Portable Solar Tracker

Tri Bui, Tuyen Bui, Christopher Davis,

Stephen Holman

Department of Electrical Engineering and

Computer Science, University of Central

Florida, Orlando, 32816

Abstract – In order to be a part of the “green

technology” revolution, a goal was established to

design and create an optimized device that would

capture, store, and eventually distribute solar energy.

A concentrator is attached in order to focus a wide

area of light on a small area of solar panels. The

device tracks the path of the sun so that the sun’s rays

are always orthogonal to the solar panels, thus

maximizing the energy captured. A microcontroller

“conducts the symphony” of the peripheral devices

that work in tandem; these devices include a display,

compass, photoresistors, motors, and battery charging.

Once charged, the energy stored in the battery is

capable of powering and charging a small electronic

device such as a cell phone or an iPod®. The device

was also designed to be structurally stable and

relatively lightweight such that it may be considered

portable.

Index terms – Reverse Current/Overcharge

Protection, Trough Concentrator, Mylar,

Photoresistor, Voltage Regulation, Dual Axis Rotation

I. INTRODUCTION

The portable solar tracker is an optimization based

project that involves challenges with power system

monitoring and management, feedback control systems,

circuit design, programming, structural reliability and

practicality, amongst other residual challenges. After

conducting research, it was discovered that there is a

significant difference in the voltage that is output

depending on the angle of the panel with respect to the

sun’s rays, and was thus decided that a primary source

of the optimization would involve the panels tracking

the path of the sun. An efficient, low power system

would be required to move and monitor the panels such

that they remained in a constant perpendicular

orientation. It was also hypothesized that an optical

concentration system would increase energy intake, so a

lightweight focusing contraption needed to be designed.

A picture of the final prototype is shown in Figure 1.

Figure 1a) Side View

Figure 1b) Top View

The device utilizes a two-button interface, one to

reset the stored information and sweep to determine the

sun’s position and another to display the information on

an LCD. The user can then use this information to

determine if there is enough energy to charge his or her

electronic device.

II. THE EFFECTS OF SUNLIGHT ON EXPOSED

SURFACES

The solar radiance given at Earth’s distance from the

Sun, is about 1,368 watts of energy in the form of EM

radiation per square meter. By positioning the

photovoltaic panel on a tracking system, you can

optimize the area that is being exposed to its fullest.

Utilizing the tracking system you can achieve an

increase in the output power by up to 20%. Ideally, the

sun and the panel would have to be orthogonal to each

other in order to achieve that efficiency. The reason is

that at any other angle, the amount of area exposed will

disperse the amount of radiance exposure on it. Figure

2 below shows the effect of exposure on a given area.

Figure 2: Effects of sunlight at 90 degree and 30 degree

III. COMPONENTS

A. Solar Panels

The solar panel array will consist of 16

monocrystalline photovoltaic cells arranged in two

ways, each will consist of 4 cells. The first is an

arrangement of 4 cells that are in parallel with each

other to produce 8 V and 220 mA. As for the other

configuration, it will be 2 cells in series in parallel with

another 2 cells that is in series, making up 16 V with

110 mA.

This is the energy conversion efficiency of the

photovoltaic cells shown in equation 1. Where P

power produced by the cell, E is the irradiance which is

1000 W/m2, and Ac is the area of the cell. With this

equation, the calculated efficiency of each solar panel

14.59 %. The solar tracker’s aim is to help either

maintain peak performance of the solar panel array by

keeping the panel orthogonal to the sun’s ray, or to

further increase the performance with the solar

concentrator. The aim is to try to reach

increase of 20 %.

Equation 1:

This is the energy conversion efficiency of the

photovoltaic cells. Where Pm is the power produced by

the cell, E is the irradiance, and Ac is

cell. The solar panel for the tracker’s fill factor came

out to be .89, which is typical for commercial grade

cells ( usually >.7).

Equation 2:

The LM2598 adjustable switching regulator, an

efficient component, will be used to regulate an input of

16V to 7.3V. A similar switching regulator was used to

light at 90 degree and 30 degree

The solar panel array will consist of 16

photovoltaic cells arranged in two

ways, each will consist of 4 cells. The first is an

arrangement of 4 cells that are in parallel with each

0 mA. As for the other

configuration, it will be 2 cells in series in parallel with

n series, making up 16 V with

This is the energy conversion efficiency of the

photovoltaic cells shown in equation 1. Where Pm is the

power produced by the cell, E is the irradiance which is

he cell. With this

equation, the calculated efficiency of each solar panel is

The solar tracker’s aim is to help either

maintain peak performance of the solar panel array by

keeping the panel orthogonal to the sun’s ray, or to

performance with the solar

he aim is to try to reach a minimum

This is the energy conversion efficiency of the

is the power produced by

is the area of the

cell. The solar panel for the tracker’s fill factor came

out to be .89, which is typical for commercial grade

The LM2598 adjustable switching regulator, an

efficient component, will be used to regulate an input of

16V to 7.3V. A similar switching regulator was used to

regulate an input voltage from the battery to the USB

output, the second switching regulator will

The reason why the switching regulator is used was

because it proves to be more efficient then the linear

regulator. Unlike the linear regulator, the constant

switching (at 150 kHz) that is produce

minimizes wasted energy in the form of heat given off

by the heat sink. Figure 3 shows the switching regulator

waveform and the characteristic switching that occurs

by the regulator. As you can see, it switches from peak

value to zero and then cycles over again.

Figure 3: Oscilloscope picture of the D

Switching regulator.

B. Microcontroller

The components of the solar tracker

controlled by the Atmega168 microcontroller

microcontroller will be powered directly by one of the

solar panel arrays and will be regulated to 5 V

operating voltage. A reset switch is attached for

necessary cases and situations. This IC has

pins, reserved for the photoresistors, and 14

that will control all the other components.

microcontroller was chosen because it has the precise

number of pins for the components that needed it. The

MCU also has 14 KB of flash memory

programming.

C. Motors

In order for the sun's rays to be orthogonal to the sun

at all times and thus provide the most power from the

photovoltaic cells, motors were required to be able to

adjust and move the solar tracker to follow the sun or

any other light source. The advantages and

disadvantages of various motor types and

configurations were analyzed including DC, Servo, and

Stepper motors. Servo motors and in particular two (2)

regulate an input voltage from the battery to the USB

output, the second switching regulator will output 5 V.

The reason why the switching regulator is used was

because it proves to be more efficient then the linear

regulator. Unlike the linear regulator, the constant

switching (at 150 kHz) that is produced by the regulator

form of heat given off

shows the switching regulator

waveform and the characteristic switching that occurs

by the regulator. As you can see, it switches from peak

value to zero and then cycles over again.

Oscilloscope picture of the D-D Adjustable

Switching regulator.

the solar tracker will be

controlled by the Atmega168 microcontroller. The

microcontroller will be powered directly by one of the

arrays and will be regulated to 5 V, the ideal

A reset switch is attached for

This IC has 6 analog

ed for the photoresistors, and 14 digital pins

that will control all the other components. This

microcontroller was chosen because it has the precise

number of pins for the components that needed it. The

flash memory to store the

In order for the sun's rays to be orthogonal to the sun

ll times and thus provide the most power from the

photovoltaic cells, motors were required to be able to

adjust and move the solar tracker to follow the sun or

any other light source. The advantages and

disadvantages of various motor types and

ns were analyzed including DC, Servo, and

Stepper motors. Servo motors and in particular two (2)

Hitec HS-322HD fixed rotation servos were selected.

Specifications that made the HS-322HD ideal for use in

this project include it's low power consumption of

7.4mA current draw idle, and 160mA/60º at 4.8V.

Because of the nature of the solar tracker project, speed

was not a consideration when selecting a motor while

torque on the other hand (3 kg.cm/41.66 oz.in at 4.8V

and 3.7 kg.cm/51.38 oz.in at 6.0V) was a major factor

in its selection because the optical configuration needed

to be moved and then held at specific angles depending

on the location of the light source. There are two

motors used in the solar tracker, one used for

controlling direction along the X-Axis, and one for

controlling direction across the Y-Axis. This is referred

to as a pan and tilt configuration. Figure 4 below shows

the motors and how the pan and tilt works on the solar

tracker.

Figure 4: Motors Demonstrating Pan and Tilt

D. Optical Configuration

The purpose of the optical configuration was to

increase the amount of light that is received incident

onto the photovoltaic cells and thus increasing the

power output by the cells. Different designs were

considered including Plane, Parabolic, and Trough

(open-cylinder) mirror configurations along with lenses

used for focusing light onto a focal point positioned

precisely on the solar panels. The trough design was

eventually determined to be the most efficient method

of increasing the power output of the solar panels after

testing several different methods and combinations of

lenses and mirrors. Figure 5 shows several terms and

properties of the concave mirror design of the trough

shape configuration chosen. The final optical

configuration was constructed using a reflective layer

of Mylar for the mirror like surface which was then laid

on a lightweight wire mesh to allow the frame to be

adjustable so that the focal point could be adjusted if

required. Once the trough was finished the solar panels

were then added both facing the vertex and facing

towards the sun. This configuration allowed the

efficiency of the optical configuration to measure at all

times. Figure 6 below shows the final optical

configuration of the solar tracker. Initial tests concluded

that depending on the time of day and the amount of

cloud cover the efficiency of the optical configuration

ranges from -5% to +5%.

Figure 5: Concave Mirror Properties and Terms

Figure 6: Final Optical Configuration

E. Compass

An additional feature of the solar tracker is to be

able to determine the direction that the tracker is facing

and display that information to the user. The compass

used in this project was the HMC6352 which works

using 2-axis magnetic sensors. Operating anywhere

from 2.7V to 5.2V the HMC6352 features heading

repeatability of 1º, heading resolution of 0.5º, and a

selectable update range from 1Hz to 20Hz. The

compass communicates using the simple I2C

communication protocol which allowed it to be

implemented using only two data lines, serial data

(SDA) and serial clock (SCA). Another reason that the

HMC6352 was the ideal candidate for the solar tracker

was the option of three different unique modes of

operation including Standby, Query, and Continuous.

Using standby mode the power consumption of the

compass was greatly reduced because in standby mode

the compass only performs calculations when data is

requested from the user by the push of a button as

opposed to continuously calculating the direction and

consuming much more power. The compass is

positioned on the frame so that the user easily knows

the direction that the tracker is facing at the push of a

button.

F. Battery

A 7.2V 3300 mAh Nickel Metal-Hydride battery

was chosen to store the energy collected by the solar

panels. Nickel Metal-Hydride exhibited qualities that

were more ideal for this project. It was the second most

lightweight battery type and exhibited the second

highest capacity both next to lithium-ion type batteries.

It was also a relatively cost effective battery type and

was obtained for about half the price of comparable

lithium-ion batteries. A lead-acid battery would have

been a little bit cheaper to purchase, but it would have

been much heavier and detracted from the “portable”

aspect of the device. A major drawback of lithium-ion

batteries that erased them from consideration was that

they exhibit thermal runaway in high ambient

temperatures, not ideal for an outdoor solar device. A

picture of the Ni-MH battery is shown in Figure 7.

Figure 7: Battery Close-up

The negative aspect of this battery type is that it

loses its overall capacity very quickly compared to

other types if it is not charged and discharged correctly.

Therefore the battery needed protection in the forms of

reverse current and overcharge protection. A diode was

installed to prevent reverse current and a relay

controlled by a signal from the microcontroller prevents

overcharge. The microcontroller must be powered on

for the battery to charge and will receive information

from the DS2438 to determine if it should. Two

methods, -∆V and dT/dt, are used to determine when

the battery is fully charged, and at this point the relay

creates an open circuit for the battery. A conceptual

diagram of this protection circuitry is shown in Figure

8.

Figure 8: Relay and Overcharge Protection

The battery has four solar panels dedicated to

powering it, with two in series and two in parallel to

guarantee that the input voltage meets the 7.2V

threshold. This is the threshold set by the adjustable

regulator, and since the sun’s availability is variable it

is best to have a nice cushion of voltage availability

before cut out.

G. Battery Monitoring

The status of the battery is monitored by the DS2438

battery monitoring chip from MAXIM-IC. The device

is an 8 pin surface mount IC that can measure the

battery’s voltage, the amount of current that comes in or

out, and other secondary information such as

temperature and relative humidity. Interfacing it with

the MCU is through a One Wire interface that requires

only a single connection, thus allowing more I/O ports

and read/write commands to perform one at a time. The

line is tied to a 4.7K pull up resistor with 5V power

supply making it an active high when idle. In this One

Wire interfacing scheme, message is sent by producing

low-duration time pulses. To send a 1, the connection is

held low for 15µs and returned to high again. For a 0,

the signal is held low 60µs.

Figure 9: One Wire interface (Reprinted with the

permission of MAXIM-IC)

Figure 10 below shows the circuit configuration needed

for battery monitoring. The positive terminal of the

battery connects to the VDD pin and the negative

terminal is wired to the ground of the DS2438. The

negative terminal of the load connects to the Vsense+

pin making all the current coming into or out of the

battery travel through the current sensing resistor before

going to ground. The voltage across the sense resistor is

use to calculate the current. CF (0.1µF ) and RF (100kΩ)

is a low pass filter that help prevent current spike for an

accurate current accumulation calculation.

Figure: 10 Battery monitoring circuit

(Reprinted with the permission of MAXIM-IC)

H. Photoresistors

To maintain the solar panels perpendicular with the

sun’s rays, photoresistors are used to determine whether

the sun has shifted from its last detected position.

Photoresistor (or photocell) is a type of device that

changes its resistance depending on the amount of

illumination exert on it. As light intensity increases, the

amount of resistance decreases. With this characteristic,

two photoresistors can be used to calculate the sun’s

position in one direction by differentiating their values.

If one resistor has a smaller value than the other

resistor, it means that the sun is locates closer to the

smaller value resistor. If their values are the same, then

it means that the sun is directly above. The solar tracker

contains four photoresistors (one for each direction) to

ensure that the panels are facing in an optimized

direction. To interface the photoresistors with the

MCU, each resistor are place in a voltage divider

configuration. The VT83N1 was chosen because it

outperformed the P203 photoresistor in sensitivity tests

as shown in Figure 11.

Figure 11: Photoresistor comparison

A 5V DC is supplies across the photoresistor and a

1kΩ resistor. The MCU’s A/D input measures the

voltage drop across the 1kΩ, quantize it and compare

with the other photoresistor quantized value

I. Pololu Push-Button Switch

Minimizing the amount of power consumed by the

device is partially handled by a Pololu Push-Button

Switch. The switch serves the purpose of powering the

LCD display and the HMC6352 electronic compass

when the data is requested from the user. Two pins on

the switch are connected to the microcontroller, an off

pin and a voltage out pin. When the off pin is driven

high the LCD display and the compass both receive

power, then are initialized and calculations are

performed on the compass. These calculations are

performed only once, then displayed for approximately

8 seconds. After the 8 seconds have elapsed the off pin

is driven low and power is turned off. The

microcontroller then checks the voltage out pin to

verify that the power is off by turning the voltage out

pin low. The microcontroller then rechecks for the

button to be pressed by the user before driving the off

pin high and performing the calculations and display

again.

J. LCD

To allow the users to know the status of the solar

tracker, an LCD screen is used to display information

such as the battery’s voltage and capacity, the direction

and angle the panels are facing. This type of LCD is a

simple 2X16 character display with no backlight and

has a Hitachi HD44780 controller. The reasons it was

chosen because of its ease in communicating with any

type of microcontrollers and its minimal power

requirements at 2.7V. Normal interfacing of the LCD

requires eight data connections for 8-bit mode, but with

software initialization, it can be interfacing in 4-bit

mode to save I/O pins of the microcontroller.

K. USB Output

To utilize the battery, it is use to charge electronic

devices through a Universal Serial Bus (USB) port. The

reason for having a USB port as a charger because

nowadays many portable electronic devices such as cell

phones, MP3 player and camera use the connection to

simultaneously transfer data and draw power from the

computer. The USB port has four connection pins, they

are: ground, D+, D-, and 5Vdd (see Figure 12). The D+

and D- are used to transfer data and they produce 2.5V

and 2V, respectively. To build a USB port that has

these characteristics, a simple voltage divider circuit

can be used. From the Figure 12b below, the voltage

across 100kΩ and 50kΩ produce 2.5V and 2V, and they

can to power the D+ and D- pin.

Figure 12: a) USB pin assignment b) Circuit

IV. PROGRAMMING

The ATmega168 microcontroller was programmed

using the Arduino development environment. Coding to

control the LCD, motors, and peripheral devices was

made easy because the Arduino environment is built to

support many of these devices and the majority of the

code is open source.

Every Arduino “sketch” (or program) consists of a

‘setup’ and ‘loop’ function. The setup runs once at the

initialization of the device, and this function contains

all of the initializations to which pins are interfacing

with which. It was also desired to have the tracker lock

on to the sun’s position upon reset of the device

(initialized by one of its two buttons) so an algorithm

was written into the setup function to accomplish this.

The loop function constantly cycles once the setup is

finished and continues as long as the microcontroller is

powered. This function contains the code to constantly

monitor the position of the sun and adjust the

orientation of the panels and concentrator when

necessary, it also calls to separate functions to perform

calculations for the temperature and battery state so that

they can be displayed and monitored. Every 20 cycles

the code will monitor the state of the battery and store

the information into an array, the new elements in the

array will be compared with old elements to determine

whether a signal needs to open or close the relay. The

code will use the aforementioned -∆V and dT/dt

methods simultaneously to determine whether or not

the battery should be charging.

voltagearray[x] = MeasADC(_1W_Pin, V_AD);

for(i = x; i > 0; i--)

if(voltagearray[i] < voltagearray[i-1])

dropping++;

temparray[y] =

MeasTemperature_2438(_1W_Pin);

for(j = y; j > 0; j--)

if(temparray[j] > pow(voltagearray[j-1], 1.25))

rising++;

if(dropping >= 3 || rising >= 3)

digitalWrite(relaypin, LOW);

A separate function will also be called upon to

display information to the LCD when the pololu switch

is initiated (the round button labeled “Display”). This

function will convert compass and battery monitoring

information to quantities that can be output in a

pleasing and useful manner to the user. A sample of the

compass code follows:

if(int(headingValue / 10) >= 337 ||

int(headingValue / 10) < 22)

lcd.print("North");

else if(int(headingValue / 10) >= 22 &&

int(headingValue / 10) < 67)

lcd.print("NorthEast");

Information about the state of the battery will also be

useful for the user so he or she can effectively charge

their electronic device via the USB output. The

following code illustrates how the current accumulated

will be calculated to determine and display the

percentage of the remaining battery capacity:

crntacc = CrntAccumulation(_1W_Pin);

capacity = crntacc / (2048 * .05);

battpercent = (capacity / 3.3) * 100;

lcd.print(crntacc);

The capacity variable is returned in amp hours.

Other various factors such as cloud cover and

position resetting in anticipation of sunset will be

anticipated by the code, but some aspects will be part of

the user’s responsibility to maintain optimal operation

of this device. A conceptual flow diagram of the entire

code structure is shown in Figure 13.

Figure 13: Program Concept Diagram

V. PRINTED CIRCUIT BOARD

To interface all the components for a clean and

efficient design, a printed circuit board was designed

and created. The group went to 4pcb.com and had the

Advanced Circuits corporation take care of the

fabrication of the board once schematics were

completed and error checked. Figures 15 and 16 exhibit

samples of the overall schematic. The board serves to

contain and easily interface all the components of the

solar tracker. The printed circuit board consists of 2

layers, a top layer for wire routing, and a bottom layer

which serves as a ground (GND) plane for the board.

Power is routed into the board in two separate places,

on one side to power the microcontroller, lcd, compass,

and servo motors. On the opposite side of the board

power comes in which then distributes to the battery

monitoring chip, the LM2598 switching voltage

regulators, the 7.2V NiMH battery, and eventually

flows to the 5.0V USB DC output. Throughout the

board are also various analog components placed in a

manner to minimize space but also allow for enough

space for soldering and additional component

integration if necessary. The board measures

approximately 9.5" x 4". Figure 14 below shows the

printed circuit board for the solar tracker. Figures

Figure 14: Printed Circuit Board

VI. STRUCTURE DESIGN

Structurally, the solar tracker will take the form of

the trough system. Instead of using the trough system

for solar-thermal energy, this design will be used for

solar concentration. The solar panel will be placed at

the focal point of the concentrator. For this design,

Mylar will be used as the reflective material. The

structure as a whole will be made from acrylic. This

material was tested to determine if it is structurally

sound. The trough will be supported by two support

Figure 15: Microcontroller Schematic close-up

Figure 16: Input Voltage Regulation close-up

bars of acrylic on each side. It will then be situated on

another beam of the same, but thicker material. The

support bar will raise the panel 6.5” from the beam. The

beam is 13” long by 2.5” wide. From there, wheels are

attached to help the base servo rotate while helping to

support the structure utilizing weight distribution. As

for the placement of various components such as the

PCB, they will be situated in a custom-made enclosure

with dimensions (14.5x8.5x4.5)”. Figure 1 depicts the

structure of the solar tracker.

1.<http://www.womdpws/icar/edu/tour/link=/earth/clim

ate/sunradiation_at_earth.html> . 3/31/10

2. Arduino Datasheet

<http://arduino.cc/en/Main/ArduinoBoardDuemilanove

>. 4/2/10

3. Equation 2 provided by: Jenny Nelson. The Physics

of Solar Cells (2003). Imperial College Press.

THE ENGINEERS

Tuyen Bui is a graduating

Electrical Engineer, and has

been working at an electronics

repair shop in recent years. After

graduating, he hopes to find a

job in digital communications.

Christopher Davis is a senior

graduating with a BSEE. After

graduation Chris plans on

traveling the world and honing

his skills as an engineer to work

in the analog devices industry

and/or pursue his Master's

degree in Electrical Engineering.

Stephen Holman is a graduating

Electrical Engineer. He wants to

eventually obtain an MBA. He

hopes to dedicate his career to

the advancement of renewable

energy sources or music

electronics.

Tri Bui is a graduating Electrical

Engineer from UCF. His career

interest is in hardware design. In

the future he hopes to obtain an

MBA or a master’s degree as an

Electrical Engineer.

4. Equation 2 provided by: Introduction to Thermal

Science : Thermodynamics, fluid dynamics, heat

transfer. Frank W,. Schmidt, Robert E. Henderson, Carl

H. Wolgemuth. --2nd ed. ISBN 0-471-54939-8. Page

92.

5. DS2438 Datasheet

<http://pdfserv.maximic.com/en/ds/DS2438.pdf>

REFERENCES