Abstract

LED lighting is an emerging lighting technique which is predicted to completely replace the

present lighting techniques. There are several approaches to white light generation. One

approach is to use a blue or UV LED to excite one or more phosphors to give white light.

This method is not efficient. This project attempts to make white light from the basic colours

Red, Green and Blue. The method followed is high frequency switching of these three

colours to give a mixing effect. Conventional RGB LEDs available in the market is used for

the same. The high frequency switching pulses for Red, Green and Blue LEDs are generated

using PIC18F4550. This is used to drive an optocoupler that works as a relay to switch the

corresponding colours giving a mixing effect.

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Chapter 1

Introduction

White light is composed of all spectral contents in the visible region. The rapid development

of light-emitting diodes (LEDs) over the last few years has opened up new opportunities in

the general illumination market. The efficacy of white light from LEDs is now over 20 lm/W,

which already exceeds that of incandescent lamps. It is forecast that LED efficacy will reach

50 lm/W in near future, which approaches that of compact fluorescent lamps. In addition,

higher power packages are becoming available that enable compact lighting systems with

LEDs. However, additional challenges remain. The general illumination market has strict

requirements on the quality of white light—lamps of the same type must all appear to have

the same colour point.

There are several approaches using LEDs to achieve white light. One approach is to use a

blue or UV LED to excite one or more phosphors to give white light. The focus here is on the

use of red, green, and blue LEDs (RGB-LEDs) to produce white light. The advantages of

RGB-LEDs are that they provide a light source that can have a variable color point, and

theoretically can provide the highest efficiency LED-based white light. The ability to change

the color point of the lamp provides a new feature to general illumination that has the

potential to generate new applications and hence new market opportunities.

A key challenge for RGB-LEDs is to maintain the desired white point within acceptable

tolerances. This arises from the significant spread in lumen output and wavelength of

manufactured LEDs, and the changes in LED characteristics that occur with temperature and

time. Maintaining the desired white point can only be achieved with feedback schemes to

control the relative contributions of red, green, and blue to the white light.

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Chapter 2

White light requirements

A key requirement of illumination relates to the spectral properties of the white light source.

Our perceived color of objects depends upon the spectrum of incident light upon them.

A red object illuminated with light that is drastically deficient in red will appear black. The

lighting industry uses a standard color rendering index (Ra) to determine the color rendition

properties of a light source. It is based on the components of eight standard spectra in the

white light source as compared to a black-body radiator with the same color temperature as

the light source. Thus, an incandescent lamp has an value Ra of 100. Typical fluorescent

lamps used in offices have an Ra of 80. The required value depends upon the application.

The illumination of goods in a retail store is typically the most demanding application for

color rendering index. The precise requirements depend upon the goods being displayed. As

the goods on display are changed, different color points may be desired. With conventional

light sources, this means that the lamp has to be changed. RGB-LEDs will allow the desired

color point to be achieved simply by adjusting the ratio of RGB illumination. Typical indoor

living space is illuminated with sources that have an Ra of 80. General outdoor illumination

such as street lighting puts the lowest demands on color rendering with Ra of 40 or less being

common.

The Ra that can be achieved with LEDs depends on the white spectrum. The white spectrum

is made up of the individual LED spectra, and thus, depends on the wavelengths selected, and

the number of different wavelength LEDs used to make white light.

RGB-LEDs can achieve the required Ra values provided that the correct LEDwavelengths are

selected. Most applications can be addressed by the selection of three different wavelengths.

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Chapter 3

Colour Stability of RGB LEDs

Conventional light sources (fluorescent, incandescent, etc) can be manufactured very

reproducibly such that the lumen output and color points are highly consistent.

As a result, the general illumination market has grown to expect this level of consistency. The

manufacturing process for LEDs, on the other hand, does not provide this level of

consistency. Nominally identical LEDs can vary in light output by over a factor of two, and

the wavelength can vary by many nanometers. Lumen output and wavelength also change

with temperature and lumen output changes over time in a way that cannot be accurately

predicted. These factors all influence the color point that is obtained by mixing the light from

a combination of different wavelength LEDs. We now discuss the quantitative effect of these

LED characteristics based on white light from RGB-LEDs.

The largest impact on color point of RGB-LEDs comes from changes in light output of the

individual LEDs. This can be as a result of aging, or from the initial spread in the

performance of the LEDs used in the lamp. Change in temperature of the LED pn junction

leads to changes in light output, wavelength and spectral width. These all influence the

resulting color point of the RGB-LED. The red LEDs (or any AlInGaP-based LED), typically

reduces its light output by 10–15% for every 10 C increase in temperature. If it were possible

to reduce the temperature sensitivity of the red LEDs, the stability of white light from RGB-

LEDs with temperature could be significantly improved.

In addition to the effects already discussed, the peak wavelength of an LED also shifts with

current. Thus, as the intensity of RGB-LEDs is adjusted by changing the amplitude of the

drive current to each of the LEDs, the color point of the combination will change. While this

effect limits the accuracy of the color point, it is typically less critical

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Changes in light output and peak wavelength with temperature, and changes in light output

over time mean that factory calibrations will not be sufficient to produce a stable white light

RGB-LED product. The large variability in the performance parameters of LEDs makes

compensation schemes based on temperature measurement and time inadequate.

3.1 Super Flux RGB LED

The RGB LED used in this project is the PIRANHA Super Flux RGB LED. It is a

7.6mmX7.6mm square LED.

The materials used is

1. AlGaInP for Red colour

2. InGaN for Green

3. InGaN for Blue

Some of the main features of this LED are

1. High Luminous output

2. Common anode

3. Superior weather resistance

4. Water clear lens

5. 5mm lens

6. Ultra brightness

7. Wide viewing angles

8. RoHs compliance

9. Ideal for backlight and Indicator purposes

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The rating and optical characteristics of Superflux RGB LED is give below.

Table 3.1 Absolute maximum rating

Table 3.2 Electrical and Optical Characteristics

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Chapter 4

Circuit Design

The main intention here is to design a circuit that gives an appearance of white light,

composed of red, green and blue components. One strategy is to switch on and off the red

green and blue LEDs at a high frequency, more than what our eye can perceive. The

combined effect as perceived by the eye will be a white light.

There are two parts in the circuit designed, an LED array, and a switching circuit.

4.1 LED array

An LED array is made of nine LEDs in a series-parallel manner. Three LEDs are connected

in series and three such series connections are connected in parallel. In each of the LEDs, the

corresponding pins for red, green and blue are shorted. The common anodes are shorted to

and are connected to +5V supply.

4.2 Switching Circuit

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In order to switch the three colour LEDs on and off at a high frequency, it is required to get

pulses that can drive the LEDs. These pulses are generated here with the help of

PIC18F4550.

4.2.1 PIC18F4550

It is a 40 pin IC with high endurance and enchanced flash programming features. It has an on

chip 10 bit ADC incorporating programmable acquisition time allowing for a channel to be

selected and a conversion to be initiated without waiting for a sampling period and thus

reducing code overhead. The flash program memory is of size 32 Kbytes. ADC is of 13

channels, and has five bidirectional ports. It also has a streaming parallel port. There is an

internal oscillator block which generates two different clock signals, either as the

microcontroller’s clock source and may eliminate need for external oscillator circuit on the

OSC1 and/or OSC2 pins. The other clock source is the internal RC oscillator which provides

a nominal 31kHz output.

In this circuit, no external oscillator is used. The PIC is operated using the oscillator within

the chip. The operating voltage of PIC18F4550 is 4.2V to 5V DC. PIC 18F4550 uses the

standard set of 75 PIC18 core instructions, as well as an extended set of eight new

instructions for the optimization of code that is recursive or that utilizes a software stack. Most

of the standard instructions are a single program memory word (16 bits) but there are four

instructions that require two program memory locations. Each single-word instruction is a 16-

bit word divided into an opcode, which specifies the instruction type and one or more

operands, which further specify the operation of the instruction.

Fig 4.1 shows the detailed pin diagram of pic18F4550 used in this project.

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Fig 4.1 PIC18F4550 pin diagram

A 5V supply is connected across VDD and VSS pin number 11 and 12 respectively and also

to pin number 1 through a resistor.

PIC18F4550 supports a total of 16384 instructions. It also has high current source/sink

capability of 25mA.

4.2.2 MCT2E optocoupler

The PIC produces a series of pulses for each of the three colours red, green and blue. Since

the LEDs are common anode type, we need a switching circuit to convert the pulses to

switching commands for the three LEDs. Here we are using an optocoupler MCT2E for this

purpose. The optocoupler figure is given below.

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Fig 4.2 Optocoupler MCT2E

These are Standard Single Channel Phototransistor Couplers. The MCT2/ MCTE family is an

Industry Standard Single Channel Phototransistor. Each optocoupler consists of gallium

arsenide infrared LED and a silicon NPN phototransistor.

This isolation performance is accomplished through double molding isolation manufacturing

process. These isolation processes and the quality program results in the highest isolation

performance available for a commercial plastic phototransistor optocoupler.

Some of the characteristics of MCT2E optocoupler is given in the following tables.

Table 4.1 Absolute Maximum Input Ratings

Parameter Test condition Value Unit

Reverse voltage 6 V

Forward current 60 mA

Surge current T <= 10 microsec 2.5 A

Power dissipation 100 mW

Table 4.2 Absolute Maximum Input Ratings

Parameter Test condition Value Unit

Collector-emitter

breakdown70 V

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Emitter-base breakdown

voltage7 V

Collector current T <= 10 ms 100 mA

Power dissipation 150 mW

4.3 Circuit

Fig 4.3 Circuit Diagram

The above figure shows the circuit diagram for this project. The LED switching pulses are

generated at high frequency using a PIC18F4550. Port D of PIC18F4550 is configured as an

output port first. Then, the pins 19, 20 and 21 corresponding to Port D RD0, RD1 and RD2

are programmed to output the pulse. Since we are using common anode type RGB LED,

these pulses cannot be used to drive the LED array directly. For this , an optocoupler MCT2E

is required. The pulses from the Port D pin RD0, RD1 and RD2 corresponding to red, green

and blue are given to one input of the optocoupler each (pin number 1). Pin number 2 of all

three optocouplers are shorted and grounded.

In the LED array, all the common anodes are shorted and connected to +5V supply. The

anodes corresponding to red, green and blue from the array are given to pin number 5 of each 11

of the 3 optocouplers. The pin 4 of all these are shorted and connected to ground through a

pot.

4.4 Working

When the program is run on the PIC (Refer Appendix for the program), it generates pulses of

7 milliseconds each. Each of these pulses are separated by 3 millisecond gap, before

switching on the next. In this way it is assured that one set of colours is turned off before the

next set is turned on. A total of 10 millisecond (7 ms for on and 3ms for off) is required for

each of the three colours. This gives a total of 30 milliseconds duration for switching all three

colours. This high frequency switching results in high speed on and off of red, green and blue

LEDs giving an appearance of white light when viewed from a distance. This is because of

the persistence of vision, or the retention of an image in the eye for 1/16 th of a second. Since

all the three colours switch within this time, it appears to be a mixture of these colours. As is

obvious, the mixing of these three basic colours result in white light.

Chapter 5

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Result and Conclusion

5.1 Switching pulses

Figure 5.1 shows the switching pulses of red green and blue LEDs respectively. These pulses

are the output of the pin numbers 19, 20 and 21 of the PIC18F4550. Refer Appendix for the

program. It is these pulses that are given to optocoupler.

Fig 5.1 Switching pulses

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It is observed from the above figure that pulses are 5V pulses of duration 7 milliseconds,

followed by a gap of 3 milliseconds, resulting in a total of 30 milliseconds duration for

switching all 3 LEDs.

White light was successfully obtained from RGB LEDs using a high frequency switching

circuit using PIC18F4550. For better results, all LEDs have to be tested for luminous

intensity of red green and blue colours and the LEDs with nearly the same output has to be

selected to make the LED array. If there is a large variation in the colour output, the result

will have a particular colour domination in the output.

The following figure shows the output obtained.

Fig 5.2 White light obtained from RGB LED

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The Program

LIST P=18F4550, F=INHX32 ;directive to define processor

#include <P18F4550.INC> ;processor specific variable definitions

CONFIG WDT=OFF; disable watchdog timer

CONFIG MCLRE = ON; MCLEAR Pin on

CONFIG DEBUG = ON; Enable Debug Mode

CONFIG LVP = OFF;

CONFIG FOSC = INTOSCIO_EC

;Reset vector

; This code will start executing when a reset occurs.

RESET_VECTOR CODE0x0000

goto Main ;go to start of main code

;Start of main program

Main:

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CLRF PORTD

MOVLW 0x00

MOVWF TRISD,0

L1 MOVLW B'00000001'

MOVWF PORTD,0

CALL DELAY1

MOVLW B'00000000'

MOVWF PORTD,0

CALL DELAY2

MOVLW B'00000010'

MOVWF PORTD,0

CALL DELAY1

MOVLW B'00000000'

MOVWF PORTD,0

CALL DELAY2

MOVLW B'00000100'

MOVWF PORTD,0

CALL DELAY1

MOVLW B'00000000'

16

MOVWF PORTD,0

CALL DELAY2

GOTO L1

DELAY1

MOVLW 0X8F

L2 DECFSZ WREG

GOTO L2

RETURN

DELAY2

MOVLW 0X0F

L3 DECFSZ WREG

GOTO L3

RETURN

;End of program

END

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References

[1] “Red, green, and blue LED based white light generation: issues and control”, IEEE

Journal 2002 Muthu, S.  Schuurmans, F.J.  Pashley, M.D. Philips Res., Briarcliff Manor

[2] MCT2/MCT2E Datasheet, Vishal Semiconductors

[3] PIC18F2455/2550/4455/4550 Data Sheet, Microchip

[4] “Programming and customizing the PIC microcontroller” Michael Predko, Mike Predko

Balu Raveendran,

National Institute of Technology Calicut,

Kozhikode, Kerala

Email:balu.raveendran@gmail.com

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