Introduction: How to Use LED's

Picture of How to Use LED's


I've been noticing that a lot of people (not people on instructables, but people interested in electronics in general) don't really know about LED's. No one in my science class at school had ever even heard of them. This is an easy introduction to LED's that should teach you the basics. It's good if you're interested but have no idea where to start. I also thought this would help out anyone wanting to enter the contest, but it took me a while to finish.

Remember, (As always) feedback and ratings (positive or negative) are always appreciated!

Step 1: Name/Background Info

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LED's. So common on Instructables. What the heck are they?

LED is an acronym for Light Emitting Diode. Well, you ask, what on earth is a diode.

A diode is a device that, in simplest terms, allows electricity to flow through one way but not the other. Those of you who are knowledgeable about mechanical things could think of it as sort of a check valve. If you have no mechanical knowledge, disregard that last sentence.

Now that you know what a diode is, an LED is just one that emitts light (But you could probably figure that out just from reading the name).

Step 2: More Important Information

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Another important thing about all LED's (and all diodes) is that every one of them has exactly two electrodes. These are important to know when you are wiring an LED into a circuit. They are the...

Anode - The p-side which is the longer leg. And the...

Cathode - Which is the n-side and shorter leg.

Since you know these terms you can remember that electricity flows easily from the anode to the cathode but not the other way around.

Step 3: Advantages of LED's

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LED's are great for many reasons. First of all, they don't heat up like regular lightbulbs do. This is great because, well, you don't burn yourself. They are also smaller than a lightbulb. Another important thing about LED's is that they run on very low amounts of electricity, which is helpful because it makes them safer to work with (you don't electrocute yourself). Most run on about 20mA.

Step 4: Tips

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Just like with everything, there are some tips that are helpful to make sure your LED's work well.

Clip the leads - Simple, I know, but people forget to. This is important because it prevents them from bumping into other parts and messing up your circuit.

Remember which electrode is which - This is a big one because if you don't it won't work at all. It's a diode; current only flows through it one way.

Read the package - Simple again, but each LED requires slightly differnt voltage and ampage.

Step 5: Resistors

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It always helps to wire a resistor into your circuit. It will make the LED last longer by dropping the voltage. There are some sites that make it easy to find which resistor you need. My favorite is here.

Step 6: Project Ideas

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Now that you have some knowledge of what LED's are you might want some project ideas. It's easy. Anything involving light. And it's not even limited to that. You can check out the LED Contest for some ideas.

The Instructable that I want to enter into the contest is a "Reminder Table" which is a little table with a number of sections (mine has four) that you put things in to remind you to do them. Each section lights up a different color. The idea is that you turn each light on when you have something that you need to remember to do, then off when you accomplish that. I want to use it to remember my practice cards, guitar picks etc.

One of my favorite ideas in the contest is the Star Wars Blaster, which opens up a whole other area of project ideas.

Step 7: Now You're Ready!

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With these "Tool Tips" (sorta) and a completed LEDucation (sorry, couldn't resist) I hope you can get out and build some cool LED projects. Remember, I really do appreciate any type of feedback or ratings.


Dennis Lyman (author)2014-04-15

Hi, I recently picked up an antique light bulb with a crusifix inside the bulb. Unfortunately, it does not work. I would like to modify the bulb and add an internal led. Any suggestions would be greatly appreciated. Denny

Mike_MK (author)2011-09-15

A few tips;
Often LED's have equal length 'legs', so to work out which one is which, look closely inside the plastic with a mag glass, the negative is the side with the bigger surface area, or looks like an upside-down 'L'.
The power supply is often 12 volts DC (a battery, or transformer). But can be 3 volts to about 30 volts DC, (Normally.)
The current limiting resistor is very necessary, if you want the LED to last any time, and a proven value to use with 12 volts, is 470 ohms. This will limit the current to 0.025 Amps, i.e. 25mA. This is using the common formula from 'Ohm's law', V/A=R, so in this case, 12/0.025=480 ohms.
It is best practice to fit it between the Positive of the LED and the power supply's positive terminal.
LED's can be bought in various shapes, round, square, rectangle, and in various colours, red, green, yellow, orange and blue. Hope the photo helps.

Elephants Are Fat (author)2009-03-25

do you where you can get a large amount of LED's?

i bought 100 with 100 resistors on ebay for about $2.00, and after shipping (hawaii) it was $7.00 they seem to work fine too- and if you dont want to buy online, you might be able to try LED christmas lights :]

go to the store dude.. u will find large amount of LED's..ok..
has very low prices on l.e.d.s compared to other stores.

indybob (author)2010-11-18

im interested in making some grow lights u sing led's any suggestions,, or books to get,,, indianabob

J3PPiSH (author)2010-08-06

Thanks I really learned from this!

mr_bandit (author)2008-11-23

The resistor is to limit the CURRENT, not the voltage. The two things this article is lacking is a schematic and a simple picture of the circuit soldered up. The long lead is on the voltage plus(+) side and the short on the minus (-) side. The resistor is in series with the LED, on either side of the LED: (+) --- resistor --- long lead --- short lead --- (-) or (+) --- long lead --- short lead --- resistor --- (-) A resistor in the range of 1K to 10K is usual. Note some LEDs connected directly to a watch battery will work without a current-limiting resistor, but it is iffy and not good practice. (more in-focus pictures would be nice ...)

NiftyJunk (author)mr_bandit2009-10-16

Yeah, its that little flower button on the camera. You know, for close ups. I was so happy when I found out what that little function does.

Schisler7 (author)2008-02-04

How much are LEDs from Radioshack? Is there any better place to get them?

kadris3 (author)Schisler72009-09-30

send me an s.a.s.e. and I'll send you some. see below. Uncle Cy

Swishercutter (author)Schisler72009-06-01

Ebay...I use Digikey for new specific parts but for the price you can't beat ebay. In my case the person sent the resistors (5 band precision ones) for free when I bought the LED's. Not that resistors are expensive but it's still nice to not have to buy some.

burzvingion (author)Schisler72008-02-05

It's usually only prudent to buy anything from radioshack if you absolutely need it immediately and there are no other brick-and-mortar stores in your area that will sell said item. Almost anything you buy there, from LEDs to batteries to plastic project enclosures, will be marked up around 500%. It's a little bit like buying groceries at a gas station, except worse, because it happens to be the only gas station in town.

Buying parts online from wholesalers and surplus outfits is usually the cheapest way to go. Standard LEDs can be had for around 5 to 10 cents each, while high brightness ones will usually be a little more. Blue/violet, white, and UV LEDs are the most expensive, but can often be found for $0.75 or less each. Some places to look include:
Then theres eBay. Search and you shall find. There are a lot of good deals and hard to find items around on eBay that most people never think to look for.

kadris3 (author)burzvingion2008-04-05

if any student is broke, but wants a few leds to play with, i would be happy to send them to him. send me a stamp and some of my slow movers, pulls, or drops will be sent for your playing pleasure. they will all light when they leave here. when an led hits the floor, i cannot tell which bag it came from. i know the color, but not the intensity. please tell me your grade or yr in school. i'm happy to help but don't appreciate getting lied to or ripped off. also on the website is ohms law instruction fr figuring resistance. questions are a click away under "ask uncle cy". u don't need a bundle of money if u know an old fart like me!

Gjdj3 (author)burzvingion2008-02-05

I normally use digi-key but I had this LED left over from an earlier project.

psycholily (author)2009-01-03

so... if i have, say, solar LED path lights, and I want to swap out the white LED for an amber one, I need to..... do something? because they're different? aargh. can anyone tell me? (also, this has been so useful to me!)

kadris3 (author)psycholily2009-09-30

on a 3 vdc supply ,you need a 47 ohm resistor.Uncle Cy

Gjdj3 (author)psycholily2009-01-03

It shouldn't be too hard to do. Amber and white have similar voltages I think, so you could probably just go right ahead and swap without any resistor. On the other hand though, if you don't want any risk of frying the LED you could just try adding a small resistor to the positive leg of the amber LED.

psycholily (author)Gjdj32009-01-04

thanks. so what am I looking for when I go to get the resistor? what value? (sorry, annoying questions, and I don't even know if i'm asking the right ones) the LEDs available to me in white are 1: 3mm 1500mcd 3.5v DC @ 30 mA 2: 3mm 5000mcd 3.4v DC @ 25 mA 3: 5mm 2000mcd 4.0v DC @ 30 mA 4: 5mm 8000mcd 4.0v DC @ 30 mA 5: 5mm 16000mcd 3.0v DC @ 20 mA and lord knows which type will be in the light in the first place. the voltages are all different. then the amber ones are: 1: 5mm 6500mcd 2.0v DC @ 20 mA 2: 5mm 7000mcd 2.0v DC @ 20 mA the lights run on AA batteries, so I assume there's already some sort of resistor in there? sorry, I'm totally confused, I think i'll give this up as a bad job and just leave the LEDs the colour they come to me as -_-;;

Gjdj3 (author)psycholily2009-01-04

Well If you were going to replace white with amber it shouldn't be too hard. There are three resistors that would probably work with the amber light. I would start with the highest first so that there's no risk of burning out the LED. Then, if it doesn't work you can try the next lower resistor. The three that would probably work would be... A. 100 Ohms B. 75 Ohms C. 51 Ohms Just solder the one you choose to the positive leg of the new LED. And remember, if you can't find an exact match on the resistor, just choose one close to it.

psycholily (author)Gjdj32009-01-04

Thanks so much! I'll give it a shot, and if mishaps occur, well, LEDs are cheap.

Gjdj3 (author)psycholily2009-01-04

Cool! Tell me how it goes.

grantdevine (author)2008-12-28

So do I need to us a resistor when only attaching one LED or is it only a reccomended thing or is it totally unnesecary

Gjdj3 (author)grantdevine2008-12-28

It depends on the power source and led. For example, 3V to an red led wouldn't require a resistor. On the other hand 9V to the same led would need a resistor otherwise the led would fry.

kadris3 (author)Gjdj32009-09-30

red LEDs usually have a nominal forward voltage of 2.1 vdc. you do need a current limiter. 47 ohms works well here. for an extra measure of safety you could use a 55 ohm or slightly larger. the higher you go the further you get from speck brightness. I hope this helps. Uncle Cy

skrubol (author)Gjdj32009-06-04

What sort of red LED's are you using that work with 3v? Vf of normal red LED's is 1.8v I believe. Some LED's have built in current limiting resistors, meant to run them at 3 or 5v, which it sounds like you have. Normal red LED's will last seconds at 3v or pop right away. You really should learn some more about how LED's work before posting information about them.

McGrep (author)Gjdj32009-06-03

Normally, I have a basic understanding of electrical components, however LEDs are especially not my cup of tea. That's why I ended up eradicating the few LEDs I had... I should have stopped with the 9V batteries when they started shorting out... that was not a brilliant idea on my part. I had dissected an old universal remote and burned up the visible LED, while using an infrared LED (with a lower voltage) connecting the cathode to the positive terminal and anode to the negative. I've got a ways to go!

grantdevine (author)Gjdj32008-12-28

That would explain why my LED's have a tendency to explode on me XD

theRIAA (author)2008-02-04

the only thing i've never figured out is, what mA is the LED running at if I don't use a resistor? is it calculable? how do you run resistors in parallel, and do you have to know the mA to drop voltage? and where can I get some of those sweet blurry LEDs?

kadris3 (author)theRIAA2008-12-05

all single LEDs draw 20ma, unless they are a special low amperage one. the voltage varies with color. think of the resistor as a current limiting device. old ohms law applies. e=ixr. e is volts. i is amps. r is resistance. since i has to be in amps, 20ma is .02. for red, yellow,amber, and orange use 2.1 volts. for white,UV,
blue and green use 3.4 volts. IR are by themselves and about 1.2 vdc. as an example 3 white leds in series are 10.2vdc. if a 12vdc supply is used 1.8 vdc, left to be dissapated, /.02 =90 ohms. 100 works great. for 13.8 vdc use 180 ohms. for a brighter light parallel up several strings. best of luck to all. Uncle Cy

wccolvin (author)kadris32009-09-29

I am confused,I thought you had to take your Source Voltage of 12 Volts and Divide it by your .020 Amps of Current to get the Resistance to supply the correct Current through your Series Circuit.that would be 12 Devided by .020 which equals 600 Ohms Resistance.I am also confused on the Power is equal to Source Voltage times your Circuit Current.So if I have a 3 Volt LED with .700 Amps of Current that would give 2.1 Watts of Resistance.That just seems like a large Wattage for only 3 Volts.Could you please help me understand where I am getting Confused.Thank you very much,Sincerely Charles.

kadris3 (author)wccolvin2009-09-30

hi Charles,
thanks for your questions. first watts is power not resistance. a 3vdc led that draws .7 amps is indeed a 2.1 watt burner.
my intent was to determine the value of the current limiter. you only use the difference between supply and forward voltage for the LEDs. if you have 3 white LEDs in series that's a forward voltage of 10.2 vdc. if the supply is 12vdc then 12 -10.2=1.8volts. 1.8 vdc / .02=90 ohms. 100 ohms would work perfectly. if the supply is 13.8 then 180 ohms is needed.
think of the string of LEDs as one entity. it is limited to .02 amps (20 ma). we only need the difference between the supply and how many volts the string of LEDs "eats". we are not figuring the resistance of the whole string, only the missing part.
P=E X I. no resistance in the formula for power. you seem to have ohms law down.
you are trying to figure the resistance of the entire string which is not what we were intending to do. you only need to figure the missing part which is easy to calculate. we know the current (.02 - limited by properly biased diodes) and the supply minus the total voltage of the Diode string. this gives us the voltage of the unknown value we seek. then basic ohms law and we have the needed resistance of the current limiter.
if you are still confused please let me know and i'll try again. thank you

skrubol (author)theRIAA2009-06-04

No, you can't calculate the current from the voltage you feed to the LED. Anyone who tells you otherwise is incorrect and setting you up for failure. You need a resistor or something to control the current to the LED, it will not run at 20mA if you just give it some voltage. If you give an LED a constant voltage, at best its light output will not be stable (as it warms up it will get brighter,) and at worst it will fail. You also won't be able to get the maximum output out of it without risking killing it. Also not all LED's are 20mA. I've got an LED that uses 2800mA, and several in the range of 350-1000mA. These are a different class of LED (power LED,) but there are all different sizes (both physically and power handling) of LED.

Gjdj3 (author)theRIAA2008-02-04

Lol. Blurry LEDs.

About the amperage. I think the best explanation I heard was at this site.

E D E N (author)2009-07-23

So, how exactly do you find the mA of an LED? Is it just from general assumption or like an actual measurement from the LED ?

Gjdj3 (author)E D E N2009-07-23

Different types of LEDs will have different mA ratings. You can generally tell by looking on the package.

E D E N (author)Gjdj32009-07-25

Alright, thanks

iain0070 (author)2008-10-19

how do u tell which is positive if you have salvaged it and both ends r th same length?

Gjdj3 (author)iain00702008-10-19

The negative side should have a flat edge and the positive side should have a rounded edge.

Swishercutter (author)Gjdj32009-06-01

You are correct. The cathode should have a flat edge. When I was working in industry (electronics manufacturing) the assemblers were told not to trust the flat side or the longer leg. So how did they tell? If you look inside the LED you can see one side is larger and looks a bit like a "flag", this side is the cathode...the flag points to the positive. So if you have salvaged parts this is a good way to tell for sure, especially when the leg is cut or the case is embedded.

Rotten194 (author)Swishercutter2009-07-08

Actually, some LED's are backwards in this respect. Most aren't though.

odonnghaille (author)2008-12-23

You can also tell the Cathode by the flat rim as oppose to the curve on the plastic above the legs.

skrubol (author)odonnghaille2009-06-04

Also, generally if you can see the interior of the LED, the cathode is usually attached to the cone shaped mirror part. Neither of these are rules though, and I've seen LED's in similar packages with no flat, and with the anode connected to the mirror.

Gjdj3 (author)odonnghaille2008-12-24

Yep, and the metal inside is a different shape for each side.

xCeasarx (author)2008-05-28

soooo...can u like take a LED from anything like a ps2 controller or cable box annnndddd...can u connect it to anything with power or energy that it can handle

vpbotbuilder (author)xCeasarx2009-06-01

Yeah, you can also use a resistor which resists the amount of current that fllows through the LED so that it can last longer

Gjdj3 (author)xCeasarx2008-05-28

Yeah, pretty much. Just don't overload it on the power or it will burn out.

stepe (author)2009-02-14

Hi! Thanks for all the great info no LED!! Unfortunately, I am new to both Instructables and crafting in general, so I have one question..I know it is silly, but..well, I would like to know where you connect LED lights to, I mean what would be the power source.. Thanks in advance!

vpbotbuilder (author)stepe2009-06-01

its not silly, when i first started using LED's, i didnt know either. You would simply connect the LED to a battery.

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