# LED lighting--some results



## lownote (Jan 3, 2008)

I've been doing a lot with LEDs lately, and while I'm nowhere near the ability of so many people here I thiught this might be useful for some people

I bought a string of "warm white" Christmas lights, seventy bulbs. The bulbs are different from the usual LED dome--they are flat on the top edge but sort of come to a funnel in the middle. See the picture. The result is they seem to cast a great deal more light to the sides. They don't have that painful squint effect when you look at them end-on





















The color is good, much less blue than the typical white LED, but I've had good luck painting them--I've been brushing on some tamiya clear yellow, as George Schreyer recommended, and the result is much more like an incandescent











The interior lighting is three of these Christmas light painted yellow


In this pic the building in the lower right corner is lit with three of the Christmas LEds dipped in Tamiya clear yellow












The "fire" in front of it is red, orange and yellow leds. The three building in the left foreground, and the station, are lit with amber 4 watt malibu lights


This combine has two Chirstmas Leds in the passenger section, both bubs painted with floquil reading yellow. It's very hard to tell them from incandescents


Click for photo
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## Mike O (Jan 2, 2008)

Great idea. Thanks for sharing. Nice pictures too. 

Mike


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## Richard Smith (Jan 2, 2008)

Lighting looks great. Very nice color.


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## joe rusz (Jan 3, 2008)

Very nicely done. And inspirational too (if I ever get off my fat butt)


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## Torby (Jan 2, 2008)

Never found a set of those in the after-Christmas sales, when I buy Christmas supplies.


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## Jerry Barnes (Jan 2, 2008)

All I saw was cool whites.


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## lownote (Jan 3, 2008)

I found them at one store I went into srt of randomly, and now I wished I'd bought more. The shape of the bulb really seems to make a difference

The downside of these is I can't get them out to the plastic holder--they're molded in. So they'd be tough to use as headlight, for example


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## Ray Dunakin (Jan 6, 2008)

Great pics! 

I bought a set of 30 warm white LEDs for future modeling use. They look similar to yours. 
How can you tell what voltage to use on the individual LED? 

I also bought a couple sets with colored "bulbs" for home use. I was surprised that none of these sets appears to have a transformer. 
How can they use straight 110v house current?


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## lownote (Jan 3, 2008)

Thanks Ray! It took me forever to get decent night shots. They key was a tripod and shooting steadily from dusk til close to darkness. Shots taken when it doesn't look dark to the eye look dark in the picture

The lights I got had three big molded plastic cylinders inline, I assume they had diodes and /or resistors to step the voltage down. I could not get one open to find out


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## Semper Vaporo (Jan 2, 2008)

Posted By Ray Dunakin on 01/15/2009 7:25 PM
Great pics! 

I bought a set of 30 warm white LEDs for future modeling use. They look similar to yours. 
How can you tell what voltage to use on the individual LED? 

I also bought a couple sets with colored "bulbs" for home use. I was surprised that none of these sets appears to have a transformer. 
How can they use straight 110v house current? 






The LEDs are probably in series, so divide the listed working voltage (110V or 120V) by the number of lights in the string and you have the working voltage for each LED. 

110V/30LEDs= 3.667V per LED


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## Dan Pierce (Jan 2, 2008)

LED's are not voltage devices!!!! 

They have a suggested voltage for maximum use, but are really current devices with a max voltage rating. 

It is very important to watch the current on LED's. 

I use a meter and set my led's to a 15 ma max current at 22 volts. (I have 24 volt supply to a TE which gives 22 volts max to the track. 

Some just tie led's direct to batteries and they work. This can be dangerous as the current is not being limited and may be well over the 15ma which causes excessive current draw on the battery and the led can get hot and burn.


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## lownote (Jan 3, 2008)

True! A good reminder


I have used this led calculator a lot: http://led.linear1.org/led.wiz 



The problem is that you don't really know what the forward current rating is for a bunch of Chrstmas light leds. I've been using the figures for Radio Shacks 5 mm white leds. But it's guesswork. I have sometimes just sat with bunch of resistors and tried different ones till I go the brightness I wanted


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## Paul Norton (Jan 8, 2008)

All the WHITE LEDs I have purchased to date have shown a 3.6 voltage drop and have a maximum current rating of 20 milliamps.

The Ngineering web site has the best tutorial on LED CIRCUITS I have ever found. They also have the CALCULATORS for determining the proper resistors required in a circuit. I have added both to my browser’s FAVORITES.


All Electronics currently have a 24 LED, 24" LIGHT STRIP, catalog # LBR-24P. It operates on 24 volts DC at 80 milliamps. Twenty-four, 5 mm, bright white LEDs are staggered in two rows with built-in resistors mounted on pc board. There are holes at both ends of strip for soldering connectors or wire leads.

The LEDs on the board are in groups of 6, so the board could be cut to provide lighting circuits of 6, 12 or 18 LEDs.


 

Although I use resistors in my LED circuits to limit the current to less than 20 milliamps, you can use a couple of AA cells to test a white LED. As long as the AA cells have a total voltage of no more than 3 volts, the maximum current to the LED will not exceed 20 milliamps.

Adding the female connector from an All Electronics two-pin plug set (catalog # CON-240) to a twin, AA battery holder makes a cheap and easy tester for white LEDs. The battery holder with switch is available from Radio Shack and most electronic shops. The leads of the LED simply slip into the connector for testing.


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## Greg Elmassian (Jan 3, 2008)

Yep, but test a red or green led and you instantly convert a LED to a DED! 

It's good to know the voltage drop when calculating the resistor, BUT, the point people are making is that CURRENT is the key. The current through a led can vary dramatically by a change of just a few hundredths of a volt. 

Coming back with a shortcut that usually works is not helping people "stay out of trouble" .... again my example, you take your tester to a red led and it burns it out. 

Finally, you will see that manufactures will specify the maximum current, but will only specify the NOMINAL forward voltage, again underscoring the importance of controlling the current no matter what the voltage. 

Let's give people the best advice, that is most bulletproof. I have hundreds of shortcuts I use myself, but would never advise beginners to use. 

In the spirit of helping others the best way, 

Greg


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## Torby (Jan 2, 2008)

Would that be a Dark Emitting Diode?


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## Greg Elmassian (Jan 3, 2008)

Yep! Ding! 

Engineers joke... 

Greg


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## Semper Vaporo (Jan 2, 2008)

I used to have a spec sheet for a "DEAD" (Darkness Emitting Arsenic Diode). It showed that this device was very useful to not illuminate Top Secret displays and controls. I don't remember what company put the spec sheet out, but I also remember a Signetics spec sheet for a WOM (Write Only Memory).

Apparently, an Engineer at Signetics was frustrated with the seemingly useless multiple approvals required for a component specification, especially because it appeared that no one actually checked them, they just hung on to them for a while and then signed them. So he created a specification for a "WOM" (Write-Only-Memory) and sent it along with other specifications to be approved. Signetics management didn't learn that it actually got published in a new catalog until customers began asking for "price and availability" quotes. Signetics re-published a corrected edition of the catalog and asked for the "erroneous" ones to be returned... which some people did and others didn't . Then, I think in 1974, Signetics did a large advertisement in "Electronics" magazine's April issue as an April Fools' joke. The spec sheet included what initially appeard to be conventional charts about the device, but were things like: "Bit capacity vs. Temp.", "Iff vs. Vff", "AQL vs. Selling Price" and my personal favourite "Number of Pins Remaining vs. Number of Socket Insertions". It listed the device, the 25120, as requiring a 6.3 VAC VFF supply, a +10V VCC, and VDD of 0V, +/- 2%. It was suggested to be useful for recording the last data of the flight path of munitions and bombs or for Top Secret data that no one was ever allowed to see.

I think the DEAD came out of this advertising sillyness along with another device, but I can't remember what the other device was. But I do remember Engineers sitting around discussing whether one could actually create a DEAD.


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## Greg Elmassian (Jan 3, 2008)

The other device was an op amp, with very funny specs.... like the rise time was "about 8:30 am", speed was in "furlongs/fortnight", and other silliness... I will look for the spec sheet, came out in 75-78 I believe, and it was from a company called "Nominal Semidestructor"... 

Regards, Greg


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## Paul Norton (Jan 8, 2008)

Posted By Greg Elmassian on 01/16/2009 3:03 PM

Yep, but test a red or green led and you instantly convert a LED to a DED! 

Coming back with a shortcut that usually works is not helping people "stay out of trouble" .... again my example, you take your tester to a red led and it burns it out. 

Let's give people the best advice, that is most bulletproof. I have hundreds of shortcuts I use myself, but would never advise beginners to use. 

In the spirit of helping others the best way, 

Greg

Somehow I don't get the feeling that your post was written "in the spirit of helping others the best way." 

In the entire post there was never anything mentioned but white LEDs. If you really wanted to help you could have just said that the tester might not be suitable for coloured LEDs, instead of bashing another person's post.


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## Greg Elmassian (Jan 3, 2008)

Jeeze... it's not bashing.... it's a suggestion... supported by facts that are indisputable and also based on many years of experience, and also coincide with the advice of others. Why do you have to up the ante? 

Do you think that all beginners or everyone realizes that red and green have different forward voltages? 

I'm suggesting caution, and you call it bashing... so bashing is when someone does not agree? I presented reasons, facts, experience...

I know, you *are *right... no one could _possibly _make that mistake, everyone _knows _that red and green leds are different and not to use your uncontrolled "Tester" on red or green leds... 


Come on Paul... think of all the people reading this... pretend you are trying to help all of them, who might read this thread a year from now, and not make it a "trick" that only works with white leds.... 

The best advice is the advice most helpful for all... and I am sticking to my opinion... 

Greg


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## lownote (Jan 3, 2008)

Is the a way to determine the correct voltage current for an LED without the package specs? I don't think so. The LEDS Radio shack sells have (most of the time) the specs printed on the package. I bought a bunch of those and used those specs as guidelines. It's working so far

LED lighting is pretty fascinating. The bulbs in this set I bought are very effective at spreading the light, and I've been able to get it to mimic incandescents reasonably well. But LED lighting still seems to be in its infancy. We're going to see LEDS getting cheaper and moving into more common use, in modeling and in daily life.


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## Paul Norton (Jan 8, 2008)

When I started using bright white LEDs for headlights they cost about $5 to $6 dollars each. Even at that price the electronic shops in the area always seemed to be out of stock. The lighting boards in my GP-9 drew more than 200 milliamps of power, so I replaced the headlights and number board lights with LEDs. That cut the current to less than 20 milliamps, but cost $40 to do so.

Now bright white LEDs are less than a $1, and considerably cheaper if you are willing to take a string of Christmas lights apart. LEDs are now all around us in truck and bus taillights, traffic lights, flashlights, etc.


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## dbodnar (Jan 2, 2008)

Posted By lownote on 01/17/2009 5:42 AM
Is the a way to determine the correct voltage current for an LED without the package specs? I don't think so. The LEDS Radio shack sells have (most of the time) the specs printed on the package. I bought a bunch of those and used those specs as guidelines. It's working so far

LED lighting is pretty fascinating. The bulbs in this set I bought are very effective at spreading the light, and I've been able to get it to mimic incandescents reasonably well. But LED lighting still seems to be in its infancy. We're going to see LEDS getting cheaper and moving into more common use, in modeling and in daily life. 




There is a very low tech method that I use from time-to-time to determine an appropriate current level that can be fed to an undocumented LED - here is an excerpt from an article that I wrote up on it:

_*Temperature:*_ 

_Inexact method - no tools or special components- can destroy LED_

_Use the above procedure checking the temperature of the LED at each step_
_The best way to check the temperature is to touch the LED with a finger. Once you are sure it is not too hot touch it to your lips or to your cheek._
_Once the LED starts to get the least bit warm use that resistance or one a bit higher in value. _
_The objective is to keep the LED from getting anything but a bit warm. Never let it get hot._
_Remember, the higher the resistance, the lower the current and the dimmer and cooler the LED will be_
 The whole article is located at:
/http://davebodnar.com/railway/LED_light_project/ 


the section quoted above is near the bottom.

dave


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## Dan Pierce (Jan 2, 2008)

The 15 milliamp i used has always worked for me even though some leds can go to 20 ma. It never hurts to stay under the specification.


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## dbodnar (Jan 2, 2008)

Posted By Dan Pierce on 01/18/2009 4:53 AM
The 15 milliamp i used has always worked for me even though some leds can go to 20 ma. It never hurts to stay under the specification.


Dan - the 15-20 ma limit is OK for many run-of-the-mill LEDs - I have been using some high output white LEDs for a number of train and non-train projects - some of them are happy with 200 ma and others go clear up to 900ma - those have an aluminum heat sink attached and get quite hot.

The world of LEDs is rapidly changing - even the ones that draw 200 ma are way too bright to view directly - those are the ones I used on the front and rear of the Mr. Rogers trolley http://www.trainelectronics.com/Trolley_MrRogers/ 


dave


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## pinewoods (Jan 20, 2009)

A simple constant current regulator for safely testing all type of LEDs can be created from an inexpensive three terminal voltage regulator.

An LM7805 or most any other type may be used. (Radio Shack , DigiKey)

Take the desired LED current, subtract the regulator quiescent current(from datasheet) and divide this into the regulator voltage. This value represents the resistor to connect between the output of the regulator and the common pin of the regulator.

5.0V(7805 output voltage) / ( 15 mA(desired LED current) - 5 mA(7805 quiescent current)) = 500 ohms

Connect the anode(+) of the LED to the common pin of the regulator instead of the output. Connect the positive power supply to the input of the regulator, the LED cathode to the negative.

Once the incoming power supply exceeds about 10V (regulator output voltage + regulator dropout voltage + LED voltage) the current through the LED will remain at a contant value. This will work for an input of up to 35VDC with a 7805. The 7805 will get a little warm.

Any color LED may be used without any danger of burning it out. Multiple LEDs in series will work as well.

Sorry if I got too deep,
Its my job.


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## paintjockey (Jan 3, 2008)

So I guess hooking up a resistor, pouring the voltage to it and waiting for it to explode isn't the right way of doing it? To be honest, all the stuff you guys just said pretty much went waaaaaaay over my head. I figured out LED lighting by trial and error, which obviously isn't the smart way but it worked. Then i realized I didn't like them and tore them all out and blew a couple up just for fun. 

But, I didn't know that different colors required different voltages. It would explain a few things..... But why is that? (and please explain it as simply as possible)


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## SteveC (Jan 2, 2008)

Posted By paintjockey on 01/22/2009 2:57 PM
_{snip...} _But, I didn't know that different colors required different voltages. It would explain a few things..... But why is that? (and please explain it as simply as possible) _{snip...}_
The variations in voltages for LEDs is because of the material that they are made from, the following link has a table showing the various materials used. The far right column provides the material that the various LED colors are made from.

LED Color Chart[/b]

I'm no expert, but if you're dealing with an unknown LED. Why couldn't you modify Paul's testing box, and for example...
[*] Use a 9v battery.
[*] In series with a 1K 1/4W resistor.
[*] Provide two terminals to plug the LED's leads into.
[*] Finally, provide a Push-On/Push-Off switch to control the circuit.
[*] Place the unknown LED in the circuit.
[*] Turn the switch On.
[*] Then measure the voltage drop across the LED.
[*] From that point you've got the required information to calculate the proper resistor value. 
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## pinewoods (Jan 20, 2009)

Without getting into semiconductor structures...

The shorter the wavelength of light emitted by the LED the more of a change in energy level of the electrons moving through the LED is required to emit the photon.

IN GENERAL. You may notice a pattern as you go through the spectrum from red to yellow to green to blue the higher the forward voltage of the LED gets. Different die chemistries may skew this a bit. White LEDs are generally blue emitters with a yellow phosphor coating on the die. 

This trend also carries true with infared and ultraviolet LEDs as well. the IR(longer wavelength than red) have a low drop(typ 1.2V) and the UV(shorter wavelength than blue) have a very high drop(as high as 4.0V).

Since the volume of electrons moving through the LED(current) is proportional to the photons emitted the more current the brighter its gets. )Until the heat generated melts everything.)


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