 2023 10:04 PM

I can see the profile of the resistor under that heat shrink. It looks like a snake that swallowed a dumbell. I have dozens of that style, too. They're perfect for a direct replacement of the old Life-Like bulbs in their locomotives.

Good Luck, Ed

  • Member since
    February 2021
  • 1,110 posts
Posted by crossthedog on Friday, May 5, 2023 9:19 PM

CSX Robert
A white LED will typically have a voltage drop of 3-3.5 volts, If they are labeled 9 to 12 volts, then they do have resistors so you shouldn't have to add any unless they are too bright.

Well that's what Ed (or was it Dave?) thought, too, just looking at my photo. But I was pretty sure that I intentionally bought LEDs that did NOT have resistors. But I've bought a lot of lights lately, trying different things. Since I have a whole pack of these I suppose I could spare one for a test. I'll hook it up without a resistor and see what happens. If it doesn't blow up, I guess that means it has one built in. And if that is the case it might explain why I thought the cafe looked a little dim when I lit it up for a test. I'll report back later. It's pizza night at our house, my wife's homemade pizza dough.

-Matt

Returning to model railroading after 40 years and taking unconscionable liberties with the SP&S, Northern Pacific and Great Northern roads in the '40s and '50s.

  • Member since
    February 2007
  • From: Christiana, TN
  • 2,134 posts
Posted by CSX Robert on Friday, May 5, 2023 8:49 PM

crossthedog
I was wondering the same thing, cuz it sure looks like they've wrapped a tiny resistor in there, but this package of LEDs I bought specifically because I wanted some withOUT built-in resistors. So I think that's just the soldered joint under there.

crossthedog
The only "specs" I have for the LEDs is that the package is labeled Warm White and 9.0 to 12.0 volts.

A white LED will typically have a voltage drop of 3-3.5 volts, If they are labeled 9 to 12 volts, then they do have resistors so you shouldn't have to add any unless they are too bright.  

  • Member since
    September 2003
  • 21,669 posts
Posted by Overmod on Friday, May 5, 2023 8:32 PM

None of the epoxies I'm familiar with do anything to resistors.  In fact the most common 'potting' materials to make circuits shock- and weatherproof are epoxies.

  • Member since
    February 2021
  • 1,110 posts
Posted by crossthedog on Friday, May 5, 2023 8:23 PM

CSX Robert
What is your supply voltage? Do you have any specs for the LEDs (forward voltage drop and maximum current)? If you don't have the specs, then what color are they (voltage drop can be estimated from the color)?

Supply is 12 volts. The only "specs" I have for the LEDs is that the package is labeled Warm White and 9.0 to 12.0 volts. I don't think the white is really warm. It's kinda limey, like a greenish or yellowish white, like a modern streetlight when it first comes on.

Returning to model railroading after 40 years and taking unconscionable liberties with the SP&S, Northern Pacific and Great Northern roads in the '40s and '50s.

  • Member since
    February 2021
  • 1,110 posts
Posted by crossthedog on Friday, May 5, 2023 8:16 PM

BigDaddy
Here is another idea. These are LED's in the ceiling of a building

Wow. That is some serious scratchbuilding.

Returning to model railroading after 40 years and taking unconscionable liberties with the SP&S, Northern Pacific and Great Northern roads in the '40s and '50s.

  • Member since
    December 2015
  • From: Shenandoah Valley
  • 9,094 posts
Posted by BigDaddy on Friday, May 5, 2023 5:58 PM

Here is another idea.  These are LED's in the ceiling of a building (built by Logan Holtgrewe of Severna Park MR fame)

It was designed for a 3V power supply.  He used K&S or is it K&N brass wire to solder the the LED's.  Remember LED's have a polarity

Henry

COB Potomac & Northern

Shenandoah Valley

  • Member since
    February 2007
  • From: Christiana, TN
  • 2,134 posts
Posted by CSX Robert on Friday, May 5, 2023 5:42 PM

crossthedog

 

 
gregc
as i suggest, try different resistor (> 62) to lower the brightness.

 

Greg, the chart doesn't make much sense to me -- pretty as it is. There seem to be all values of resistor for sale out there. How low could I go (10?) before I'm not providing enough resistance? I made the mistake (twice!) of testing an LED without a resistor in the mix and in both cases I instantly blew its brains out. So is there a lower threshhold the resistor should stay above?

 

-Matt

 

What is your supply voltage?

Do you have any specs for the LEDs (forward voltage drop and maximum current)?

If you don't have the specs, then what color are they (voltage drop can be estimated from the color)?

 
  • Member since
    February 2021
  • 1,110 posts
Posted by crossthedog on Friday, May 5, 2023 5:08 PM

gregc
as i suggest, try different resistor (> 62) to lower the brightness.

Greg, the chart doesn't make much sense to me -- pretty as it is. There seem to be all values of resistor for sale out there. How low could I go (10?) before I'm not providing enough resistance? I made the mistake (twice!) of testing an LED without a resistor in the mix and in both cases I instantly blew its brains out. So is there a lower threshhold the resistor should stay above?

-Matt

Returning to model railroading after 40 years and taking unconscionable liberties with the SP&S, Northern Pacific and Great Northern roads in the '40s and '50s.

  • Member since
    February 2007
  • From: Christiana, TN
  • 2,134 posts
Posted by CSX Robert on Friday, May 5, 2023 5:05 PM

I would probably just use a dab of hot glue but expoxy would be fine. 

  • Member since
    February 2021
  • 1,110 posts
Posted by crossthedog on Friday, May 5, 2023 4:40 PM

CSX Robert
To do two resistors, use the circuit on the right

Ah. I would not have been able to cook that up on my own. Thanks, Robert.

Still waiting to hear from Dave or anyone -- anyone? Beuller? anyone? -- whether or not epoxy will damage a resistor. I'm considering using Brent's weensy resistors on magnetic tape, but more likely I will use the standard resister and try to glue it to the wall or floor. Would epoxy corrode the cylinder? If epoxy is not a good idea, how would you affix this kind of resistor to styrene?

-Matt

Returning to model railroading after 40 years and taking unconscionable liberties with the SP&S, Northern Pacific and Great Northern roads in the '40s and '50s.

  • Member since
    February 2007
  • From: Christiana, TN
  • 2,134 posts
Posted by CSX Robert on Friday, May 5, 2023 4:07 PM

crossthedog
Okay, if I remove the lines between the anode and cathode for each LED, is that the same as this? If I got it wrong this time I'm giving up until after lunch and a nap. :)

That is not what you want.  To do two resistors, use the circuit on the right and to do one rsistor use the circuit on the left:

  • Member since
    July 2006
  • From: 4610 Metre's North of the Fortyninth on the left coast of Canada
  • 9,352 posts
Posted by BATMAN on Friday, May 5, 2023 3:48 PM

Try using this, I am so stupid at this stuff it hurts, but this Wizard got me through.Laugh

http://www.hebeiltd.com.cn/calculator/v5/led.php 

 

Brent

"All of the world's problems are the result of the difference between how we think and how the world works."

  • Member since
    July 2006
  • From: 4610 Metre's North of the Fortyninth on the left coast of Canada
  • 9,352 posts
Posted by BATMAN on Friday, May 5, 2023 3:41 PM

crossthedog
how to affix the copper strip, how to solder to it, where to get it,

The copper tape is sticky on one side.

To solder, get a small bead of solder on the iron and touch the bead to the copper tape, do not touch the iron to the tape. It really is quite easy.

I bought several rolls of different widths of copper tape from China for pennies on the dollar for what they want for it here.

 I order a lot from China with an average delivery time of 11 days. Type in what you need on eBay and the Chinese companies come right up.

I also use these to control the power by connecting an old wall wart to it. I ordered a bunch from China for $2.00ea. You can adjust the brightness somewhat with these, after a couple of years I have not lost an LED yet.

 

Here is my first experimental try to see if it would work, you can see the chip resistors. First, attach the wire to the resistor and then hold the resistor to the bead of solder by holding the wire and a quick touch does the trick. 

This is pretty messy as I was trying different methods including painting it all after the soldering was done. I got much neater in short order.

 

Painted with cheap Walmart craft paint.

 

Brent

"All of the world's problems are the result of the difference between how we think and how the world works."

  • Member since
    February 2021
  • 1,110 posts
Posted by crossthedog on Friday, May 5, 2023 3:31 PM

tstage
As far as a hand-drawn diagram, there should be no line betwen the anode & cathode of each LED.

Okay, if I remove the lines between the anode and cathode for each LED, is that the same as this? If I got it wrong this time I'm giving up until after lunch and a nap. :)

Returning to model railroading after 40 years and taking unconscionable liberties with the SP&S, Northern Pacific and Great Northern roads in the '40s and '50s.

  • Member since
    February 2021
  • 1,110 posts
Posted by crossthedog on Friday, May 5, 2023 3:11 PM

York1
have added foundations to the buildings and houses I now build. The wires run through the floor, and the resistor and soldering all sits under the building. The foundation is a flat piece of styrene.

John, I think you gave this valuable input when I was trying to wire the Perkins, and it was in large part your testimony that made me put a resistor and most of the small-to-larger wire connections under the lower floor. But that building already had a void under the lower floor, this one does not. So I like your idea of just building it up from ground level. This will incidentally raise the building to a better height for accommodating a sidewalk. As it is, the building would sit too low if I want to add any street material and then sidewalks even higher. So this is a good idea.

I was hoping Dave would chime back in and answer my question about what to use to affix a resistor to the styrene kit wall or to an added styrene floor/foundation.

Which brings me to...

BATMAN
I found working with wires in a building a pain as well so I used copper tape. Note the tiny chip resistor above each light.
Brent, I think you've shown me this photo before, and I think it's a good solution. But it looks like awfully tiny, precision work, something that intimidates me a bit. So many questions about it -- how to affix the copper strip, how to solder to it, where to get it, what kind, etc.. And your  roundhouse is large and has its roof off. So it seems like it would be easier to get in there with (hot!) tools.

 

Returning to model railroading after 40 years and taking unconscionable liberties with the SP&S, Northern Pacific and Great Northern roads in the '40s and '50s.

  • Member since
    February 2007
  • From: Christiana, TN
  • 2,134 posts
Posted by CSX Robert on Friday, May 5, 2023 3:03 PM

crossthedog
Robert, I always have trouble picturing parallel circuits; where would the resistor be in a parallel setup?

In the situation betamax described is the resistor is not in parallel - it's a parallel/series circuit.  The LEDs are wired in parallel as I mentioned, but the pair of LEDs is wired in series with the single resistor.

crossthedog
I think of parallel as a loop with a bulb on it and a short-cut across the middle of that loop with another bulb on it

That is correct.

crossthedog
But where could a resistor be that it wouldn't affect both LEDs if it failed?

The resistor would effect both LEDs, what I said was if the LEDs are in series a failed LED won't damage the other LED (it would still effect it, as in it wouldn't light until the damaged LED was replaced, but it wouldn't damage it).

  • Member since
    July 2006
  • From: 4610 Metre's North of the Fortyninth on the left coast of Canada
  • 9,352 posts
Posted by BATMAN on Friday, May 5, 2023 2:37 PM

I found working with wires in a building a pain as well so I used copper tape. Note the tiny chip resistor above each light. I had nothing but grief putting more than one LED per resistor, so one resistor per LED it was. 

Once everything is soldered up you can paint the copper tape and it disappears.

Brent

"All of the world's problems are the result of the difference between how we think and how the world works."

  • Member since
    February 2018
  • From: Flyover Country
  • 5,557 posts
Posted by York1 on Friday, May 5, 2023 2:27 PM

crossthedog
The issue is just how difficult I find it to solder and get wires to behave in the space within a structure like this. The tiny wires break, the larger ones act like springs and push against stuff and each other.

I have the same issue.

I have added foundations to the buildings and houses I now build.  The wires run through the floor, and the resistor and soldering all sits under the building.

The foundation is a flat piece of styrene.  I then build up the foundation by gluing more stryene around the outside edges of the foundation on the bottom.

That leaves a void under the building floor which allows wires, resistors, and soldering connections to all fit.

York1 John       

  • Member since
    February 2021
  • 1,110 posts
Posted by crossthedog on Friday, May 5, 2023 2:12 PM

tstage
Is there some kind of room constraint that adding an extra resistor is an issue?

Hi Tom. The issue is just how difficult I find it to solder and get wires to behave in the space within a structure like this. The tiny wires break, the larger ones act like springs and push against stuff and each other. Clipping them short for cleaner wiring makes it even harder to solder them. I'm very happy to add a resistor for each LED, it's just dashed difficult for me to fit it all in there.

tstage
As far as a hand-drawn diagram, there should be no line betwen the anode & cathode of each LED. But, I assume you know that and it was a quick diagram.

No, I didn't know that, and I don't even know what anode and cathode are, but that's why I posted the diagram (yes, that was quick). I hoped someone would correct it. But Tom, if I remove the short lines at the bottom of each of the LED mini-paths, doesn't that make it a series instead of parallel? How would I make this diagram parallel?

Returning to model railroading after 40 years and taking unconscionable liberties with the SP&S, Northern Pacific and Great Northern roads in the '40s and '50s.

Moderator
  • Member since
    June 2003
  • From: Northeast OH
  • 17,249 posts
Posted by tstage on Friday, May 5, 2023 1:44 PM

Matt,

Is there some kind of room constraint that adding an extra resistor is an issue?  Resistors are cheap.  I would add a resistor for each LED...

As far as a hand-drawn diagram, there should be no line betwen the anode & cathode of each LED.  But, I assume you know that and it was a quick diagram...

Tom

https://tstage9.wixsite.com/nyc-modeling

Time...It marches on...without ever turning around to see if anyone is even keeping in step.

Subscriber & Member Login

Login, or register today to interact in our online community, comment on articles, receive our newsletter, manage your account online and more!

Users Online

There are no community member online

Search the Community

ADVERTISEMENT
ADVERTISEMENT
ADVERTISEMENT
Model Railroader Newsletter See all
Sign up for our FREE e-newsletter and get model railroad news in your inbox!