30 LED Projects.pdf

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CONTENTS

Battery Monitor MkI MkII

Bi-Coloured LED

Bike Flasher

Bike Turning Signal

Bi-Polar LED Driver

Constant Current

Constant Current 7805 drives 1watt LED

Dice

Domino Effect - The

Driving A Bi-Coloured LED

Driving White LEDs

Equal Brightness

Fading LED

Flashing A LED

Flashing Railroad Lights

Flickering LED

Kitt Scanner

Knight Rider

LED and Piezo - simplest circuit

LED Chaser

LED Detects Light

LED Dice

LED Dimmer

LED FX

LED Night Light

LEDs on 120v and 240v

LED Zeppelin

Lights - Traffic Lights

Low Fuel Indicator

Mains Night Light

Phone Light

Police Lights 1,2,3

Powering A Project

Railroad Lights (flashing)

RGB LED Driver

RGB LED Flasher

Resistor Colour Codes

Roulette

Shake LED Torch

Simplest circuit - LED and Piezo

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

White LED Flasher

1 watt LED - a very good design

2 White LEDs on 1.5v Supply

3x3x3 Cube

4 way Traffic Lights

8 Million Gain!

10 LED Chaser

10 LEDs on a 9v Battery

120v and 240v LEDs

to Index

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 or OVERHEAD-VOLTAGE . And the resistor is called the

CURRENT-LIMIT resistor.

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.

A large Head Voltage is also needed when a plug-pack (wall wart) is used. These devices consist of a

transformer, set of diodes and an electrolytic. The voltage marked on the unit is the voltage it will deliver

when fully loaded. It may be 200mA, 300mA or 500mA. When this current is delivered, the voltage will be 9v

or 12v. But if the current is less than the rated current, the output voltage will be higher. It may be 1v, 2v or

even 5v higher.

This is one of the characteristics of a cheap transformer. A cheap transformer has very poor regulation, so to

deliver 12v @ 500mA, the transformer produces a higher voltage on no-load and the voltage drops as the

current increases.

You need to allow for this extra voltage when using a plug-pack so the LEDs do not take more than 20mA to

25mA.

TESTING A LED

If the cathode lead of a LED cannot be identified, place 3 cells in series with a 220R resistor and illuminate

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