Introduction: Practical Guide to LEDs 1 - Pick Your LED!

Picture of Practical Guide to LEDs 1 - Pick Your LED!

You've heard about LEDs. Chances are you've already tinkered with them. But there are so much details you probably don't know about. Sadly the resources available are often incomplete or just unpractical. This guide takes you all the way from a beginner level to adept skills!

This is chapter 1 of a short series. Use the table of contents below to browse the content I've already published.

In this chapter we'll focus on how to choose the right LED for your project. Along the way you'll learn a few nifty things that'll come in handy later on.


  1. Pick your LED!
    1. Brief Introduction to LEDs
    2. Polarity
    3. Properties & Units
    4. Colors
    5. Appearances
    6. Power Rating
  2. Essential Circuits
  3. Switching & Dimming
  4. Matrix & Multiplexing (new!)
  5. High Power & Lighting

Step 1: Brief Introduction to LEDs

Picture of Brief Introduction to LEDs

LEDs are everywhere, and not without reason. Compared to traditional light sources they are highly efficient, resulting in less power consumption and less heat for the same brightness. They are available in all sizes, whether it be a tiny package suitable for mobile devices or in large clusters to illuminate rooms. Instead of fading on and off like lightbulbs they can be switched instantly, making them suitable for displays and precise brightness control. Powered correctly, they can last for decades without replacing.

All those benefits come at the cost of increased complexity of the circuit. The term "LED" is short for "Light Emitting Diode" and as all semiconductor based components they show highly non-linear characteristics. This means you can not treat an LED like an ordinary lightbulb and connect it straight to a power supply!

The good news: The circuit required is not that complicated either.

The circuit you will build must be tailored to the LED it is supposed to drive, so it is wise to pick first which LED is best suited for your project.

Step 2: Polarity

Picture of Polarity

LEDs, like any other diode, allow current flow only in one direction. This is why you need to pay close attention to the polarity of every LED. The anode needs to be connected to positive (+), the cathode to negative (-) potential. There are several ways of identifying the polarity (follow in order):

  1. Check the datasheet, if you have any
  2. The cathode has a shorter pin
  3. The case at the side of the cathode is flat
  4. There's a '+' or '-' mark somewhere close to a pin
  5. The cathode is usually the bigger metal part inside of the LED for manufacturing reasons

Very small LEDs have other forms of marks, such as tiny dots, printing on the bottom or asymmetric cases. Unfortunately this is not standardized so you have to read the datasheet.

If you are still not sure you can always connect a CR2032 battery directly to it and try it out! In fact this is a popular way to build simple projects such as LED Throwies. This works because a CR2032 can not provide enough power to destroy an LED.

Step 3: Properties & Units

Picture of Properties & Units

For your design you may require to pick specific LEDs and develop a circuit for them. Datasheets often contain a ton of information so it can get hard to pick out the most important bits. Here's a short overview about the key parameters:

NameSymbolUnitExplanation (Summary)
forward voltageUFVtypical voltage across the LED when IF flow through it
forward currentIFArecommended max. current through the LED
peak currentIPEAKAmaximum current for a fraction of a second
reverse voltageURVmaximum voltage when connected in reverse without damage
viewing angleφ°the width of the light beam
luminous intensityIvmcdbrightness of the brightest spot
luminous fluxFlm (lumen)amount of light produced
dominant wavelengthλDnmmeasure for the dominant color of the light
color temperatureKindicates the 'warmness' of a white LED
color rendering indexCRIindicates how natural colors of illuminated objects are

Step 4: Colors

Picture of Colors

To get colored light with traditional bulbs a color filter (such as dyed glass) is required.

LEDs don't need such a filter, they produce only one wavelength (= color) in the first place. The color depends on the type of semiconductor used. Different semiconductors have also different electric properties, most notably the forward voltage.

The forward voltage can vary a lot, so it's always best to look in the datasheet for the exact values. When no data about the LED is available, the forward voltage can be approximated with the following table.

ColorTyp. forward voltage
Bright Green3.2-3.8V

You should never connect an LED directly to a voltage source, even if the voltage is identical to the LEDs forward voltage. Read "Chapter 2: Essential Ciruits" to learn how to connect an LED properly.

Step 5: Appearances

Picture of Appearances

LEDs come in a huge variety of styles. We'll take a look at the most common features:

Mounting Types
There are three common way in which LEDs can be mounted:

  1. Through Hole (THT)
  2. Surface Mounted (SMD)
  3. Screw mounted or thermal adhesive

THT components might be what you are most familiar with: Their pins go through holes in a PCB and are soldered on the other side.
SMD parts are soldered directly to the surface of a PCB. With this method they can be made much smaller than THT parts and are perfect for tiny projects, such as wearables. On the flipside they are more difficult to solder.
Some LEDs, especially power LEDs are mounted on an aluminum or ceramic substrate to improve the cooling. They are meant to be mounted with either screws or a thermal adhesive.

The LED chip itself produces a wide beam of light (about 120°). To make it suitable for spotlights a clear dome is placed right above the chip to act as a lens. Some LEDs, so called diffused LEDs, don't have any beam at all, they evenly scatter the light in all directions. This is very useful for indication LEDs as the light is visible from any direction.

Multi-Color/ RGB LEDs
You may have seen LEDs with multiple chips for different colors into one case. With such LEDs you can not only select the color you want, but also mix the light to get any color in between! We'll come back to this in "Chapter 3: Switching & Dimming".
Often it is not possible to have two dedicated pins for each LED chip, in such case either the anodes or cathodes are connected internally to only one common pin.

LED Matrix
Similar to Multi-Color LEDs these contain multiple LEDs in one package, but usually all the same color. They are also available as both, common anode and common cathode configurations. They can be used to display text or numbers and are readable from quite afar. However the high LED count usually requires a micro-controller and multiplexing. In "Chapter 4: Matrix & Multiplexing" we'll dig deeper into this.

7 Segment Display
While numbers can be displayed with a simple LED matrix, a seven segment display provides a much more elegant solution with way less LEDs you need to control. Alphanumeric displays are based on the same idea, but with a total of 14 or 16 segments they can show in addition all uppercase English letters. Electrically this kind of display is identical to the common LED matrix.

Integrated circuit
No matter the light effect you want, with some effort everything is possible. For the most common effects, such as blinking or color cycling, you don't even need to invest that much time. Some LEDs come equipped with a tiny circuit to do just that.
A very special LED is the WS2812/ WS2812B, sold as "Neopixel" by Adafruit. The chip inside takes serial data from a micro-controller (such as an arduino) and controls the RGB LED to match that!

No matter which LED you decide to get, check the datasheet briefly, so you don't miss anything important.

Step 6: Power Rating

Picture of Power Rating

The most common LEDs are made to handle around 20mA. Depending on their purpose LED with different current requirements are also available.

Low current LEDs are specifically made for battery powered devices, such as wearables. They are made to be highly efficient, even at 2mA they are still fairly bright. Be aware that even slightly larger currents might damage them.

When normal LEDs can't provide the amount of light you need, consider stepping up to high power LEDs. The very common 1W LED delivers as much light as 20 5mm THT LEDs at a fraction of the price. If that's not enough there are modules with dozens of LEDs in one package rated at a up to 100W total!
All that light comes at a cost, though: It will get hot. So hot that it will damage the LEDs (and burn your fingers). An appropriate heatsink is required for cooling . Be sure to get LEDs pre-soldered on an aluminum PCB for easy mounting. Driving the LED gets also more complicated. The circuit must not only be rated for higher currents and voltages, but it also often requires its own cooling. The whole 'Chapter 5: High Power & Lighting' will be dedicated to this topic.

Quite often you'll see circuits which exceeding the LEDs current rating to get a little more light. While this might work well for some time it will reduce the LEDs efficiency, reliability and lifetime. Gross overloads might result in a "burnt" LED as shown in the comparison picture above. The picture was taken from this instructable, thanks @openproducts for letting me use it!


OzzieJohn (author)2017-06-17

Great read and I hope you don't mind but I'm copying it for future reference.

nqtronix (author)OzzieJohn2017-06-19

Sure, go ahaed, you're free to use it within the BY-NC-SA licence! Basically as long as you don't claim it that it's your work and you don't use it commercially, you can do whatever you want.

OzzieJohn (author)nqtronix2017-06-19

No problems mate it will be used by only me. I have also put your name as the author.

bobolson99 (author)2017-02-15

Sir, this is precisely the information I need. I've read all of your Instructables thus far, and you should write an Instrustable on how to write an Instructables! Your presentation is perfect...all of the needed information presented very logically! Very well explained! Thank you.

TheHomeschoolHouse (author)2016-10-22

Great introduction to LEDs! I look forward to reading your next LED guides!

Rayregula (author)2016-10-07


"This is chapter 1 of a short series. Use the table of contents below to browse the content I've published yet. New chapters are added every sunday!"


This is chapter 1 of a short series. Use the table of contents below to browse the content I've already published. New chapters are added every Sunday!

nqtronix (author)Rayregula2016-10-07

That was embarrassing, I fixed it. Thank you.

sheba92766 (author)2016-09-24

Having absolutely no experience with any of this & the desire to learn to solder & add leds to projects, I'm very happy to have found this. Most of it still looks like, "LEDs are made jibberish blah blah blah lights rabble rabble anode", but I'm hopeful now. Thank you very much for taking the time to create this. One day soon, my home-made lightning cloud will thank you.


nqtronix (author)sheba927662016-09-25

Those lightning clouds look fantastic and are a perfect application for LEDs. I'd love to see yours when you get around to make it. Best luck :)

Newoldbuilder (author)2016-09-25

Thank you for this ible. It was well written and very informative. I learned quit a lot from this and will be reading all your other ibles. p.s. I voted.

Barry_L (author)2016-09-21

Brilliant tutorial! Thanks for sharing :)

nqtronix (author)Barry_L2016-09-24

Cool to see you here, Barry :D I'm glad you like it :)

sirfixalot7797 (author)2016-09-22

thanks good details also can we salvage leds from other broken items.

Majik01 (author)2016-09-16

@nqtronix, PhilKE3FL, LaurJ.

Going by all your advice I located these:

They meet your advice(s) although not flat top 5mm they have a φ of 120°. So using 2 rows of 100 of these what will be my "best choice" circuit? Thanking you in advance.

nqtronix (author)Majik012016-09-17

These LEDs are fairly powerful, about 20x as much as a normal LED. Make sure they aren't blinding before you build your project. For mounting and better heatsinking you should get some with a "star pcb"

You'll also need a few transistors to swtich the LEDs.

Flat 5mm LEDs may work as well:

As for the circuit PhilKE3FL and LaurJ. might be a better help than me, so ask them. Unfortunatly a @name does not notify useres yet. You need to reply to one of their comments or write them a PM instead. ;)

Majik01 (author)nqtronix2016-09-18

Thank you. I will leave a message for them.

jwzumwalt (author)2016-09-17

This series is one of the better instructables I have seen. Each article is the right size and the topic stands on its own. It goes from basic to expert, good pictures and graphics, and well thought out outline of the material - Great job!

Majik01 (author)2016-09-17

Voted !!!

nqtronix (author)Majik012016-09-17

Thanks! :D

SherylinRM (author)2016-09-17

Voted for you.

Thanks for this :)

nqtronix (author)SherylinRM2016-09-17

Thank you! :)

PhilKE3FL (author)2016-09-16

"This means you can not treat an LED like an ordinary lightbulb and connect it straight to a power supply!"

"If you are still not sure you can always connect a CR2032 battery directly to it and try it out! In fact this is a popular way to build simple projects such as LED Throwies. This works because a CR2032 can not provide enough power to destroy an LED."

Here you make a statement and then contradict it and, you even explain HOW to run an LED without a circuit of any kind, just the power source.

The truth is you can connect an LED to a power source AS LONG AS the power source's voltage is limited to slightly below the LEDs running voltage, to be on the safe side.

(Note, when LEDs first came out we were using them with sources of 5 V or higher and a current limiting resistor was absolutely needed to protect the LED, today in many cases a 3 V or 4.5 V LED includes a limiting resistor designed for a supplied voltage of 3 or 4.5 Volts respectively.)

I've even run a 3V LED from a "dead" 9V battery, as long as it's resting voltage is less than 4 Volts the voltage across the LED drops to the LED voltage of about 3V and the LED will run the "dead" 9V battery down to under 2V when I throw it away.

If the dead 9V battery is about 6 volts I have put two 3V LEDs in series OR I've used a dropping (current limiting) resistor in series with one LED.

Look up the specs on the LED for the current the LED needs, or start off assuming 20 mA current & do the calculation. ALWAYS use a higher valued resistor if you don't have one of the needed value.

Example, your calculation says that you need a 200 Ohm resistor to limit the current to 20 mA (E = I * R) so we've started with E = 4V in this case and 4V/0.02A (20 mA) = 200 Ohm. Now, you notice that you have 220 and 180 Ohm resistors in your collection. Use the 220 Ohm or higher resistor and NOT the 180 Ohm or lower resistor. Depending on the power source I would start with something close to 300 Ohms and see if the brightness is sufficient, if not, use lower & lower resistor values until you reach 200 Ohms - assuming you have that value.

nqtronix (author)PhilKE3FL2016-09-16

Sir, did you realize this is a multi-chapter series and this is only meant to be the introduction? Literally all of this is covered in the next chapter which you didn't even bother to flick through. I do not mind criticism; in fact I had a few mistakes, which have been kindly corrected by readers.

If you believe you can do a better work than I do, please feel free to create your own instructable. Hack, I even don't mind if you advertise for your own homepage with a slightly offtopic circuit (No circuit is worth the effort compared to a 10ct 2032 battery, sorry). But please stop hijacking my 'ibles trying to teach. It adds clutter and confusion I spend hours to avoid.

Thanks for your understanding.

PhilKE3FL (author)nqtronix2016-09-16


Calm down, I copied your statements, one from page 1 the other from page 2. I think you're helping others that do not know as much about LEDs as you do, please continue to do so. And yes I did know it was a multi-installment but I admit to not having tried to find the next installment.

As to any circuit it is, of course, more complex than a simple 2032 battery, or similar 3V battery.

As for being worth the effort, I disagree. The circuit I show on my Joule Thief page is for using a "dead" AA or AAA 1.5 Volt battery for driving an ultra-bright white LED. I've used this circuit in old 1 battery flashlights to replace the dim incandescent bulb with an ultra-bright white LED. On top of that, this circuit is usable until the battery gets down to 0.5 Volts or even lower depending on the efficiency of the circuit used, which I also explain on my web site.

As for doing better? Who's to say, I have no interest in usurping your turf, only making suggestions in the hope of helping and pointing out that you contradicted your own statement. This was NOT done to point out a flaw, but because you may be asked the same question by others who do not know as much as you. If you have already covered the point I brought up, simply saying I missed it would have been sufficient. Sorry for trying to help and to extend your work for interested people.

Is there a link to your next installment from this first installment? I admit I did not read enough to find it if there is.

I'm sure you realize that looking up "Joule Thief" circuits here on Instructables will probably bring up 100 or more articles from different authors, right? Everyone has an idea on how to do it better, my pages on my amateur radio web site go into the experiments I performed, and my mistakes made. I documented everything, mistakes and all, that lead me to my conclusions. Am I right? In some way, probably, but I'm also sure there are others who know more and may be able to correct or add to my small attempts in understanding this one simple circuit. Is what I've done better? Perhaps or perhaps not, better, after all, is in the eye of the beholder and what they are interested in, or what they're after.

nqtronix (author)PhilKE3FL2016-09-16

A joule thief has its applications, they are perfect for flashlights as you've stated, but as a simple polarity tester it seems overkill to me. I choose lithium coin cells wherever they can be used, mainly because of their shelf life of 20+ years and low leakage risk.

You can browse the chapters with the table of contents in the first step, just click on one of the non-indented organge links and it'll take you to the right instructable.

I briefly checked the link you've posted before my last comment and I was not that impressed. You main Joule Thief artical on the other hand is very professional, far beyond the usual "It works" one can find around here. Being for engineering I love data and real measurements over all the theory, I also understand how much time this takes. Thank you for all the effort put into this!

I guess I did you wrong, you now seem like a decent guy to me, not like a I-know-everything-better internet stranger. Sorry for all the fuss, I hope we have things sorted out now. :)

PhilKE3FL (author)nqtronix2016-09-16

Yes, I agree about the circuit being a bit of over kill for a simple tester, on the other hand I had the circuit already built so decided to use it for this task.

One of the main problems of using a Joule Thief circuit is that the first we use old, "dead" batteries and then on top of hat the battery lasts so long that leakage is a REAL issue. I will be adding some information on how to clean a battery holder once a battery leaks in it. But, for those wanting the info RIGHT NOW simply soak it in some clear vinegar until it stops bubbling. The wait about another minute or two just-to-be-sure you've gotten all the leakage cleaned off.

No problem with your misinterpreting my first comments as criticism & not as helpful info. I love to be corrected or have people add stuff because that's how we learn, when we make mistakes we learn far more than when we do it right.

Please keep your Instructables coming!

LaurJ (author)PhilKE3FL2016-09-16

"This works because a CR2032 can not provide enough power to destroy an LED." is the explanation. He also mentions on a later page that the reason is the internal resistance of the CR2032 battery, which is enough to limit current. In fact, your statement that " can connect an LED to a power source AS LONG AS the power source's voltage is limited to slightly below the LEDs running voltage...", while not wrong, does not explain the situation here. It's possible to run a red led with a forward voltage drop of 1.8 V from the 3V CR2032, because the voltage (or more precisely, emf) is not enough to characterize the behaviour of a power source in the face of a load.

dj_segfault (author)2016-09-15

You don't say what you want to control these lights with, but that's a lot of lights to individually control. However, you don't need a microprocessor to do it. There's a kind of chip called a "decade counter" that sequences through 10 outputs being high one after another. But they also have the ability to daisy chain these chips so when one chip has put it's last output high, it can signal the next chip to put its first output high. If you connect 10 of them then you can sequence all 100 pairs of lights.

For an example circuit using the most common one, the 4017, see

Majik01 (author)dj_segfault2016-09-15

I did not state what I would control it with because I have no idea what I'm doing!!! Trying to teach myself how to do this is like trying to teach a cat quantum mechanics!!! Would it be possible to wire it in series so I would have less wiring and each "side" and would 1 controller be used for both "sides"?

LaurJ (author)Majik012016-09-15

You likely will not get away without wiring at least one wire for each LED group (the LEDs that will be on at the same time), plus one common wire for the cathode. Assuming 10 groups, each group will have 20 LED-s, and using 20mA (the most common, and can be reasonably bright) LED-s that's 400 mA of current. With AWG 22-24 wire, at the end of the 100' you'll have around 1 V of voltage drop, so you need to consider that when adding current limiting resistors to the LED-s and selecting your power source. Double that, to include the cathode wire resistance, now that I think of it.

Also, the cable will cost some. From farnell, if you buy by 100m or 50 m, you'll get around 1.5€/m for 12-core 0.22mm2 (~24AWG), plus shipping.

For controlling, I suggest either a 555 circuit with a potentiometer to control the speed (if you want to eyeball it) or some cheap MCU if you want to dial in an approach speed at a few-% accuracy (at which point you don't really need the 4017 sequencer).

Majik01 (author)LaurJ2016-09-16

@ LaurJ I was an electrician for 40+ years so I understand voltage drop etc.. I had to retrain myself in the IT business so I have 1000' of Cat6 4 pair cable (24 awg). I have 400-500 5mm white LED's.I do not have a data sheet on them. So I don't know the Uf, If or the φ value. I can use Mr. nqtronixs' wonderful guide to "guess" @ the values or do you suggest I buy new LED's w/data sheet? My dad and I used to build Heathkit projects so I know how to solder good. May I ask from you a circuit diagram (in any file format)? Will I need 2 circuits, 1 for each side of my runway or will one circuit handle it? Here is a link to show you what I'm trying to reproduce:

Please see @ 5:42 and 10:00. The difference will be that my runway lighting system will strobe on each side. Thanking you in advance.

nqtronix (author)Majik012016-09-16

Try if the beam of the LEDs is wide enough for your taste, if so keep them. A save assumption for white LEDs is 3.7V @ 20mA. One circuit will be sufficient for both sides, just put the LEDs of each side in parallel, each with it's own resistor.

Majik01 (author)nqtronix2016-09-16

A 555 circuit with a potentiometer? What value resistor for each? Can you provide a circuit diagram in any file format? Thank you.

Majik01 (author)dj_segfault2016-09-15

So the daisy chain is a series circuit right? And I need 100 3mm-5mm white LED's per side for a total of 200 3mm-5mm LED's. The total length per side is 100' with 1 LED per foot.

Renauld (author)2016-09-16

Thanks for the info. It fills quite a few holes in my knowledge.

PhilKE3FL (author)2016-09-15

Or, you can build a joule thief circuit with an open place for the LED. Mark it so you know which is + and which - then plug the unknown LED in & push the test button. If you have it correctly installed it will light up, if not it won't light, or it is dead so flit it around & try again to be sure. See my LED tester on my web site.

PhilKE3FL (author)PhilKE3FL2016-09-16

Sorry about that, "flit" should be flip, flip the LED & plug it in the other way to test it again to be sure it is not dead & you only had it in the wrong way for the first test.

LaurJ (author)2016-09-15

A technical optoelectronics note about the need for a plastic encapsulation: While the dome's shape can be modified to alter the final light distribution, the need for encapsulating the chip at all (besides mechanical protection) is due to the fact that the refraction index of the semiconductor material is very high. The plastic acts as a middleman (compare RF impedance matching by quarter-wave transformer, although here there's usually no "quarter-wave" about it), with a refractive index roughly the geometric mean of the ones of the semiconductor and air. This maximizes light transfer from the chip into air. Without it, much more of the light would be internally reflected back into the chip at the semiconductor-air interface, and since the semiconductor also has high absorption, would be rendered into heat, reducing the efficiency of the LED. In fact, this is still one of the largest obstacles in achieveing higher efficiency.

PhilKE3FL (author)LaurJ2016-09-16

Thank-you LaurJ for the helpful physics information! I didn't know that and I've worked in both ElectroOptics & optics fields! I never even bothered to wonder why - my bad!

nqtronix (author)LaurJ2016-09-15

Wow, I didn't know that! Do you have a link to the appnote?

LaurJ (author)nqtronix2016-09-16

App note? You mean my notes from the University? :D

nqtronix (author)LaurJ2016-09-16

Nevermind, I assumed that "note" reffered to one of those "app(lication) notes" manufactures publish. I'll have an optoelelctonics course next semester.

kode1303 (author)2016-09-05

If someone have made a guide to LEDs like this, I have not seen it. Very useful! Thanks.

nqtronix (author)kode13032016-09-06

I'm glad you like it!

alokmishra (author)nqtronix2016-09-15

Lovely, u r a good writer, try a topic in literature also... Thanks.

nqtronix (author)2016-09-15

Just to make sure I get your idea correctly (The video is broken): You want to place 2x 100 single colored LEDs 1" = 2.54 cm apart, which should display a "chasing" animation, right?

In that case you want to use small LEDs with a wide angle. Depending on your soldering skills I'd suggest 0603, 0805 or 1206 SMD LEDs. 200pcs. should cost no more than 5$ on ebay or aliexpress.

If it is possible that every 10th LED or so is on at the same time, you can wire them into 10 groups and simplify the circuitry greatly. Those 20 LEDs per group will draw about 0.4A, so you'll need a transistor or mosfet to controll them. How teo do that is discribed in detail in chapter 2 and chpater 3.

Your control circuitry can be either be an arduino switching 10 of it's IOs or you modify circuit like this LED Chaser to suit 10 (groups) of LEDs.

Majik01 (author)nqtronix2016-09-15

That would be 1' not inch. This link is what I had in mind. Except the guy only has 9 rows of 10, not 10 of 10. Hope this helps you answer.

Thank you.

Majik01 (author)Majik012016-09-15

Sorry this is 9 rows of 9. If I could build/wire 10 rows of 10 in-line to make 100' with 100 LED's strobing or chasing.

nqtronix (author)Majik012016-09-15

Yes, this would totally be possible. For LEDs spaced apart 30cm may want to pick larger LEDs with a wide beam, such as 5mm LEDs with a flat dome.

dj_segfault has posted a link for very straight forward circuit, is should be perfect for this application.

soumitraseth (author)2016-09-15



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