Talking Elektronics - 30 LED Projects.pdf - 00 - Księgozbiór Elektronika - neo666x

For our other free eBooks,

50 - 555 Circuits

1 - 100 Transistor Circuits and: 101 - 200 Transistor Circuits

100 IC Circuits

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

For more articles and projects for the hobbyist: see TALKING ELECTRONICS WEBSITE

email Colin Mitchell: talking@tpg.com.au

CONTENTS

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

INTRODUCTION

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.

CONNECTING A LED

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.

CHARACTERISTIC VOLTAGE DROP

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 10,000 to 50,000 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:

HEAD VOLTAGE

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 = 11.9v

Dropper resistor = 0.1v

As soon as the supply drops to 11.8v, 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 11.5v, 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.

TESTING A LED

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.

CIRCUIT TO TEST ALL TYPES OF LEDs

IDENTIFYING A LED

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:

Talking Elektronics - 30 LED Projects.pdf

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