Can two LEDs share a resistor?

[crossthedog]

Once upon a time there was a two story cafe.

Forget that, I'm ditching the storytime conceit. Here's the deal: I put a cardboard floor in to separate the two stories, and then separated some of the windows off with walls, so that a warm LED upstairs will light some but not all windows, while the cafe below will be brightly lit with another LED.

I've tested this holding two resistors and four wires with just my two hands (difficult? yup.) and it looks fantastic.

Now, take a look in the lower window. You can see the wires for both lights coming down the wall.

This is where I want to put resistance. In the other two buildings I've done so far, I put just one resistor in the basement because there was only LED that needed resistance.

They work fine. I know how to do it. But it seems like a lot of work and would be a mess of soldering and wiring to put two resistors in here.

So my question is, is there any way to do this with just one resistor? The resistor I have here is for 3v LEDs:

This is what I'm thinking, but I'm not sure whether this would be enough resistance, and if it is, where would the resistor be placed? Could I just make one long loop from the power source with a resistor anywhere in that loop?

Thanks in advance,

-Matt

[CSX Robert]

crossthedog

Could I just make one long loop from the power source with a resistor anywhere in that loop?

Yes you can, as long as your supply voltage is enough.  Two 3 volt LEDs will drop 6 volts total, so it wouldn't work with 5 volts but would with 9 or 12 volts.

crossthedog

but I'm not sure whether this would be enough resistance

Actually, it may be too much resistance. With the LEDs dropping more volts, the resistor will drop less.  Less volts through the same resistance gives less amps, which means dimmer LEDs, so you may have to lower the resistance.  Note that it won't neccesarily be half the resistance, it will depend on the supply voltage. I would, however, just start with the resistor you have and see if it's bright enough.

[hon30critter]

crossthedog

So my question is, is there any way to do this with just one resistor?

Hi Matt,

As has been stated, yes you can use one resistor for two LEDs. In fact, you can use a single resistor for up to about six LEDs, but calculating the resistor value can be somewhat complex.

My suggestion is to use one resistor for each LED. In your particular situation using one resistor per LED would be particularly advantageous. If I understand your comments correctly, you want relatively dim lights on the second story and brighter lighting on the main floor. If you use one resistor for each LED you will be able to control the brightness for each LED. If you use one resistor for both LEDs their brightness will be the same. I would suggest trying a 10,000 ohm resistor for the upper floor and a 1000 ohm resistor for the main floor. If you find the second floor to still be too bright then try a 20,000 ohm resistor. I have used resistors up to 30,000 ohms where I wanted to simulate the light of a kerosene lantern.

The other issue that you pointed out was that the wires were visible through the windows. There are ways to prevent this. One is to simply paint the wires with the same colour as the walls. Or, you could run the wires so that they are not visible through the windows. That takes a bit of planning before you start to assemble the buildings, but it is easy to do.

I'm going to go out on a limb here, but I suspect that you are installing the LEDs in the buildings and you are then installing the resistors in the wires leading to the building. I would suggest that you wire in the resistors as close as possible to the individual LEDs. That will allow you to use just one pair of wires to power all the lights in the structure.

I may be totally out to lunch on all of the above. If so, I apologise.

Cheers!!

Dave

[betamax]

Use one resistor per LED.

Logic: The resistor is calculated to provide current limiting to two LEDs, which is twice the current as one LED. Should one LED fail, the current will increase to the remaining LED, which will soon fail as a result.

Besides, how much money is saved by eliminating a resistor?

LEDs and Resistors

[gmpullman]

Looks like there's already a resistor under the shrink wrap in your second photo? Probably a 680 or 1KΩ.

So you want to further reduce the intensity of the light from each individual LED by adding resistance in series?

I have 'ganged up' resistors for this purpose, especially in signal LEDs. The resistor can be in either leg but I usually try to keep them on the minus side as is a good practice for decoder function wiring since the anode is usually common. (blue + wire)

Good Luck, Ed

[gregc]

surely multiple LEDs can be wired in series with a single resistor if the supply voltage is high enough as CSX Robert said.   you could probably wire ~6 LEDs in series with a 12V supply, avoiding unnecessary work (soldering)

it seems today's LEDs are extremely bright and am using resistors in the 10s of Ks with 3.3V, so start with a 1K resistor (~10ma w/ 12V) and try other values until satisfied with the brightness.

[crossthedog]

CSX Robert

Actually, it may be too much resistance. With the LEDs dropping more volts, the resistor will drop less. Less volts through the same resistance gives less amps, which means dimmer LEDs, so you may have to lower the resistance.

Robert, this surprises me completely. I'd have thought it went the other way, that if I strung more LEDs in a series with one resistor, each one would be brighter. This is very enlightening. (<-- see what I did there?)

hon30critter

In your particular situation using one resistor per LED would be particularly advantageous. If I understand your comments correctly, you want relatively dim lights on the second story and brighter lighting on the main floor. If you use one resistor for each LED you will be able to control the brightness for each LED. If you use one resistor for both LEDs their brightness will be the same.

Well, not exactly the same brightness. I dipped the upstairs bulb in paint and then wiped it a bit, so it now has a warm gloaming look. But you're right in that when I tested it, the lower lamp had a brighter cafe color but it was not as bright as I thought it should be.

hon30critter

The other issue that you pointed out was that the wires were visible through the windows. There are ways to prevent this.

Wires showing through the window is not a problem for me; I showed that to demonstrate where I want to put the resistors, which is on the back lower walls. And your idea of painting them bright wall colors is a good one.

hon30critter

I suspect that you are installing the LEDs in the buildings and you are then installing the resistors in the wires leading to the building. I would suggest that you wire in the resistors as close as possible to the individual LEDs. That will allow you to use just one pair of wires to power all the lights in the structure.

No, I actually plan to have the resistors inside the building, but fastening them to something is problematic. Will epoxy damage the resistors? Thanks for the detailed ideas, as always.

betamax

Logic: The resistor is calculated to provide current limiting to two LEDs, which is twice the current as one LED. Should one LED fail, the current will increase to the remaining LED, which will soon fail as a result. Besides, how much money is saved by eliminating a resistor?

Max, I'm not trying to save money, I'm trying to avoid soldering in tight spaces with tiny wires, which activity I dis-love quite a bit. Thanks for the warning about the surge in current should one LED fail.

gmpullman

Looks like there's already a resistor under the shrink wrap in your second photo?

Ed, 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. But good to know I can gang the resistors up, as you've suggested. Thanks.

gregc

it seems today's LEDs are extremely bright and am using resistors in the 10s of Ks with 3.3V, so start with a 1K resistor (~10ma w/ 12V) and try other values until satisfied with the brightness.

Greg, funny you say that. I think the LEDs are often too bright for interiors, too. I like them for locomotive headlights, but yeah, normally way too bright for a lamp upstairs. However, in this case, I want the lower light to be a lot brighter than it is. When I tested the lamp in the cafe, it was the right hue, but it seemed a little dim. Is there any way to know from looking at the photo of my resistor above what strength it is? These came with some 3v loading dock lamps I bought from WeHonest; they're the only ones I "currently" have.

Thanks all.

-Matt

[gregc]

crossthedog

Is there any way to know from looking at the photo of my resistor above what strength it is?

i'm seeing blue red black gold which mean a 62 Ohm resistor which results in a 25 ma for a 1.4V LED driven by 3V or 16 ma for a 2V LED driven by 3V.    max LED current use to be 20ma and i think they are plenty bright today at much lower currents.

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

 

[wvgca]

you can, but it's not really good practise to run more than one LED per resistor .. and with one resistor per LED you can easily adjust the brightness of each indivual LED, and resistors are really cheap, maybe a penny each ...

[CSX Robert]

crossthedog

Thanks for the warning about the surge in current should one LED fail

What betamax is describing is if you have two LEDs in parallel running off one resistor. If you have both LEDs and the resistor all in series (which is what I understaood you to mean), then if one fails it will break the circuit.  The other one still won't light until the bad one is replaced, but it won't get damaged.

[CSX Robert]

wvgca

you can, but it's not really good practise to run more than one LED per resistor

No, actually, it's often a very good practice.  It's true that you can't adjust their brightness individually, but as long as you're ok running them at the same current then it's better to run multiple ones in series off a single resistor if you have the supply voltage to do it.  You get less current draw for the same light output and less power wasted as heat through the resistors.  With just a few LEDs, it won't make much difference, but when you get into dozens or hundreds of LEDs, it can make a big difference.

[crossthedog]

CSX Robert

What betamax is describing is if you have two LEDs in parallel running off one resistor.

Robert, I always have trouble picturing parallel circuits; where would the resistor be in a parallel setup? 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. But where could a resistor be that it wouldn't affect both LEDs if it failed? If you feel an urge to draw a diagram, please indulge it.

Thanks,

-Matt

[crossthedog]

hon30critter

I would suggest that you wire in the resistors as close as possible to the individual LEDs. That will allow you to use just one pair of wires to power all the lights in the structure

Dave, I like the idea of having just two wires incoming to the structure, but again, having trouble picturing the wiring path and where the resistors would be in the diagram. After all the input I'm leaning toward separate resistors for each floor. Would that look like this?

[tstage]

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

[crossthedog]

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?

[York1]

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.

[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. 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.

[CSX Robert]

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).

[crossthedog]

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.

 

[crossthedog]

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. :)

[BATMAN]

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.

 

[BATMAN]

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

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

 

[CSX Robert]

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:

[crossthedog]

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

[CSX Robert]

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

[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

[CSX Robert]

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)?

 

[BigDaddy]

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

[crossthedog]

BigDaddy

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

Wow. That is some serious scratchbuilding.

[crossthedog]

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.