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FREQUENTLY ASKED QUESTIONS
Answers to the questions I am most asked on my e-mail hotline.
Last update on 12-17-08.


This handy answer locator may help you find your answer.

Q: What the heck are LEDs and how do they work?
Q: What are the "usual" wavelengths & colors LEDs come in?
Q: How do I hook them up to a battery?
Q: How come my white LED doesn't work?
Q: Do I connect my batteries in series or parallel to run LEDs on them?
Q: Can I run a 3.6 to 4.0 volt LED with 4.8 volts?
Q: I bought a flashlight you reviewed, but can't find batteries for it. Help!
Q: How do you power LEDs when you test them?
Q: I'm using white LEDs in macro (close-up) photography but I get these ugly blue rings. Help!
Q: I want to use my LED light in the winter, but the batteries poop out in the cold
Q: Where did all these blinding blue & green LEDs start popping up from?
Q: I want to build a circuit to pulse LEDs or drive full color LEDs. Help!
Q: How do those white LEDs work anyway?
Q: How do I change the green LEDs in my cell phone to blue ones?
Q: I want to replace the taillights in my motorcycle or car with LEDs. How do I do that?
Q: Where do I get 2mm by 5mm rectangular blue LEDs?
Q: Where can I find blue SMD LEDs (size 0603) for my cell phone?
Q: I need a 780nm near-infred LED. Do they exist, and if so, where can I get them?
Q: Where can I find UV (ultraviolet) LEDs?
Q: How come blue & white LEDs have two wires inside and red LEDs have only one?
Q: What are the LuxeonŠ LED bin codes?
Q: How do you measure laser power without a laser power meter?
Q: How do you make LEDs blink? New!


Use your browser's BACK button to return to this list if you need to look up another answer.




Q: What are LEDs anyway?

LEDs (Light Emitting Diodes) are those little colored lights you see in electronic equipment, household appliances, toys, on signs, and many other places. Red, yellow and green ones are the most common, since they have been around the longest. Other colors, like turquoise, blue, pure-green and white are much newer, so you may not see many of them around yet. But you will.
LEDs are different from ordinary light bulbs because they do not have a filament to break or burn out. They generate very little heat, and are ideal for putting lights into battery-operated equipment like telephones, toys, and portable computers.

An LED is basically a really fancy diode. Diodes only let current (electricity) to flow in one direction and not the other. LEDs are diodes too, but they have the unique "side effect" of producing light while electricity is flowing through them.

In the simplest terms, an LED is made with two different kinds of semiconductor material: one type that has too many free electrons roaming around inside, and another that doesn't have enough. When an electron from one material (the donor) gets pushed across a thin barrier and gets into tiny spaces in the other (the holes), a photon or particle of light is produced.
The color of light depends on a number of factors, including the type of material they make the LED with and the material's quantum bandgap (how much energy each electron needs to pack in order to cross the barrier).
A smaller bandgap that fairly weak electrons can cross gives you infrared or red light, while a large bandgap that needs really strong electrons gives you light that has a blue or violet color to it.

Things that go on inside of an LED are a little more complicated than this, but you get the idea.

Here are some graphics that show basically the inside workings of most common types of LEDs:



Cree-type LEDA mystery LED
Top left: Inside workings of a Gallium Nitride blue or green LED.
Top center: Insides of an ordinary red LED.
Top right: An obsoleted custom model red LED (a Fairchild FLV-104), circa 1973.
Below left: Cree gallium nitride (blue or green) on silicon carbide substrate.
Below right - a very mysterious red LED that showed up in a garbage parts bag.
Not unlike the FLV-104, this one shows an unusual split triad die (emitting surface).

Different LEDs will have differing shapes and configurations of die cups and leadframes that support them; these variations aren't depicted. There are also other variations of LED chips, and case styles; they are not depicted here for simplicity's sake.



Q: What are the different wavelengths & colors LEDs come in?

A: Years ago, you could only find them in infrared, red, yellow, and yellowish-green. Now, as of just a matter of DAYS ago (early December 2001), it can now be said that LEDs come in every color of the rainbow and in the invisible regions at both ends!
So here is a listing of known available LEDs, sorted by wavelength (in nanometers). The color description (for visible models) is as you would see the beam shined on a white surface a foot or two away.

NOTE: There is no guarantee that your eyes will "see" color the same way mine do. Use this as a general guide only.

Values for Vf (forward votage) are for each color; these assume 5mm through-hole LEDs in all cases.
If (forward current) is 20mA unless otherwise stated.

  • MID-INFRARED
    4600nm
    4300nm
    3800nm
    3600nm
    3300nm
    3100nm
    2900nm
    2700nm
    2300nm
    2200nm
    2100nm
    2000nm
    1900nm
    1800nm
    1700nm
    1600nm

  • LOWER-INFRARED
    1300nm; Vf=1.0 volts
    1020nm
    980nm
    960nm
    950nm
    940nm; Vf=1.5 volts
    920nm; Vf=1.5 volts
    905nm
    880nm
    870nm
    850nm nearly invisible, a very dull red glow can sometimes be observed when viewed directly; Vf=1.5 volts
    840nm nearly invisible, a very dull red glow can sometimes be observed when viewed directly; Vf=1.5 volts
    810nm nearly invisible, a very dull red glow can sometimes be observed when viewed directly; Vf=1.6 volts

  • NEAR-INFRARED
    780nm very dim cherry red when viewed directly; Vf=1.7 volts
    770nm dull, deep cherry red when viewed directly; Vf=1.7 volts
    740nm deep cherry red; Vf=1.7 volts

  • RED
    700nm deep red; Vf=1.8 volts
    660nm pure red; Vf=1.9 volts
    645nm pure bright red; Vf=2.0 volts
    630nm 'HeNe laser' orangish-red; Vf=2.0 volts
    620nm distinctly orange-red; Vf=2.1 volts

  • ORANGE
    615nm reddish orange; Vf=2.2 volts
    610nm pure orange; Vf=2.2 volts
    605nm amber; Vf=2.3 volts

  • YELLOW
    590nm sodium yellow; Vf=2.3 volts
    585nm yellow; Vf=2.3 volts
    575nm pure lemon yellow, bordering on becoming greenish; Vf=2.4 volts

  • YELLOW-GREEN
    570mm very yellowish green; Vf=2.4 volts
    565nm yellow-green; Vf=2.4 volts
    555nm yellowish lime green; Vf=2.4 volts
    550nm emerald green; Vf=2.4 volts
    540nm (none seen, no descriptor available); Vf=3.6 volts

  • GREEN
    530nm pure, non-whitish emerald green; Vf=3.6 volts
    525nm pure, slightly whitish green; Vf=3.6 volts
    515nm (none seen, no descriptor available); Vf=3.6 volts

  • BLUE-GREEN
    505nm greenish blue / turquoise; Vf=3.6 volts
    500nm greenish cyan; Vf=3.6 volts
    495nm turquoisish, slightly whitish sky blue; Vf=3.6 volts

  • BLUE
    475nm Bright, slightly greenish-tinted azure blue; Vf=3.6 volts
    470nm Bright blue; Vf=3.6 volts
    460nm Bright, less greenish blue; Vf=3.6 volts
    450nm Pure blue; Vf=3.6 volts

  • BLUE-VIOLET
    444nm Deep blue / violet-blue; Vf=3.6 volts
    430nm Whitish violetish blue; Vf=3.8 volts
    420nm Deep violetish-blue; Vf=3.8 volts

  • PURPLE (Phosphor-based)
    Color appears to be a bluish-purple; Vf=3.6 volts

  • VIOLET
    416nm Bluish-violet; Vf=3.8 volts
    410nm (none seen, no descriptor available); Vf=3.8 volts
    405nm Pure violet; Vf=3.8 volts
    400nm Deeper & dimmer violet color than 405; Vf=3.8 volts

  • NEAR-UV
    395nm Deep royal purple with reddish tinge; Vf=3.8 volts

  • ULTRAVIOLET - UVA
    385nm Dimmer royal purple with whitish tinge; Vf=3.8 volts
    380nm Whitish purple; Vf=3.8 volts
    370nm Nearly invisible, can appear a dull, deep purple when filtered with Wood's glass; Vf=3.8 volts If=10mA
    365nm Invisible - LED output appears as a very dim whitish violet
    350nm Invisible - LED output appears like the 365nm LED above, but dimmer

  • PINK (Phosphor-based); Vf=3.6 volts
    Color appears bluish-pink, hot "barbie pink", to coral pink

  • WHITE (Phosphor-based); Vf=3.6 volts
    White LEDs come in a wide color range from lemon yellow to purplish white. The most common appear on the target as a light bluish cross-shaped hotspot surrounded by a "overcast sky white" outer corona.
    Color temperatures for common types range from the low 4000°s to near 12000°K, with the most commonly found LEDs being in the 6500° to 8000°K range.



Q: I want to hook up an LED to a battery. How do I do that?

A: LEDs are easy to hook up. But unlike a regular light bulb, you usually need to put a resistor in the circuit to prevent your LED from becoming a crispy, smoking stinky black blob on the end of your wire.

For a single regular red LED and two "AA" cells (a common setup), use a resistor as close to 30 ohms as you can find. If you use 4 cells (6 volts), use a 180 ohm resistor. For 12 volts (such as in your car), 470 ohms will work nicely.
Wire the resistor in series between the battery and the LED; don't wire it across the (+) and (-) of the battery or other power source you're using; as it won't do any good there.
It doesn't matter where the resistor is put, just as long as it is in there (and in series with the LED) somewhere in the circuit.

A visit to Resistors Without Tears (my designation) can help you pick the right resistor for any LED you might want to use.

LEDs are also polarized - in other words, they will only light up when connected one way. If you reverse any of the wires, the LED won't light at all. Remember, LEDs are diodes, and those things only let electricity through in one direction.
In most LEDs, the longer of the two leads is the positive (+) side. If the leads are the same length, look at the insides. One part will be fat and kind of bowl-shaped; the other side is a lot skinnier. The skinny side is (+), the fat, bowl-shaped part is (-).

Note, that a few types of LEDs - usually certain types of red ones, the bowl side is (+) and the rod side is (-). But for most other types, the bowl side being (-) is usually the case.

This simple diagram shows how a typical LED should be hooked up to a battery.

how to wire an LED



When you're using the newer, super bright green, aqua, blue, or white LEDs, you should be careful to not shock them with static electricity or hook them up backward; these kinds of LEDs are more sensitive to this kind of abuse than any of the other colors. They should be kept in anti-static bags, like those metallic or pinkish ones that computer cards come in. If you don't have any of these bags, simply wrapping them in kitchen foil will do.
Although some of the newest ones are a bit more static resistant than they used to be, you can still blow them up if you aren't careful.



Q: I bought a white LED, but it doesn't work well on 2 "AA" cells. How come?

A: White LEDs need more voltage to light up than the kind of LEDs you might have monkeyed with in the past. A regular LED might light up fine at 2.4 volts, but a white LED needs 3.6 volts and sometimes even a little more. That's why they run so dim when you try to run them from just two batteries. Add a third battery (don't forget the resistor) and it will work fine.

Same goes for blue LEDs - even more so for those Radio Shack blue LEDs. Those things can sometimes need 4 volts or more to work really well, and they can sometimes be hooked directly up to 5 volts with no resistor! Try that with a regular LED, and prepare to plug your ears and sweep up the remains of it afterwards.

There are now some very small circuits that can be used to step up 1 or 2 batteries high enough to run blue or white LEDs.

As for which resistor to use: A white (or blue, aqua, or true-green) LED on 3 "AA" cells should work fine with a 50 ohm (or as close as you can find) resistor. With 6 volts (4 "AA" cells), 120 ohms; and for 12 volts (automobile, etc.) use around 420 ohms. If you can't find the exact resistance - and you probably won't for some of these - just pick one that's as close as you can find. Nothing will blow up if you miss the mark by 10 or 20 ohms.
For a single LED, a 1/4 watt resistor is fine, but you can use 1/2 or 1 watt if that's all you can find. The LED doesn't care.


Q: If I connect four 650mAh NiMH batteries in series, I will have 4.8volts at 650 mAh. If I connect four of these batteries in parallel I will have 1.2volts at 2600 mAh. Which setup would be better to power one LED while maximizing brightness and time?

A: Do it in series. Putting them in parallel gives the 1.2 volts as you found, but 1.2 volts isn't enough voltage to light up any LED. Don't forget the resistor. 50 ohms or so should work fine.



Q: I have seen the Nichia data chart and it says that max DC Forward Voltage VF[V] is 4.0volts at 20 mA for the blue, green and white LED's... so would it be bad to use 4.8 volts?

A: The resistor takes care of that. When you put a resistor in a circuit with an LED, the resistor has a voltage drop across it; effectively removing that "extra" voltage from the LED. If you try to run an LED without a resistor, the full battery voltage (4.8 in this case) will be present across the LED; causing it to use much higher current than normal, and blowing it out.



Q: I bought a flashlight you reviewed, but I can't find the "AAAA" batteries it needs.

A: You can buy "AAAA" cell batteries at most Radio Shack stores, or buy them online from a place like CheapBatteries.com or from the same outfit you bought the light from.

Coin cell batteries, like the kind used in some keychain flashlights, can be bought either from the place you got the light from, or from an internet electronics supplier like Hosfelt Electronics or All-Electronics. Both of these places also have mail-order catalogues if you don't have regular access to the net or don't have a credit card.

In an emergency, you can also find "AAAA" cells inside of Duracell 9-volt batteries, but beware, they can have a tendency to "pop" after you have the outer casing off. They may also need to be insulated (taped around) if you use them in a metal flashlight. Definitely for emergency use only.



Q: How do you power the LEDs when you test them?

A: I use a special programmable current-limiting power supply, but you can just use coin cells. Two watch batteries will do in a pinch, and the LED can be tested on them without a resistor. A good choice for casual testing is to keep a pair of CR-2016 lithium coin cells handy. For a red, orange, yellow or yellow-green LED, test it with just one of these batteries. For true-green, blue-green, blue, or white LEDs, use two.

Note: It's probably not a good idea to just leave the LED cooking on these batteries, but for short-term testing (a few seconds to a minute or so), no harm will be done.



Q: I want to use white LEDs for photographing or videotaping insects, plants, electronic parts, and other close-up subjects, but all of the white LEDs I've tried have this blue circle in them that ruins the picture. Any suggestions?

A: Try using Nichia's rectangular white model, NSPWF50S. This LED has a very wide, even beam that doesn't have that obnoxious blue ring in its beam. Since all white LEDs tend to have a bluish cast on film or videotape, you may need to adjust your camera's white balance or even use an orange-tinted filter to compensate.
The beam angle is very wide, around 140 by 120 degrees, so they won't be very good much over 1-2 feet away from the subject. They should work great for close-ups (a foot or less) though.

You will probably have to buy these directly from Nichia, since electronics places don't seem to carry them yet. I have some info on my Where To Buy LEDs page.


Q: I want to take my "AA" LED flashlight with me on winter activities, but the batteries just don't handle the cold. What should I do?

A: Use lithium batteries. Energizer makes a lithum 1.5 volt "AA" cell, model L91. They're expensive - around $2.50 apiece, but they work well in temperatures that alkaline batteries quit working in. They should work reasonably well down to 30 or 40 below zero.
You will notice after awhile that your flashlight may have a funny odor inside of it; this is perfectly normal when using this kind of battery.


Q: Where the heck did all these blinding blue LEDs start coming from?

Shuji Nakamurua
A: You have this man to thank for them. Back in the early 1990s, Shuji Nakamura was the top research and development guy for a small Japanese chemical outfit called Nichia (NEE'chee'ah). They made phosphors for TV tubes and fluorescent light bulbs, among other things. He was the one who figured out how to deposit certain types of chemicals onto artificial sapphire that led the way for today's blindingly bright blue, blue-green, pure green, and white LEDs.

Nichia Chemical (now Nichia America) leads the world in fabricating the best blue & green LEDs and the only commercially successful blue-violet semiconductor lasers.

In late 1999, Shuji took his show "on the road" by taking an engineering position at the University of California, Santa Barbara to work on new designs for his gallium nitride LEDs. But Nichia America is still where to turn for the best and brightest nitride-type LEDs.
For the record, Nichia also manufactures phosphor-based white LEDs. Those things use a blue LED chip coated with a special phosphor like that inside your TV picture tube.

Scientific American has an article in their August 200 edition which you may find worth reading.
Shuji's picture was taken from that magazine for educational purposes, not to make a profit. :)


Q: I want to build a circuit to pulse LEDs or drive full color LEDs. Help!

A: This is one type of question I don't have the answer for. I have neither the resources nor the knowledge to construct or design electronic circuits using LEDs. Look for newsgroups catering to electronics hobbyists or look in electronics hobbyist magazines like Poptronics or Nuts & Volts.
Sorry about that. I deal with just the bare LEDs themselves, not hobbyist or experimentor's circuitry for driving them in fancy ways like chaser lights, pulse width modulation power supplies, or PIC-based RGB drivers.


Q: How do those white LEDs really work anyway?

A: Before the days of super bright blue LED chips, white LEDs were actually made with four seperate LED chips enclosed in a single LED body: a red chip, a yellow-green chip, and a pair of light blue silicon carbide chips. If you screwed with the current going to each chip long enough, you could get the LED to emit a dim but reasonable approximation of white.

Nichia's bright blue breakthrough in the mid 1990s changed all of that.
Today's white LEDs are made with a single blue LED chip that has been covered with a special material. This material (called a phosphor) glows a yellowish color when exposed to the blue light from the LED chip.

As you can see in the picture to the right, this phosphor appears as a dull yellowish junk covering the surface of the LED.

The overall result is a super bright LED that produces a white to bluish white light. Generally speaking, you treat white LEDs exactly the same as blue ones. They have the same voltage and current requirements, because they use an almost identical chip as the blue LEDs do.

For the more technically minded, the best type of white LEDs generally appear at coordinates 310x by 320y on the standard CIE chromaticity chart, and have a color rendering index of 85 or greater. Color temperature can range from 5,500°K to as high as 8,000°K, with a few odd samples going even higher. Generally, a good white LED will have a color temperature of 6,500°K, which is approximately the same as noonday sun at the Earth's equator.



Q: How do I change those yucky yellow-green LEDs in my Nokia phone to blue ones?

A: This requires a little skill and patience to do, but it can be done. The best way to describe this is to let someone else do it.
Go to B.B.'s Nokia Pages / Blue LED section to find out how to change LEDs in your phone. This page shows you everything, including what kind of LEDs you should look for, how to take your phone apart, and how to put it back together again without having screws left over.

Sorry, but I do not stock the type of blue LED used in this retrofit. These people do, however:
ALL4CELL.COM
They also stock blue SMD LEDs in the 0603, 0805, and 1206 case sizes.
They perform installations, and offer help for do-it-yourself types.



Q: How should I change the taillights in my car or motorbike with LEDs?

A: This kind of LED application is a bit beyond my league. First, you would need to know (or find out) the D.O.T. standards for vehicle lighting in your state, then calculate how many LEDs would be needed to meet this speficiation. Remember, when you put the taillight lens back on, the light output will drop, so you will have to find some way to measure or compare an LED replacement with the lens in place.
This is not my area of expertise. The only suggestion I have is to hit up a search engine like Yahoo or Excite and punch in some keywords like "motorcycle LED red" or "taillight LED" and see what comes up. Somewhere you should be able to locate listings of manufacturers who actually do this for a living, and they would know the applicable rules for vehicle lighting. You could either buy their product directly, or ask them some questions (ie. how many, how bright, etc.) so you could make your own.



Q: Where do I get 2mm x 5mm rectangular blue LEDs?

A: LEDTronics (http://www.ledtronics.com) has these rascals.
A data sheet on them can be found at http://datasheets.led.net/Pages/430nm_ultra_blue_discrete_leds/53b.htm. Contact Jordon Papanier (jpapanier@ledtronics.com) at LEDTronics if you need any additional info.



Where can I find blue SMD LEDs (size 0603) for my cell phone?

A: In small quantities, I'm not really sure. But ISP Korea sells these things - see my SMD LED page for a bit of info on this component and a link to the supplier.
These put out 55mcd at 470nm in a very wide 110° beam. As always, wider beams always mean lower mcd numbers.

You can also get these at all4cell.com
They stock blue SMD LEDs in the 0603, 0805, and 1206 case sizes.
They perform installations, and offer help for do-it-yourself types.



I need a 780nm near-IR LED, but I've never seen them anywhere. Do they even exist?

A: 780nm is a very unusual wavelength to find in an LED. This is usually only found in small diode lasers, like the kind in old-fashioned CD players. Nonetheless, there is a 780nm LED out there. Go to Roithner Lasertechnik and check their IR stuff out.
I have one of these on my IR LED page if you're interested in seeing the beam profile and other basic specs.



Where can I find UV (ultraviolet) LEDs?

A: Right now, the only place to buy these is directly from Nichia, their manufacturer.
Expect to pay at least $33 apiece in small quantities (under 10 pcs.) and expect to see a small amount of paperwork, related to non-disclosure and liability. I have fairly complete contact info for Nichia on my Where To Buy page, or you can just fire off an e-mail to info@nichia.com.

As an update, as of December 2001, near-UV LEDs that emit in the deep violet part of the spectrum have become available. See my Violet LED and UV LED pages for additional information and sources.

As of 02-13-08 you can also obtain Fox Group 350nm and 360nm LEDs from DComponents Corp.





How come blue and white LEDs have two little wires inside, and red LEDs only have one?

A: "Regular" LEDs like red and yellow are made from a chip of junk like gallium arsenide phosphide, which conducts electricity. So when they make the LED, the reflective silver cup makes up one of the two connections; the current flows through this chip, through the P-N junction layers (the thin part that actually creates the LED's light), and finally out through the thin wire on top of the chip.

White LEDs - which are really just blue LEDs in sheep's clothing - are made using a chip of artificial sapphire, which is an insulator. So they have to be made differently. The active material of this LED is actually just a thin layer of chemicals on top of this slab of sapphire. One wire goes to the top layer of this (much like the wire in a red LED does) and is usually placed at one corner, and the other wire goes to the opposite corner that is stripped of everything except a conducting layer a few tens to perhaps a few hundred atoms thick.


But since the whole working LED ( a *very thin* critter indeed!) is built entirely on top of this insulating sapphire chip, the "bottom" (the conducting layer just above the sapphire) in this case is still on the top surface of the sapphire chip itself.



What are the LuxeonŠ bin codes?

LuxeonŠ LEDs are sorted into "bin codes" that determine brightness and color tint, which are explained in more detail on this website.



How do you measure laser power without a laser power meter?

First off, you should use a solar cell with a matte dark brown finish, ***NOT*** the kind that has a shiny, "fractured" blue appearance to it.

Connect the solar cell directly to the DMM, set the DMM to a low milliamp scale, irradiate it with the laser while slowly waving the laser beam on the face of the cell, and observe the DMM's display.
Note the highest reading you see, and apply the following mathematical formula to it:
Current in amps as shown on the DMM multiplied by 1239.7 divided by 532 divided by 0.97. The result should be your laser power in milliwatts.

The "532" in this formula should be substituted with the laser's wavelength in nanometers; this will allow you to measure laser power at any visible wavelength.

This method isn't "laboratory" accurate, but it will get you in the ballpark: +-5% or so anyway.



How do you make LEDs blink?

From my email, comes this information furnished by D.H.:

I highly recommend "Arduino" at http://arduino.cc - that project has a standard board (which is available in many form-factors) and standard software. It's easy to program, and there are forums for help...

Any Linux, Windows, or Mac (os x) with USB port and a $35 board is all you need, and you have full-microcontroller programming avaiable.

Instructions for connecting an LED (with resistor to limit current) are all included in one of the "sample" programs for blinking a led... and you can directly blink up to 20 LED's trivially.... so do more, you need additional driver chips... but that all can be built on top of the existing basic arduino...

I personally use the RBBB from http://moderndevices.com.




Have a question that isn't answered here?
Then go ahead and ask on my e-mail hotline.








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