A PORTABLE RGB LED LIGHT SOURCE WITH COLOR CONTROL MECHANISM FOR

BACKLIGHTING APPLICATION

SAIFUL AZRUL BIN SAARIF

UNIVERSITI SAINS MALAYSIA 2016

A PORTABLE RGB LED LIGHT SOURCE WITH COLOR CONTROL MECHANISM FOR

BACKLIGHTING APPLICATION

by

SAIFUL AZRUL BIN SAARIF

Thesis submitted in fulfillment of the requirements for the degree of

Master of Science

September 2016

ii

ACKNOWLEDGEMENT

I would like to express my sincere and heartfelt gratitude to my supervisor

Dr. Arjuna bin Marzuki for guiding me through with his expertise till the successful

completion of this project. This research would not have been successful without his

valuable guidance and constructive criticisms.

I also would like to express my sincere thanks to Mr. Suardi bin Kaharuddin

and the Final Year Project (FYP) students, Nur Adibah binti Rozali, for their helps

through the project development. Not to forget to Mr. Abdul Latip bin Hamid, Mrs.

Sharina binti Mustapa Albakari, Mrs. Roziana binti Mat Said, Mrs. Zammira binti

Khairuddin and to all the lab technicians that I’ve met, thank you for the support

throughout my research.

Special thanks to my wife, Siti Rohaya binti Abdul Aziz for her advice,

encouragement and understanding, also thanks to my parents and my parent’s in-law

for their continuous support in going through this research. Thanks to all those who

had been supporting me in one-way or another during my research. They helped me

out far more than they ever realized, their supports are very much appreciated.

Last but not least my obligations and an appreciation go to the School of

Electrical and Electronic Engineering and Institute of Postgraduate Study (IPS)

Universiti Sains Malaysia (USM) for allowing me to use their facilities throughout

my research. Special thanks to Mr. Mohd Rahmat bin Ariffin for his assistant in

student matters.

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TABLES OF CONTENTS

Page

ACKNOWLEDGEMENT ii

TABLE OF CONTENTS iii

LIST OF TABLES vii

LIST OF FIGURES ix

LIST OF ABBREVIATIONS xii

LIST OF SYMBOLS xiv

ABSTRAK xv

ABSTRACT xvi

CHAPTER ONE – INTRODUCTION

1.1 Overview 1

1.2 Problem Statement 2

1.3 Objectives 3

1.4 Scope of Work 3

1.5 Thesis Organization 4

CHAPTER TWO – LITERATURE REVIEW

2.1 Introduction 5

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2.2 RGB LED Application 5

2.3 Color 6

2.3.1 Metrics to Describe Color 9

2.3.2 Additive Color Mixing by RGB LED 11

2.4 Driving and Dimming the LED 12

2.5 Color Control Techniques 15

2.5.1 LED Temperature Detection 16

2.5.2 Optical Sensor 20

2.5.3 Combine Method 22

2.6 Elimination of Ambient Light Disturbance 25

2.7 Summary 26

CHAPTER THREE – METHODOLOGY AND IMPLEMENTATION

3.1 Introduction 27

3.2 Color Control Mechanism Design Flowchart 27

3.3 Hardware Development 29

3.3.1 Specifications 29

3.3.2 Hardware Block Diagram 30

3.3.3 LED Characterization 32

3.3.4 Photosensor Characterization 32

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3.3.5 Microcontroller Characterization 33

3.4 Software Development 35

3.4.1 System Parameter 35

3.4.2 Closed-Loop System Flowchart 38

3.4.3 PWM Frequency and ADC Acquisition Time 41

3.4.4 Source Code Implementation 43

3.5 Control Mechanism Development 43

3.5.1 LED Junction Temperature 43

3.5.2 LED Radiant Intensity 45

3.5.3 Ambient Light Disturbance Elimination 47

3.5.4 RGB LED Input Circuit 47

3.5.5 Sensor Output Circuit 50

3.5.6 Resistor-Capacitor Filter 53

3.5.7 LED Forward Voltage with Resistor-Capacitor Filter 55

3.5.8 Transimpedance Amplifier with Resistor-Capacitor Filter 56

3.6 System Development 57

3.7 Summary 58

CHAPTER FOUR – RESULTS AND DISCUSSION

4.1 Introduction 59

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4.2 Maintaining Color Point 59

4.2.1 RGB LED Characterization 59

4.2.2 PWM Direct Connection to the RGB LED 63

4.2.3 PWM through Resistor to the RGB LED 64

4.2.4 Additive Color Mixing 65

4.2.5 Photodiode Amplifier Feedback Resistance 66

4.2.6 LED Forward Voltage 73

4.2.7 Open-Loop System 80

4.2.8 Closed-Loop System 81

4.3 Ambient Light Disturbance Elimination Results 90

4.4 Summary 91

CHAPTER FIVE – CONCLUSION AND RECOMMENDATIONS

5.1 Conclusion 92

5.2 Recommendations and Future Work 93

REFERENCES 94

APPENDICES

vii

LIST OF TABLES

Page

Table 2.1: Wavelength of distinct colors 6

Table 2.2: Radiometric and photometric unit of measurement 8

Table 3.1: Specifications of microcontroller 29

Table 3.2: Specification of RGB LED 30

Table 3.3 Hardware Components 31

Table 3.4: Reference and random reading parameters description and

abbreviation

37

Table 3.5: Obtained calculated resistance 50

Table 3.6: Parameters values for initial gain amplification 51

Table 4.1: Temperature Influenced on LED Forward Voltage 60

Table 4.2: Temperature Influenced on LED Radiant Intensity 61

Table 4.3: Microcontroller port test result without connection to LED 63

Table 4.4: Microcontroller port connection to LED 64

Table 4.5: Microcontroller port connection to potentiometer and LED 64

Table 4.6: PWM values for RGB LED additive color mixing 65

Table 4.7: RGB LED radiant output for 250 kΩ gain amplification results 67

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Table 4.8: ADC results of the RGB LED mixed-color radiant output 69

Table 4.9: Red LED radiant output ADC results 70

Table 4.10: Green LED radiant output ADC results 71

Table 4.11: Blue LED radiant output ADC results 71

Table 4.12 Cree RGB LED forward voltage ADC results 74

Table 4.13: Forward voltage ADC results with RC Filter 75

Table 4.14: Open-loop test of LED forward voltage 80

Table 4.15: Open-loop test of LED radiant output 81

Table 4.16: Cree LED forward voltage ADC results in closed-loop system 81

Table 4.17: Seoul LED forward voltage ADC results in closed-loop system 82

Table 4.18: Cree LED radiant intensity ADC results in closed-loop system 84

Table 4.19: Seoul LED radiant intensity ADC results in closed-loop system 85

Table 4.20 Control mechanism performance for Cree RGB LED 87

Table 4.21 Control mechanism performance for Seoul RGB LED 87

Table 4.22: LED radiant output with ambient light 90

Table 4.23: LED radiant output with ambient light elimination 91

ix

LIST OF FIGURES

Page

Figure 2.1: Illustration of electromagnetic radiation solid angle 7

Figure 2.2: Distinct color areas in CIE 1931 chromaticity diagram 9

Figure 2.3: Δuʹvʹ shifts in CIE 1976 chromaticity diagram 10

Figure 2.4: Color gamut location creates by RGB LED 12

Figure 2.5: Illustration of PWM duty cycle 14

Figure 2.6: Junction temperature measurement for RGB LED 17

Figure 2.7: Junction temperature measurement in system 18

Figure 2.8: Thermal resistance model 19

Figure 2.9: Optical feedback system for backlight application 20

Figure 2.10: Control structure of the RGB LED lighting system 22

Figure 2.11: Improved control structure of the RGB LED lighting system 24

Figure 2.12: Light measurement in a PWM single period 26

Figure 3.1: Flowchart of designing color control mechanism 28

Figure 3.2: Block diagram of the proposed design system 30

Figure 3.3: LED placement position 32

Figure 3.4: TIA 33

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Figure 3.5: Color error in different PWM resolution 34

Figure 3.6: Control mechanism closed-loop block diagram 36

Figure 3.7: Closed-loop system flowchart 39

Figure 3.8: Radiant output intensity reading for RGB LED 46

Figure 3.9: Ambient light disturbance reading 47

Figure 3.10: PWM output to R, G and B LED 48

Figure 3.11: PWM output to resistor in series with R, G and B LED 50

Figure 3.12: Photodiode and TIA connection 51

Figure 3.13: Simple RC filter 53

Figure 3.14: RC filter with 0.47 ms time constant 54

Figure 3.15: RC filter with 1 ms time constant 55

Figure 3.16: RC filter to LED input 56

Figure 3.17: RC filter to TIA output 57

Figure 3.18: PCB integration for the system 58

Figure 4.1: RGB LED Forward Voltage and Temperature 60

Figure 4.2: RGB LED Radiant Intensity and Temperature 62

Figure 4.3: Additive Color Mixing by RGB LED 66

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Figure 4.4: Mixed-color Radiant Output of RGB LED 68

Figure 4.5: Oscilloscope Test Results for Implemented RC Filter 69

Figure 4.6: RGB LED radiant output result in ADC values 73

Figure 4.7: Forward voltage of RGB LED Cree LED 75

Figure 4.8: RGB LED forward voltage ADC result with RC filter 76

Figure 4.9: Forward voltage changes versus PWM duty cycle 78

Figure 4.10: Forward voltage changes versus Temperature 79

Figure 4.11: Cree RGB LED forward voltage compensation 83

Figure 4.12: Seoul RGB LED forward voltage compensation 83

Figure 4.13: Regulated Cree RGB LED radiant intensity 88

Figure 4.14: Regulated Seoul RGB LED radiant intensity 89

xii

LIST OF ABBREVIATIONS

ADC Analog-to-Digital Converter

CCFB Color Coordinate Feedback

CCFL Cold Cathode Fluorescent Lamp

CCR Continuous Current Reduction

CCT Correlated Color Temperature

CIE International Commission on Illumination

CRI Color Rendering Index

DC Direct Current

DIP Dual Inline Package

EEPROM Electrically Erasable Programmable Read Only Memory

FFB Flux Feedback

I/O Input and Output

LCD Liquid Crystal Display

LED Light Emitting Diode

LSB Least Significant Bit

PC Phosphor-Coated

PCB Printed Circuit Board

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PIC Peripheral Interface Controller

PID Proportional Integral Derivative

PWM Pulse Width Modulation

RAM Random Access Memory

ROM Read Only Memory

RC Resistor-Capacitor

RGB Red Green Blue

SSL Solid State Lighting

TFF Temperature Feed Forward

TIA Transimpedance Amplifier

UART Universal Asynchronous Receiver/Transmitter

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LIST OF SYMBOLS

cd Candela

μ Micro

f Frequency

F Farad

Hz Hertz

° Degree

Ω Ohm

∆ Delta

λ Lambda

τ Time Constant

K Kelvin

C Celsius

ω Omega

sr Steradian

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PUNCA CAHAYA DIOD PEMANCAR CAHAYA MERAH HIJAU BIRU

MUDAH ALIH DENGAN MEKANISME KAWALAN WARNA UNTUK

APLIKASI LAMPU BELAKANG

ABSTRAK

Tesis ini mengemukakan pembangunan mekanisme kawalan warna untuk sumber

cahaya oleh Diod Pemancar Cahaya (LED) Merah Hijau dan Biru (RGB).

Pencampuran secara penambahan daripada LED asas tersebut memerlukan

mekanisme kawalan kerana warna yang dihasilkan tertakluk kepada variasi. Ini

adalah disebabkan peningkatan suhu ambien dan pemanasan sendiri oleh LED

tersebut. Titik warna akan berubah kerana penurunan keluaran keamatan sinaran

tiap-tiap LED adalah tidak sama. Pengawal mikro dicadangkan sebagai unit pusat

untuk mendapatkan set titik permulaan pencampuran warna seterusnya mengawal

setiap LED apabila sisihan titik warna baru berubah melebihi had. Kaedah ini

bergantung pada jangkaan voltan ke depan terhadap setiap LED dan sisihan ukuran

sinaran oleh penderia optik. Mekanisme kawalan warna yang telah dibangunkan

boleh mengekalkan gabungan titik warna dengan berkesan apabila tertakluk kepada

ujian suhu ambien terpecut menerusi penenggelam haba yang dilekatkan pada LED.

Sistem ini diuji pada dua jenis LED RGB. Sistem ini berupaya untuk mengurangkan

83 % ralat relatif untuk LED jenis pertama dan 78.6 % ralat relatif untuk LED jenis

kedua. Ralat relatif diperolehi semasa proses mengukur dengan mekanisme kawalan

tertutup mengurangkan ralat sistem kawalan terbuka daripada set titik permulaannya.

Gangguan cahaya ambien juga berjaya dihapuskan semasa penderia beroperasi.

xvi

A PORTABLE RGB LED LIGHT SOURCE WITH COLOR CONTROL

MECHANISM FOR BACKLIGHTING APPLICATION

ABSTRACT

This thesis presents the development of color control mechanism for Red, Green and

Blue (RGB) Light Emitting Diode (LED) light source. The additive mixing by the

primary LED requires a control mechanism since a color produced is subjected to

variation. This is due to the influenced of elevated ambient temperature and self-

heating of the LEDs. The color point will vary because the individual LEDs output

radiant intensity degradation is not the same. A microcontroller is proposed as a

central unit to obtain an initial set point of the color mixing thus controlling each

LED when the difference of new color point exceeded tolerance. The method relies

on the forward voltage prediction to the individual LEDs and the radiant intensity

measured by an optical sensor. The developed color control mechanism can

effectively maintain the mixed color point when it is subjected to an accelerated

ambient temperature test through heating the heat-sink attached to the LED. The

system is tested on two types of RGB LED. The system is able to reduced 83% of

relative error for the first type LED and 78.6 % of relative error for the second type

LED. The relative error is obtained during the measuring process with the closed-

loop control mechanism reducing the open-loop control system error from its initial

set point. The ambient light disturbance is also successfully eliminated during the

sensor in operation.

1

CHAPTER ONE

INTRODUCTION

1.1 Overview

Solid-State Lighting (SSL) devices such as a light emitting diode (LED) and

an organic LED (OLED) have shown a great potential to enhance the performance of

artificial lighting. LED light source has shown its significant economic benefit due to

their lower operating condition and longer lifetime’s service (Azevado et al., 2009).

Thus, the LEDs are better in energy consumption compared to the other conventional

light sources such as incandescent light bulb, halogen light bulb and most compact

fluorescent light (CFL). Although the LED luminaires lag slightly behind fluorescent

light luminaires, continuous innovation has shown competitive improvement in its

efficacy (Bardsley et al., 2014).

Despite that the CFL shows a good light efficacy, it is not environmental

friendly since mercury is added in the fabrication process and poses a danger to our

environmental health. Hence, LED is a greener choice for lighting purposes (Kim

and Schubert, 2008). Moreover, LED shows inherent controllability that their

luminance can be controlled or dimmable compared to other conventional light

sources. This SSL device also offers saturated colors, robustness and small size

device (Jacobs, Jie and Hente, 2008). Other advantages of LEDs are they are anti-

vibration and shock resistant devices (Bergh et al., 2001).

There are two methods to generate white light for general illumination for

example by the phosphor coated (PC) white LED or by mixing the light output of

red, green, and blue (RGB) LED. The PC white LED suffers from shorten lifetime

2

because of phosphors deterioration and efficiency dropped caused by the Stoke-shift

characteristic of the down conversion which a phosphor absorbs shorter wavelength

energy and emits it at a longer wavelength (Zukauskas et al., 2002).

Theoretically, color mixing from RGB LED could provide better efficiency

and enable color adjustability. At present, this method is most admirable feature for

future LED lamps or other applications such as backlighting because of it

individually controllable. In turns, various shade of white or color can be created by

tuning the ratios of each RGB LED strings (Ying, Tang and Huang, 2006), (LED

Color Characteristic, 2012). In order for the RGB LED light source to achieve

reliable luminous output and color stability for its application, various color control

feedback are applied (Deurenberg et al., 2005). The control system in return could

stabilize the luminous and color output that are influenced by ambient temperature

and also self-heating of the LEDs.

1.2 Problem Statement

In general, luminous output and wavelength of LEDs are affected by

temperature changes, driving current and ageing. The changes in LEDs junction

temperature will result in a degradation of lifetime, efficiency and its forward voltage

(Manninen and Orrevetelainen, 2007). As for the RGB LED, it is subjected to

different luminous output degradation rate of individual LED due to the self-heating

and ambient temperature changes. Thus, it will impact on its color point (Chhajed et

al., 2005). Therefore, it requires color control mechanism system that able to tune the

electrical power of each RGB LED to regulate the luminous output and its color

balance.

3

Since the optical sensor operates based on the input optical signal, the

environment illuminant might affect the accuracy of the detected output. Thus, it is

required to prevent the input optical signal from being affected by external

environment so that be reliable and usable to mobile outdoor (Muthu and Gaines,

2003).

Most of the explored control techniques are dependent on the International

Commission on Illumination (CIE) standard to express a color. However, every

human interprets a color differently with their own perception. Because of that, there

is not entirely necessary for a system to produce an accurate color. Therefore, the

aim of this research is to develop an embedded control mechanism on an arbitrary

color point input according to the user desire with using a simple microcontroller.

1.3 Objectives

This project is carried out for the following objectives:

i. To develop an embedded color control mechanism system to maintain color

point within minimum accuracy of at least 20 LSB employing 10-bit ADC

converter for portable application.

ii. To eliminate the effect of ambient light disturbance when sensing in

operation.

1.4 Scope of Work

The scope of the project is to design and implement the control mechanism to

regulate the radiant intensity output losses of RGB LED and maintain its color point.

4

1.5 Thesis Organization

Chapter 1 comprises of the overview of the project which introduce the

feature and future of RGB LED lighting, problem statement, objectives, scope of the

project and thesis organization.

Chapter 2 summarizes the literature review for the RGB LED light source

and it’s luminous and color output control system design. Several LED driving

techniques will be explained in this chapter. Various type of color control feedback

or mechanism used to stabilize the RGB LED color light output will be discussed.

Some background on the RGB LED and its application is studied as well.

Chapter 3 reveals the methodology used for this project. The road map and

flow chart of the design will be discussed within this chapter. This chapter will

describe the proposed block diagram of the color control circuit, parameters involved

in the design, closed-loop flowchart design for the source code implementation and

hardware design. The hardware integration is described and implementation using

the optical sensor and forward voltage prediction is explained. In addition, the

mechanism source code implemented in C programming language is described.

Chapter 4 indicate the results and discussions. The ambient temperature test

on the RGB LED light source will be analyzed. This result implies the design of the

color control mechanism. All other practical results and analysis from each approach

of the methodology are discussed.

Chapter 5 is the conclusion for the achievement of the objectives.

Furthermore, this chapter includes a brief statement of successfulness and future

work.

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CHAPTER TWO

LITERATURE REVIEW

2.1 Introduction

In this chapter, some background on the RGB LED and its application is

studied. The perception of color and additive color mixing by using the RGB LED is

discussed. In addition, a review is done on the methods of driving and dimming the

LED. The color control feedback techniques are studied and elaborated to gather the

information in order to implement the control mechanism system.

2.2 RGB LED Application

RGB LED can provide white light sources with variable colour point in

consideration of tunable white point, wider color gamut, faster response, longer

lifetime, high dimming ratio and higher luminance efficacy make RGB LED suitable

as a backlight for high end Liquid Crystal Displays (LCD) applications (Muthu et al.,

2002), (Hsieh et al., 2009). The advantages of LCD are light weight, high resolution

and good color performance. LCD used for a wide range of display applications in

consumer products such as notebooks, PC monitors, televisions, cellular phones and

multimedia products (Cho and Kwon, 2009), (Nam et al., 2009). As display size and

resolution are increases, it is also fundamental to optimize display backlighting.

Backlighting module is important to drive light source in LCD technologies in

consideration of LCD display quality since LCDs are not self-luminance display

devices (Liu et al., 2010).

For general lighting illumination purposes, RGB LED has shown an

advantage over Phosphor-Coated (PC) white LED as its luminous output can be

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tuneable. It could provide different white color temperature such as cool white, warm

white and can be set to other white light according to the user mood (Lim et al.,

2006).

2.3 Color

Color is defined as perceptual results of light in the visible region of

spectrum. The human eye is sensitive to a small frequency band, known as visible

light, ranging from approximately 390 nm to 720 nm. Human eye perceives certain

color corresponding to certain wavelength within the electromagnetic spectrum. For

instances, human will interpret visible light of the largest wavelength as red while

that of smallest wavelength as blue, other colors will be interpreted according to their

corresponding wavelength as shown in Table 2.1.

Table 2.1: Wavelength of distinct colors (Schubert, 2006)

Color Wavelength

(nm)Color

Wavelength (nm)

Ultraviolet < 390 Yellow 570 – 600

Violet 390 – 455 Amber 590 – 600

Blue 455 – 490 Orange 600 – 625

Cyan 490 – 515 Red 625 – 720

Green 515 – 570 Infrared > 720

The radiometry is the detection and measurement of electromagnetic

radiation across the total spectrum. Some terms used in the radiometric measurement

such as the radiant power, radiant intensity, irradiance and radiance. The radiant

power is radiant energy emitted over a surface in unit time interval. For the radiant

intensity, it is defined as a result of the radiant power emitted by a source in an

extremely small solid angle. The irradiance is a result of radiant power incident on a

7

surface, whereas the radiance is a measure of radiant power at the receiver

(Wyszecki and Stiles, 1982).

Figure 2.1 depicts the illustration of solid angle of the electromagnetic

radiation. As depicted, the solid angle cuts off an area on the surface of a sphere, S

centered at C and radius of r. The solid angle size is obtained by

(2.1)

The measurement unit of solid angle is in steradian unit denoted as sr.

Figure 2.1: Illustration of electromagnetic radiation solid angle (Wyszecki and Stiles,

1982)

On the other hand, photometry is a radiometric power that is scaled by the

spectral response of the human eye. Photometry is most likely equivalent to

radiometry except the measurement is within the visible wavelength range. Table 2.2

lists some units of measurement of the radiometric and photometric in comparison.