Introduction: All You Need to Know About LEDs

About: I'm an Electronics Engineer who likes to make Doze Lamps, Lumen Powered Thingamajigs, Almighty Brainy Buttons, Tweeting Weather Stations and share them on Instructables.

A Light Emitting Diode is an electronic device that emits light when current is passed through it. LEDs are small, extremely efficient, bright, cheap, electronic components. People think that LEDs are just common light emitting components & tend to overlook the Interesting Facts & Features of LEDs. In this instructable I will teach you 'All You Need to Know About LEDs' which includes their Working, Current and Power Ratings, Builds, Types, Resistor Calculator for LEDs, Uses, Testing & a Simple LED circuit.

Here's a Link to the 'LED Resistor Calculator' free android app: LED Resistor Calculator . This app helps you calculate the appropriate resistor value required for an LED.

History Of The LED

Captain Henry Joseph Round was one of the early pioneers of radio and received 117 patents. He was the first to report observation of electroluminescence from a diode, leading to the discovery of the light-emitting diode. Vladimirovich Losev observed light emission from carborundum point-contact junctions. In course of his work as a radio technician, he noticed that crystal diodes used in radio receivers emitted light when current was passed through them. In 1927, Losev published details in a Russian journal about his work on light-emitting diodes. A couple of years later Nick Holonyak, Jr. invented the first visible-spectrum (red) LED in 1962 while working as a consulting scientist at a General Electric Company laboratory in Syracuse, New York.

Parts List:

Step 1: Composition & Working


  1. Dremeling an LED.
  2. Top view of the Electodes of the LED. (larger- cathode, smaller- anode).
  3. Closeup of Anode and Cathode of LED. (LED sliced in half).
  4. Anode and Cathode of LED removed from plastic shell.


Most common LED's are made up of Gallium (Ga), Arsenic (As), and Phosphorus(P). Modern LEDs are not just GaAsP types - other semiconductor brews are abound! These Semiconductors are also used in various other Electronic components.


A LED is a P-N Junction diode that emits Light. When an LED is in forward bias it emits light instead of heat generated by a normal diode. When the P-N junction is in forward bias, in case of a LED some of the holes combine with the electrons of N- Region and some of the electrons from N combine with hole from the P- region. Each recombination radiates light or Photons.

LED's do have a polarity and hence do not work if they are connected in Reverse bias. The easiest method to check the polarity of common LED is by holding the LED close to your eye. You will see that there are two electrodes. The thicker one is the Cathode(-). Light is emitted from the Cathode. The thinner electrode is the Anode(+).[Although this method of checking polarity won't work for some LED's like high efficiency LEDs,etc where the opposite is true]. Generally LED's are manufactured so that the length of the leads of the Cathode and Anode differ. Due to this LED's are manufactured with the Anode(+) lead longer than the Cathode(-) lead. This also makes it easier to determine the polarity.Note: Some manufacturers do keep both electrode leads the same length. Inorder to TEST the polarity you will need to use a Multimeter.

Step 2: Current & Power Ratings, Haitz's Law

IMAGE: LED Symbol.

Common IR LEDs may work down to ~1.5V but common Red LED's need ~1.8V, Common Green LED's need ~2V & common blue & whites LED's (which of course are blue with a phosphor coating) need a good 3V.

LEDs don't have a "voltage rating"; they are current-driven. The brightness is roughly proportional to the current, and not directly proportional to the voltage. At any particular current, they will have a forward voltage, but that is secondary to the current, which is the main factor that must be controlled.

Current Ratings

Current ratings of LED's are similar too Voltage Ratings. LED's generally have a standard current rating. Most LED's require about 5-25 mA. The current required by an LED sometimes depends on the Colour of the LED. If you supply excess current the LED will burn and get damaged. On the other hand if you supply very low current the LED will not produce its maximum output. Modern ultrabright red/green LEDs can give acceptable output (for status use etc) on as little as 1mA

Power Ratings

An LED's can have various power ratings depending upon their Type, Build and Current Ratings, etc. LED's also come in 'High Power LED' packages. LED's are less ineffecient than conventional light bulbs such as CFL's and Incandescent Bulbs.

Haitz's Law

It states that every decade, the cost per lumen (unit of useful light emitted) falls by a factor of 10, and the amount of light generated per LED package increases by a factor of 20, for a given wavelength (color) of light. It is considered the LED counterpart to Moore's law, which states that the number of transistors in a given integrated circuit doubles every 18 to 24 months. Both laws rely on the process optimization of the production of semiconductor devices.

Step 3: Build


  1. Basic LED.
  2. Dome LED.
  3. SMD LED (Large).
  4. SMD LED (Small).
  5. Display LED used in 7-Segment Display.

LEDs are produced in a variety of shapes and sizes. The color of the plastic lens is often the same as the actual color of light emitted, but not always. For instance, purple plastic is often used for infrared LEDs, and most blue devices have colorless housings. Modern high power LEDs such as those used for lighting and backlighting are generally found in surface-mount devices (SMD) packages. Some LED's have diffused plastic lenses.

Basic LED

The basic LED is one of the most widely used LED's. Due too it's popularity its pice is relatively cheaper compared to othe LED's. It looks very basic and the design is very simple.

Dome LED

This is a type of LED which is shaped liked a 'Dome'. This shape is designed too increase the area to which the light is transmitted. In other words the Angle of Emission( Circumfernce) of Light from the LED is greater than the Basic LED. This is generally controlled by how far they place the light emitter from the dome. The spec sheets almost always give you a "half-power angle" (the angle off-axis at which you're seeing only half the brightness). If you want a much wider emission angle you can cut the dome off with a dremel tool. If you care, you can then file or polish the end, but it's not necessary. The closer you cut it to the emission device, the wider angle you'll get. But be careful not to cut too close because there's a tiny wire in there that usually cannot be seen by eye. Although this type of LED is slightly more expensive than a basic led.


This type of LED is generally very small in size. SMD means Surface Mounted Device. And as its name suggests, this LED is soldered onto the surface of the PCB unlike conventional 'through- hole' components. These LED's are Generally soldered by Machines( Precise Soldering Robots) and are extremely difficult to solder by hand (Although it isn't impossible to Solder SMD LED's by Hand). Inorder to solder SMD LED's by hand you just need a fine tipped soldering iron, some thin solder, a bright light, and possibly a magnifier and some good and precise soldering skills.

Display LED

This type of LED is mainly used in displays as its shape is flat.

Step 4: Types


  1. Dome LED's.
  2. IR LED's.
  3. 7 Segment Display LED
  4. Tri colour LED (colour changing LED).

Colour LED

Coloured & White LED's are mainly used in Indicators, Lamps, Lighting Equipment, etc. They are one of the most commonly used LED's

Colour Changing LED (Tri/Bi Colour LED)

In this type of LED, the colour emitted by the LED changes within a specific period of time. A tiny Integreated Circuit (IC) is embedded into this LED inorder to control the time delay between transitioning the various colours. Tri/bicolor LEDs do not change color they are actually two separate LEDs (often a red and a green) in one package. You turn one or the other to produce two colors and both to make a third.

Infrared (IR) LED

This type of LED beams infrared rays of light. These infrared rays cannot be seen by the Human Eye. This type of LED generally works on a transmission frequency of 38KHz.The designer modulates the LED as a way for the receiver to discriminate it from other IR sources. LED's are also modulated at very low frequencies to simply show a blinking LED, and are often modulated at relatively high frequencies with varying duty-cycle to effectively control their brightness. And then some are modulated at much higher frequencies to send data (as used in fiber optics for example). Its mainly used in Remote Controlled and Small Range Communication devices. You can test a IR LED by viewing it under a Camera whilst a current is appliead across the LED. In other words cameras can detect IR rays emitted from the LED. Cameras that don't have an IR block filter can generally see near IR quite well (and tend to be cheap cameras and particularly security cameras). But it should be mentioned that even some cell phone cameras don't see IR LED's very well at all because of their IR block filter.

7 Segment Display LED

A 7 segment display LED is an LED consisting of 7 display LED's connected in the form of an 8. Its used in calculators, displays, etc. An LED similar to this is also used to display alphabets.


UV LED's emit Ultra Violet rays of light. These rays have various applications such as Sterilization, Water purification, etc.

Step 5: Resistor Calculator for LED's


  1. Various resistors and a LED.
  2. LED Resistance Calculator App Logo.

So the most common question asked about LED's is the appropriate resistor to use along with. The reason a resistor is used along with LED's is to protect them from excess current which can burn and damage the LED. But choosing the right LED isn't that simple. Why? Well if you choose a very high resistance, the LED will not emit its maximum light. And if you a low resistance there are chances of the LED getting Damaged.

So a simple formula was invented:

Resistance = (Source Voltage - LED Voltage) / (LED Current / 1000).

*Keep in mind, the LED Current is in milliamps(mA).

Inorder to make this calculation easier you can use this free Android App LED Resistance Calculator. It is an app designed especially for this Instructable. Other features and more electronics related functions and calculators will be added to this app. The app was developed by BluBot Technologies . You can check out his Instructables and contact him through his Orangeboard @Nathan Neal Dmello .He also undertakes various other projects in developing Apps, Websites, Computer Programs, etc. You can contact him through his website.

Step 6: Uses


  1. TV Remote without button pressed.
  2. TV remote with button pressed and IR LED flash detected.
  3. Strip of Dome LED's from an Emergency Flashlight.
  4. LED Flash of a Smartphone Camera.
  5. LED power indicators of a Laptop.

LED's are used everywhere. From your phone flash, to your cars music system, to your garden lights, to your TV display. Basically their adaptive nature and effeciency has given them a place in most electronic gadgets.

Some of the most known uses are:

  1. Lighting.
  2. Displays.
  3. Indicators.
  4. Decorative Lights and Objects.
  5. Remote Control.
  6. Sterilization.
  7. Purification of Water.
  8. Dentistry & other Medical applications.

Step 7: Testing & Circuit


  1. Multimeter used to test LED.
  2. Simple circuit using LED.


A classic quick tester for colour,brightness & polarity is just a 3V lithium coin cell(e.g. CR2032). Only touch lower voltage LEDs to this briefly of course, or they may overheat !

Some LED's can be tested inorder to check whether it is working properly using a multimeter & by following the steps:

  1. Set the dial of the multimeter to the 'Continiuity' function.
  2. Now connect the Anode(+) of the LED to the RED/Positive/(+) probe of the multimeter & connect the Cathode(-) of the LED to the BLACK/Negative/(-) probe of the multimeter.
  3. If the LED is working the Multimeter will start to make a 'Beep' sound. And a value will display on the screen of the multimeter. In addition to this the LED should light up.

*Testing an LED using the continuity function of a multimeter usually won't work because most multimeters apply only a low voltage, less than 1V, for the resistance and continuity tests. If it does, the multimeter won't make a continuous beep; it may make one short beep. Many multimeters have a diode test function, indicated by a diode symbol, which applies up to 2V across the diode. This will reliably tell you the polarity of many LEDs but not necessarily blue and white LEDs with high forward voltages.

You can also Test the LED and any other component with the help of this circuit:- Electronic Sensor Component Tester


This is one of the most basic and versatile circuit you can find which uses an LED in it. The reason it is a great circuit to start of with is that it can also check the working of any other Electronic components or Electronic Sensors. You can also check out a detailed tutorial which will help you to make this circuit: Electronic Sensor Component Tester

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