LED Projects Eletronica

LED Projects Eletronica

(Parte 1 de 3)

For our other free eBooks, 50 - 5 Circuits 1 - 100 Transistor Circuits and: 101 - 200 Transistor Circuits 100 IC Circuits

For a list of every electronic symbol, see: Circuit Symbols.

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email Colin Mitchell: talking@tpg.com.au

Battery Monitor MkI MkII Bi-Coloured LED Bike Turning Signal Bi-Polar LED Driver Dice Domino Effect- The Driving A Bi-Coloured LED Driving White LEDs Fading LED Flashing A LED Flashing Railroad Lights Kitt Scanner Knight Rider LED Chaser LED Detects Light LED Dice LED Dimmer

Police Lights 1,2,3 Powering A Project Railroad Lights (flashing) RGB LED Driver RGB LED Flasher Resistor Colour Codes Roulette Shake LED Torch Solar Garden Light Solar Tracker The Domino Effect Traffic Lights Traffic Lights - 4 way Turning Signal Up/Down Fading LED Up/Down Fading LED - 2 White LED on 1.5v Supply

LED FX LED Night Light LEDs on 120v and 240v LED Zeppelin Lights - Traffic Lights Low Fuel Indicator Mains Night Light

White LED Flasher 2 White LEDs on 1.5v Supply 3x3x3 Cube 4 way Traffic Lights 8 Million Gain! 10 LED Chaser 120v and 240v LEDs

This e-book covers the Light Emitting Diode.

The LED (Light Emitting Diode) is the modern-day equivalent to the light-globe. It has changed from a dimly-glowing indicator to one that is too-bright to look at. However it is entirely different to a "globe." A globe is an electrical device consisting of a glowing wire while a LED is an electronic device. A LED is more efficient, produces less heat and must be "driven" correctly to prevent it being damaged. This eBook shows you how to connect a LED to a circuit plus a number of projects using LEDs. It's simple to use a LED - once you know how.

A LED must be connected around the correct way in a circuit and it must have a resistor to limit the current. The LED in the first diagram does not illuminate because a red LED requires 1.7v and the cell only supplies 1.5v. The LED in the second diagram is damaged because it requires 1.7v and the two cells supply 3v. A resistor is needed to limit the current to about 25mA and also the voltage to 1.7v, as shown in the third diagram. The fourth diagram is the circuit for layout #3 showing the symbol for the LED, resistor and battery and how the three are connected. The LED in the fifth diagram does not work because it is around the wrong way.

When a LED is connected around the correct way in a circuit it develops a voltage across it called the CHARACTERISTIC VOLTAGE DROP. A LED must be supplied with a voltage that is higher than its "CHARACTERISTIC VOLTAGE" via a resistor - called a VOLTAGE DROPPING RESISTOR or CURRENT LIMITING RESISTOR - so the LED will operate correctly and provide at least 10,0 to 50,0 hours of illumination. A LED works like this: A LED and resistor are placed in series and connected to a voltage. As the voltage rises from 0v, nothing happens until the voltage reaches about 1.7v. At this voltage a red LED just starts to glow. As the voltage increases, the voltage across the LED remains at 1.7v but the current through the LED increases and it gets brighter. We now turn our attention to the current though the LED. As the current increases to 5mA, 10mA, 15mA, 20mA the brightness will increase and at 25mA, it will be a maximum. Increasing the supply voltage will simply change the colour of the LED slightly but the crystal inside the LED will start to overheat and this will reduce the life considerably. This is just a simple example as each LED has a different CHARACTERISTIC VOLTAGE DROP and a different maximum current. In the diagram below we see a LED on a 3v supply, 9v supply and 12v supply. The current-limiting resistors are different and the first circuit takes 6mA, the second takes 15mA and the third takes 31mA. But the voltage across the red LED is the same in all cases. This is because the LED creates the CHARACTERISTIC VOLTAGE DROP and this does not change.

It does not matter if the resistor is connected above or below the LED. The circuits are the SAME in operation:

Now we turn our attention to the resistor. As the supply-voltage increases, the voltage across the LED will be constant at 1.7v (for a red LED) and the excess voltage will be dropped across the resistor. The supply can be any voltage from 2v to 12v or more. In this case, the resistor will drop 0.3v to 10.3v. This is called HEAD VOLTAGE - or HEAD-ROOM. The following diagram shows HEAD VOLTAGE: The voltage dropped across this resistor, combined with the current, constitutes wasted energy and should be kept to a minimum, but a small HEAD VOLTAGE is not advisable (such as 0.5v). The head voltage should be a minimum of 1.5v - and this only applies if the supply is fixed. The head voltage depends on the supply voltage. If the supply is fixed and guaranteed not to increase or fall, the head voltage can be small (1.5v minimum).

But most supplies are derived from batteries and the voltage will drop as the cells are used. Here is an example of a problem: Supply voltage: 12v 7 red LEDs in series = 1.9v Dropper resistor = 0.1v As soon as the supply drops to 1.8v, no LEDs will be illuminated. Example 2: Supply voltage 12v 5 green LEDs in series @ 2.1v = 10.5v Dropper resistor = 1.5v The battery voltage can drop to 10.5v But let's look at the situation more closely. Suppose the current @ 12v = 25mA. As the voltage drops, the current will drop. At 1.5v, the current will be 17mA At 11v, the current will be 9mA At 10.5v, the current will be zero

You can see the workable supply drop is only about 1v. Many batteries drop 1v and still have over 80% of their energy remaining. That's why you need to design your circuit to have a large HEAD VOLTAGE.

If the cathode lead of a LED cannot be identified, place 3 cells in series with a 220R resistor and illuminate the LED. 4.5v allows all types of LEDs to be tested as white LEDs require up to 3.6v. Do not use a multimeter as some only have one or two cells and this will not illuminate all types of LEDs. In addition, the negative lead of a multimeter is connected to the positive of the cells (inside the meter) for resistance measurements - so you will get an incorrect determination of the cathode lead.


A LED does not have a "Positive" or "Negative" lead. It has a lead identified as the "Cathode" or Kathode" or "k". This is identified by a flat on the side of the LED and/or by the shortest lead. This lead goes to the 0v rail of the circuit or near the 0v rail (if the LED is connected to other components). Many LEDs have a "flat" on one side and this identifies the cathode. Some surface-mount LEDs have a dot or shape to identify the cathode lead and some have a cut-out on one end. Here are some of the identification marks:


A LED is described as a CURRENT DRIVEN DEVICE. This means the illumination is determined by the amount of current flowing through it. The brightness of a LED can be altered by increasing or decreasing the current. The effect will not be linear and it is best to experiment to determine the best current-flow for the amount of illumination you want. High-bright LEDs and super-bright LEDs will illuminate at 1mA or less, so the quality of a LED has a lot to do with the brightness. The life of many LEDs is determined at 17mA. This seems to be the best value for many types of LEDs.

1mA to 5mA LEDs Some LEDs will produce illumination at 1mA. These are "high Quality" or "High Brightness" LEDs and the only way to check this feature is to test them @1mA as shown below.

THE 5v LED Some suppliers and some websites talk about a 5v white or blue LED. Some LEDs have a small internal resistor and can be placed on a 5v supply. This is very rate. Some websites suggest placing a white LED on a 5v supply. These LEDs have a characteristic voltagedrop of 3.6v and should not be placed directly on a voltage above this value. The only LED with an internal resistor is a FLASHING LED. These LEDs can be placed on a supply from 5v to 12v and flash at approx 2Hz. NEVER assume a LED has an internal resistor. Always add a series resistor. Some high intensity LEDs are designed for 12v operation. These LEDs have a complete internal circuit to deliver the correct current to the LED. This type of device is not covered in this eBook.

LEDs IN SERIES LEDs can be placed in series providing some features are taken into account. The main item to include is a current-limiting resistor. A LED and resistor is called a string. A string can have 1, 2, 3 or more LEDs. Three things must be observed: 1. MAXIMUM CURRENT through each string = 25mA. 2. The CHARACTERISTIC VOLTAGE-DROP must be known so the correct number of LEDs are used in any string. 3. A DROPPER RESISTOR must be included for each string. The following diagrams show examples of 1-string, 2-strings and 3-strings:

LEDs IN PARALLEL LEDs CANNOT be placed in parallel - until you read this: LEDs "generate" or "possess" or "create" a voltage across them called the CHARACTERISTIC VOLTAGEDROP (when they are correctly placed in a circuit). This voltage is generated by the type of crystal and is different for each colour as well as the "quality" of the LED (such as high-bright, ultra high-bright etc). This characteristic cannot be altered BUT it does change a very small amount from one LED to another in the same batch. And it does increase slightly as the current increases. For instance, it will be different by as much as 0.2v for red LEDs and 0.4v for white LEDs from the same batch and will increase by as much as 0.5v when the current is increased from a minimum to maximum. You can test 100 white LEDs @15mA and measure the CHARACTERISTIC VOLTAGE-DROP to see this range. If you get 2 LEDs with identical CHARACTERISTIC VOLTAGE-DROP, and place them in parallel, they will each take the same current. This means 30mA through the current-limiting resistor will be divided into 15mA for each LED. However if one LED has a higher CHARACTERISTIC VOLTAGE-DROP, it will take less current and the other LED will take considerably more. Thus you have no way to determine the "current-sharing" in a string of parallel LEDs. If you put 3 or more LEDs in parallel, one LED will start to take more current and will over-heat and you will get very-rapid LED failure. As one LED fails, the others will take more current and the rest of the LEDs will start to self-destruct. Thus LEDs in PARALLEL should be avoided. Diagram A below shows two green LEDs in parallel. This will work provided the Characteristic Voltage Drop across each LED is the same. In diagram B the Characteristic Voltage Drop is slightly different for the second LED and the first green LED will glow brighter. In diagram C the three LEDs have different Characteristic Voltage Drops and the red LED will glow very bright while the other two LEDs will not illuminate. All the current will pass through the red LED and it will be damaged. The reason why the red LED will glow very bright is this: It has the lowest Characteristic Voltage Drop and it will create a 1.7v for the three LEDs. The green and orange LEDs will not illuminate at this voltage and thus all the current from the dropper resistor will flow in the red LED and it will be destroyed.

1. Add up the voltages of all the LEDs in a stringe.g: 2.1v + 2.3v + 2.3v + 1.7v = 8.4v

THE RESISTOR The value of the current limiting resistor can be worked out by Ohms Law. Here are the 3 steps: 2. Subtract the LED voltages from the supply voltage. e.g: 12v - 8.4v = 3.6v 3. Divide the 3.6v (or your voltage) by the current through the string. for 25mA: 3.6/.025 =144 ohms for 20mA: 3.6/.02 = 180 ohms for 15mA: 3.6/.015 = 250 ohms for 10mA: 3.6/.01 = 360 ohms This is the value of the current-limiting resistor.

Here is a set of strings for a supply voltage of 3v to 12v and a single LED:

Here is a set of strings for a supply voltage of 5v to 12v and a white LED:

Here is a set of strings for a supply voltage of 5v to 12v and two LEDs: Here is a set of strings for a supply voltage of 5v to 12v and two LEDs:

LED series/parallel array wizard

The LED series/parallel array wizard below, is a calculator that will help you design large arrays of single-colour LEDs. This calculator has been designed by Rob Arnold and you will be taken to his site: http://led.linear1.org/led.wiz when you click: Design my array The wizard determines the current limiting resistor value for each string of the array and the power consumed. All you need to know are the specs of your LED and how many you'd like to use. The calculator only allows one LED colour to be used. For mixed colours, you will have to use the 3 steps explained above.

Source voltage diode forward voltage diode forward current (mA) number of LEDs in your array

View output as: ASCII schematic wiring diagram help with resistor colour codes

Design my array

Resistor Calculator

Use this JavaScript resistor calculator to work out the value of the current-limiting resistor:

LED forward voltage drop=3.6 LED current in milliamps=25

Power dissipated by LED(watts)=

Current-limiting resistance in Ohms= Closest 5% Resistor= Resistor wattage= Actual current= Power dissipated by resistor (watts)=

LED VOLTAGE AND CURRENT LED characteristics are very broad and you have absolutely no idea of any value until you test the LED. However here are some of the generally accepted characteristics:

SOLDERING LEDs LEDs are the most heat-sensitive device of all the components. When soldering surface-mount LEDs, you should hold the LED with tweezers and "tack" one end. Then wait for the LED to cool down and solder the other end very quickly. Then wait a few seconds and completely solder the first end. Check the glow of each LED with 3 cells in series and a 220R resistor. If you have overheated the LED, its output will be dim, or a slightly different colour, or it may not work at all. They are extremely sensitive to heat - mainly because the crystal is so close to the soldering iron.

HIGH-BRIGHT LEDs LEDs have become more efficient over the past 25 years. Originally a red LED emitted 17mcd @20mA. These LEDs now emit 1,000mcd to 20,000mcd @20mA. This means you can lower the current and still produce illumination. Some LEDs operate on a current as low as 1mA

LEDs as LIGHT DETECTORS LEDs can also be used to detect light. Green LEDs are the best, however all LEDs will detect light and produce a voltage equal to the CHARACTERISTIC VOLTAGE-DROP, providing they receive sufficient light. The current they produce is miniscule however high-bright and super-bright LEDs produce a higher output due to the fact that their crystal is more efficient at converting light into electricity. The Solar Tracker project uses this characteristic to track the sun's movement across the sky.


There are hundreds of circuits that use a LED or drive a LED or flash a LED and nearly all the circuits in this eBook are different. Some flash a LED on a 1.5v supply, some use very little current, some flash the LED very brightly and others use a flashing LED to create the flash-rate. You will learn something from every circuit. Some are interesting and some are amazing. Some consist of components called a "building Block" and they can be added to other circuits to create a larger, more complex, circuit. This is what this eBook is all about. It teaches you how to build and design circuits that are fun to see working, yet practical.

(Parte 1 de 3)