CHAPTER 1

INTRODUCTION

Guitar is one of the most popular stringed instruments in the music industry

today. It consists of six strings, each tuned to a particular musical note. Musical notes

are basically sound waves of a certain frequency which are attained in guitar by

vibrations of these six strings and they differ with the tension in these strings. Hence

tuning the guitar strings to the preset values becomes a very important task for a

guitarist.

Tuning is generally done with the use of guitar tuners which display the

current note being played. A guitarist observes the tuner and changes the tension in

the strings with the help of a tuning key until the correct note is attained.

Figure 1.1: A typical guitar tuner and tuning key

The objective of the project is to make the instrument self-tuning, meaning

that the frequency of vibration of the string must be picked up by a guitar pickup to be

transformed from a signal to a mechanism for tensioning which will provide

tightening and loosening of the string to generate a different pitch.

To accomplish this task a simple feedback loop is required where the

tensioning mechanism is the actuator (servo motor), the sound produced by the

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vibration of the string is the measured output and the user selected note to be played is

the set-point.

The self-tuning guitar will be extremely useful for the musician who wants to

effortlessly tune their guitar. All of the processing is done on the guitar, so there is no

external device that needs to be connected and attached to the guitar. The fiddling

with the tension in the strings is left to the guitar and more specifically the motors on

the guitar. People who regularly play guitar or who are just starting out will find this

system very beneficial to use just before playing.

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CHAPTER 2

LITERATURE SURVEY

1. Thomas Johnsen et al (2008) proposed the design of a self-tuning guitar which

had the ability to detect the frequency at which an individual string was vibrating and

then adjusting the tension on the string until it vibrated at a predetermined desired

frequency. The string frequency was captured using the guitar’s standard output jack,

and then a microcontroller was attached to the back of the guitar which interpreted the

frequency and sent commands to a collection of stepper motors on the head in order to

adjust the string that is being played. The unit’s controls consisted of a six-position

switch which was used to select the string and a set of the LEDs that indicated the

tuning status.

The process was carried out in five steps. The very first step involved

acquisition of the sound signal from guitar pickups through the output jack on the

guitar. Next, the signal from the guitar was amplified with a DC bias of 2.5 V. After

amplification, the overtones were removed using a low pass filter in the third step and

the signal is then sent to the microprocessor in the fourth step.

Now, the output of the LPF was an analog signal. In order to do calculations

within the microprocessors the analog values needed to be converted to digital values.

In order to do this, the A/D Converter sampled the incoming data and recorded the

values in the A/D registers. A “zero” value of 2.5V was used in calculations so

attenuation in the signal could be ignored. In order to calculate the frequency, the time

elapsed in between the “zero” value points in the waveform were measured with the

uP timer.

The last step involved changing the tension in the string using stepper motors.

Reasons for the selection of stepper motors instead of servo was also explained as it

had the capability of being precisely controlled and they allowed more than one full

revolution in both forward and reverse directions.

2. The project 1 of SENG 466 (2013) consisting of Michael Sykes, Bruno Sampaio,

Radost Rychterawas was to design a self-tuning monochord guitar which detected a

3

few notes around middle C and tuned the string. The instrument was be self-tuning

such that the frequency of vibration of the string must be picked up by a sensor to

transform from a signal into a mechanism for tensioning which will provide

tightening and loosening of the string to generate a different pitch. To accomplish the

model, the mechanical design first had to be mounted, tensioned and then picked so as

to ensure proper coupling of the electrical system.

The electrical system design’s objective was to tune the guitar string to a set

note by filtering the fundamental frequency and adjusting the system accordingly.

This is achieved by implementing a feedback design to read the current frequency and

compensate for error. A motor, controlled by the microcontroller, adjusts the tension

to match the new set note. Replacement of the microphone was also explained as it

produced unexpected noise and therefore a magnetic coil was selected as a

replacement. The stepper motor was used to adjust the tension of the string to increase

or decrease the frequency of vibrations, and thus the fundamental note output. Some

of the special considerations like noise and interference, fine adjustments of the

system were explained

The frequency acquisition and its corresponding calculations were also

explained on the stepper motor to be used and it was displayed on the LCD screen.

The system also was built with a “user interface” which connects the user to the

instrument through and LCD screen and switches buttons to select note.

Recommendations regarding accuracy of mechanical parts, adding a high Q

band pass filter, using a multi turn potentiometer were also suggested.

3. Samir Rawani et al (2002) made a model of a self-tuning guitar. It was operated

by a computer chip, which read the frequency of a plucked string and sent the signal

of the desired frequency to six stepper motors, one on each string. Each motor was

fastened to a worm gear and as the motor spun, the nut slid up and down the worm

gear and tightening and loosening of the string took place.

The computer chip used was a Motorola MC68HC11 which read the

frequency output from the guitar’s pick-up, and in turn sent the information to the

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motors, and kept reading at set intervals of time and relaying information to the

motors until the string has reached the desired tuning.

To power the device they used a rechargeable battery, similar to those used in

computer laptops, they claimed that it needed to be recharged every 15-20 tunings.

The feature which made their design special, and ultimately marketable, was

the ability to calibrate the guitar to alternate tunings, which are a must for any

experimental guitarist. The potential market of the model was also explained in detail.

4. The Automatic Guitar tuner project by an anonymous user on PBworks.com used

three prototypes based on which the actual model was made. First the frequency of

the signal was obtained through a series of filters. After its failure, Fast Fourier

Transforms were used to determine the frequency of the signal. After that failed, an

Auto Power Spectrum Analysis was done and a servo motor was added which proved

to be a success and it improved the accuracy of frequency calibration.

All the problems in the three stages were rectified and the final model was

designed upon. The frequency calculation was very accurate and reliable. When used

alone (without the servo) the frequency was obtained with each strum varying

somewhere between hundredths of a percent to at most half of a percent. There was

more feedback into the system with the presence of the servo and the sequence

structure which decreased the reliability of the frequency calculation, however the

system would self-correct itself since the vast majority of frequency calculations were

correct. Also to increase the speed of the system the number of samples was reduced

to 1/4 of the sampling rate. This decreased the precision of the frequency calculation,

since the values obtained now had to be multiplied by a factor of 4 and any slight

variations or errors also multiply by 4. Still in the end the device was able to work for

any string. There is a drop down menu for the string and the tuning and then upon the

user hitting go and plucking the motor will turn the peg until the guitar is "in tune".

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CHAPTER 3

SYSTEM DESIGN

The design of the guitar is centered on the use of the Arduino Mega

microcontroller to handle the sensor and motors for tensioning application. The

following sections outline the mechanical and electrical designs for the project.

3.1 Mechanical Design

The mechanical design approach taken to develop the robotic instrument was

centered on ensuring coupling of the electrical systems. The guitar string required a

way to be mounted, tensioned and picked.

3.1.1 String Mounting

The single-string unit needed support and tension which was similar to a

traditional guitar for repeatability of the sound and feel of the string. Like most

components of this project, the string mounts went through numerous design

iterations until the design was perfected.

The initial design involved a plywood piece with wood pieces mounted

perpendicular to the surface. The wood pieces were installed with grooves at the top

to support the string and restrict motion side to side as the string was plucked. This

design was initially chosen as it was simple and quick to construct for testing. It was

soon found that the string made a very noisy signal when played with the wood

pieces. The sound had a notable “twang” and was difficult for the sensor to process as

there was a significant amount of noise. The observation was made that the string was

not sitting in the grooves exactly and was as a result vibrating between the wooden

walls increasing the noise in the signal and giving a poor guitar sound.

At this point, it was observed that the string mounts and guitar bridge were

found to support the string at a single point rather than other a certain length axially as

the previous design had. The new design consists of two nuts (placed near the

headstock on guitar) mounted on the wooden pieces and can be seen in Figure 3.1

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below. The new design was found to give a much better sound than the original

design as well as a cleaner signal which was taken in by the sensor.

Figure 3.1: String Mounting

3.1.2 Tensioning System

An important criterion of the project was to have the system be self-tuning,

meaning that a mechanism was required to accept a value from the microcontroller

indicating which direction the tension is to be changed in. This was accomplished by

using a tuning peg directly from the guitar which takes a rotational input and

translates it to linear motion of the string as it wraps around the peg. The user tab of

the tuning peg was attached to the servo motor using a customized coupling as seen in

Figure 3.2. The coupling, which had to be customized, needed a strong but light

material. Hence, for this purpose polypropylene or PP was used.

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Figure 3.2: Tensioning System

3.1.3 Servo Motor

A servomotor is a rotary actuator that allows for precise control of angular

position, velocity and acceleration. It consists of a suitable motor coupled to a sensor

for position feedback. It also requires a relatively sophisticated controller, often a

dedicated module designed specifically for use with servomotors.

Servomotors are not a different class of motor, on the basis of fundamental

operating principle, but use servomechanism to achieve closed loop control with a

generic open loop motor. In other words, a servomotor is just a regular motor with a

sensor installed, typically to measure angular position during operation. Hence, given

the precision that it offers, a servo motor becomes an obvious choice for our

application.

Servos are controlled by sending an electrical pulse of variable width, or pulse

width modulation (PWM), through the control wire. There is a minimum pulse, a

maximum pulse, and a repetition rate. Servomotors can usually only turn 90 degrees

in either direction for a total of 180 degree movement. The motor's neutral position is

defined as the position where the servo has the same amount of potential rotation in

the both the clockwise or counter-clockwise direction. The PWM sent to the motor

determines position of the shaft, and based on the duration of the pulse sent via the

control wire; the rotor will turn to the desired position.

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Figure 3.3 on the next page shows the basic block diagram of a servo motor.

Figure 3.3: Servo Motor Block Diagram

One main disadvantage of a servo motor is that it does not offer continuous

rotation. In fact most of the servo motors available in the market restrict their

movement up to half rotation or 180 degrees. That is a major drawback for us,

although since the output of the servo motor is highly accurate it is given preference.

Hence, we use servo motor in our project to achieve fine tuning and it can be easily

used to account for detuning which occur due to changes in temperature, humidity and

the mere act of playing the guitar which drives it out of tune.

The servo motor we use is V0150 by Vega Robotics which offers a torque of 15

kg/cm at 4.8 to 6 V.

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Figure 3.4: V0150 Servo Motor

3.2 Electrical Design

A guitar string is basic in construction but inherently complex in dynamics. Its

vibration produces frequencies that cover the entire spectrum and include a

fundamental plus all harmonics (overtones). The system design objective is to tune

the guitar string to a set note by filtering out the higher frequencies or overtones and

adjusting the system accordingly. This is achieved by implementing a third order low

pass Butterworth filter, which filters out the overtones, controlled by the

microcontroller, adjusts the tension to match the new set note and a user interface

creates a modular and portable system.

Figure 3.5 shows the frequency spectrum of the note E2, the note at which the sixth

string of the guitar is set. It can be clearly observed that the signal contains higher

frequency elements other than the standard known frequency which is 82.41 Hz.

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Figure 3.5: Frequency Spectrum of guitar string tuned to E2 (82.41 Hz)

3.2.1 Sound Sensor

The sensor board initially contained an electret microphone; however, on

testing the microphone picked up an unexpected noise which produced a dirty signal

from the sensor module. The intrinsic property of the electret microphone is that it

picks up noise other than the vibrational string sounds. The noise factor limited the

usability of the microphone due to the complex software required to filter the signal

and therefore a magnetic coil was selected as a robust replacement.

Figure 3.6 displays a noisy waveform at the 329.63 Hz input frequency. The

noise was consistent for every new signal and persisted almost throughout the entire

waveform. However, replacing the microphone with the guitar pickup coil produced a

very clean square wave signal at the fundamental frequency expected. Hence, a guitar

pickup seemed like a perfect replacement for the system.

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Figure 3.6: Noisy output from microphone

3.2.1.1 Guitar Pickup

A guitar pickup basically consists of a magnet with a wire wrapped around it,

a coil. When a piece of metal, such as the guitar string right above it, moves, it

changes the magnetic flux of the magnet, and this induces a voltage in the coiled wire

around the magnet. The induced voltage will oscillate exactly in the same way as the

string above it oscillates mechanically - for this reason the pickup translates the

mechanical oscillation, the sound, into a voltage oscillation - which is easier to

transmit through wires.

Figure 3.7: Guitar pickups

The sensitivity of a pickup is mainly described by the number of coil windings

around the magnet, the strength of the magnet and the distance to the guitar string.

Usually a compromise between these parameters must be found. High magnetic fields

caused by more powerful magnets, or by placing the magnet closer to the guitar

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string, dampen the guitar string's oscillation - the pickup starts acting as an

electromagnetic brake. A high number of coil windings will make the pickup too

sensitive, so that any other disturbing fields get captured by the pickup, also, the

increased voltage could lead to a distorted sound.

Figure 3.8: Working of a guitar pickup

The entire guitar pickup, that visible thing under the guitar strings, is actually

a casing containing an array of magnetic poles - usually one per string - which are

connected to a master magnet that has the coil wrapped around it. Sometimes the

individual poles are visible as little circular bumps under each guitar string.

3.2.1.2 The Pickup As Circuit

From an electrical standpoint, a magnetic guitar pickup is equivalent to the

circuit in Fig. 3.9.

A real coil can be described electrically as an ideal inductance L in series with an

ohmic resistance R, and parallel to both a winding capacitance C. It is a bit simplified

compared to the reality but it gives a basic idea to start with. For a humbucker, two of

these circuits have to be connected in series. Since both coils (with precise

manufacturing) have practically identical properties, you may use the same simple

replacement circuit for the electrical examination. You then have to use twice the

values for the inductance and the resistance and half of the value for the capacitance

as compared to one coil.

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Figure 3.9: Electrical Scheme of a Guitar Pickup

Many people measure only the resistance and think they know something about a

pickup. But this is a fundamental error. By far the most important quantity is the

inductance, measured in Henries. It depends on the number of turns, the magnetic

material in the coil, the winding density and the overall geometry of the coil. The

resistance and the capacitance don’t have much influence and can be neglected in a

first approximation. When the strings are moving, an AC voltage is induced in the

coil. So the pickup acts like an AC source with some attached electric components.

The output from a pickup is much cleaner than what we obtained when using a

microphone. Figure 3.10 shows the signal obtained from a pickup.

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Figure 3.10: Clean output obtained from pickup

3.2.2 Low Pass Filter

As stated earlier, the vibration of a guitar string produces frequencies that

cover the entire spectrum and include a fundamental plus all harmonics (overtones).

Thus for computation of the fundamental frequency, all the upper harmonics are to be

eliminated. This is easily done with the help of a low pass filter.

A simple passive RC Low Pass Filter, or LPF, can be easily made by

connecting together in series a single resistor with a single capacitor as shown below.

In this type of filter arrangement the input signal is applied to the series combination

(both the Resistor and Capacitor together) but the output signal is taken across the

capacitor only. This type of filter is known generally as a “first-order filter” or

“one-pole filter”, because it has only “one” reactive component, the capacitor, in the

circuit.

But the main disadvantage of Passive Filters is that the amplitude of the output

signal is less than that of the input signal, ie, the gain is never greater than unity and

that the load impedance affects the filters characteristics. With passive filter circuits

containing multiple stages, this loss in signal amplitude called “Attenuation” can

become quiet severe.

Figure 3.11: First Order RC Low Pass Filter

One way of restoring or controlling this loss of signal is by using amplification

through the use of Active Filters. As their name implies, Active Filters contain active

components such as operational amplifiers, transistors or FET’s within their circuit

15

design. They draw their power from an external power source and use it to boost or

amplify the output signal.

Figure 3.12: Low Pass Active Filter with Amplification

The frequency response of the circuit will be the same as that for the passive

RC filter, except that the amplitude of the output is increased by the pass band gain,

AF of the amplifier. For a non-inverting amplifier circuit, the magnitude of the

voltage gain for the filter is given as a function of the feedback resistor divided by its

corresponding input resistor value and is given as:

Now, an ideal filter would give us specifications of maximum pass band gain and

flatness, minimum stop band attenuation and also a very steep pass band to stop band

roll-off (the transition band) and it is therefore apparent that a large number of

network responses would satisfy these requirements.

Not surprisingly then that there are a number of “approximation functions” in

linear analogue filter design that use a mathematical approach to best approximate the

transfer function required for the filters design. One such filter, which is perfectly

suited for this application is Butterworth Filter.

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The frequency response of the Butterworth Filter approximation function is

also often referred to as “maximally flat” (no ripples) response because the pass band

is designed to have a frequency response which is as flat as mathematically possible

from 0Hz (DC) until the cut-off frequency at -3dB with no ripples. Higher frequencies

beyond the cut-off point rolls-off down to zero in the stop band at 20dB/decade or

6dB/octave. This is because it has a “quality factor”, “Q” of just 0.707.

Note that the higher the Butterworth filter order, the higher the number of

cascaded stages there are within the filter design and the closer the filter becomes to

the ideal “brick wall” response.

Figure 3.13: Frequency Response Curve of a Butterworth filter

For our project a third order filter seemed sufficient as the filter eliminated all

the upper harmonics above second overtone. Even the magnitude of the second

overtone was reduced to almost one-fifth of the input magnitude. By providing

sufficient pass band gain in the circuit, the effect of second overtone in the signal can

be neglected.

The third order Butterworth low pass filter was designed by cascading an

active second order filter and passive first order RC filter. The circuit was initially

17

designed and simulated on National Instruments Multisim 13.0 (Figure 3.14) and

tested with a signal generator and CRO later before testing it with the actual signal.

Figure 3.14: Simulation of Low Pass Filter on Multisim 13.0

3.2.3 Frequency to Voltage converter

To avoid the unnecessary complexities in programming the microcontroller, a

suitable method needed to be found out to reduce the coding redundancies in our

program. One way to do that was to compute the frequency of the input signal outside

of microprocessor. In order to that, the frequency must be represented to the

microprocessor in analog form, possibly as DC voltage. Hence, usage of a frequency

to voltage converter seemed an obvious choice.

A frequency to voltage converter is a circuit which maps the frequencies of

input signal to corresponding outputs. Texas Instruments manufacture an IC, called

LM2917, which is a Frequency to Voltage converter IC.

The LM2907, LM2917 series are monolithic frequency to voltage converters

with a high gain op amp/comparator designed to operate a relay, lamp, or other load

when the input frequency reaches or exceeds a selected rate. The tachometer uses a

charge pump technique and offers frequency doubling for low ripple, full input

protection in two versions (LM2907-8, LM2917-8) and its output swings to ground

for a zero frequency input.

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The op amp/comparator is fully compatible with the tachometer and has a

floating transistor as its output. This feature allows either a ground or supply referred

load of up to 50 mA. The collector may be taken above VCC up to a maximum VCE

of 28V.

The two basic configurations offered include an 8-pin device with a ground

referenced tachometer input and an internal connection between the tachometer

output and the op amp non-inverting input. This version is well suited for single speed

or frequency switching or fully buffered frequency to voltage conversion applications.

The more versatile configurations provide differential tachometer input and

uncommitted op amp inputs. With this version the tachometer input may be floated

and the op amp becomes suitable for active filter conditioning of the tachometer

output.

Both of these configurations are available with an active shunt regulator

connected across the power leads. The regulator clamps the supply such that stable

frequency to voltage and frequency to current operations are possible with any supply

voltage and a suitable resistor.

Figure 3.15: Test Circuit of LM2917

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The LM2907 series of tachometer circuits is designed for minimum external

part count applications and maximum versatility. In order to fully exploit its features

and advantages let's examine its theory of operation. The first stage of operation is a

differential amplifier driving a positive feedback flip-flop circuit. The input threshold

voltage is the amount of differential input voltage at which the output of this stage

changes state. Two options (LM2907-8, LM2917-8) have one input internally

grounded so that an input signal must swing above and below ground and exceed the

input thresholds to produce an output. This is offered specifically for magnetic

variable reluctance pickups which typically provide a single-ended ac output.

Following the input stage is the charge pump where the input frequency is

converted to a dc voltage. To do this requires one timing capacitor, one output

resistor, and an integrating or filter capacitor. The output circuit mirrors this current

very accurately into the load resistor R1, connected to ground, such that if the pulses

of current are integrated with a filter capacitor, then VO = ic × R1, and the total

conversion equation becomes:

VO = VCC × fIN × C1 × R1 × K, where K is the gain constant—typically 1.0

There are some limitations on the choice of R1 and C1 which should be considered

for optimum performance. The timing capacitor also provides internal compensation

for the charge pump and should be kept larger than 500 pF for very accurate

operation. Smaller values can cause an error current on R1, especially at low

temperatures. Several considerations must be met when choosing R1. The output

current at pin 3 is internally fixed and therefore VO/R1 must be less than or equal to

this value. If R1 is too large, it can become a significant fraction of the output

impedance at pin 3 which degrades linearity. Also output ripple voltage must be

considered and the size of C2 is affected by R1.

Hence after effectively choosing C1 and R1, the final circuit diagram of the

frequency to voltage converter was made and is shown in Figure 3.16 on the next

page.

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Figure 3.16: Zener Regulated Frequency to Voltage Converter

3.3 Software Design – Arduino Mega 2560

3.3.1 Introduction

Arduino is a single-board microcontroller, intended to make the application of

interactive objects or environments more accessible. The hardware consists of an

open-source hardware board designed around an 8-bit Atmel AVR microcontroller, or

a 32-bit Atmel ARM. Current models feature an USB interface, 6 analog input pins,

as well as 14 digital I/O pins which allow us to attach various extension boards.

Introduced in 2005, it was designed to give students an inexpensive and easy way to

program interactive objects. It comes with a simple integrated development

environment (IDE) that runs on regular personal computers and allows us to write

programs for Arduino using C or C++.

1) Hardware: An Arduino board consists of an Atmel 8-bit AVR microcontroller with

complementary components to facilitate programming and incorporation into other

circuits. An important aspect of the Arduino is the standard way that connectors are

exposed, allowing the CPU board to be connected to a variety of interchangeable

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add-on modules known as shields. Some shields communicate with the Arduino board

directly over various pins, but many shields are individually addressable via an I²C

serial bus, allowing many shields to be stacked and used in parallel. Official Arduinos

have used the megaAVR series of chips, specifically the ATmega8, ATmega168,

ATmega328, ATmega1280, and ATmega2560. A handful of other processors have

been used by Arduino compatibles. Most boards include a 5 volt linear regulator and a

16 MHz crystal oscillator (or ceramic resonator in some variants), although some

designs such as the LilyPad run at 8 MHz and dispense with the onboard voltage

regulator due to specific form-factor restrictions. An Arduino's microcontroller is also

pre-programmed with a boot loader that simplifies uploading of programs to the

on-chip flash memory, compared with other devices that typically need an external

programmer. This makes using an Arduino more straightforward by allowing the use

of an ordinary computer as the programmer.

At a conceptual level, when using the Arduino software stack, all boards are

programmed over an RS-232 serial connection, but the way this is implemented varies

by hardware version. Serial Arduino boards contain a level shifter circuit to convert

between RS-232-level and TTL-level signals. Current Arduino boards are

programmed via USB, implemented using USB-to-serial adapter chips such as the

FTDI FT232. Some variants, such as the Arduino Mini and the unofficial Boarduino,

use a detachable USB-to-serial adapter board or cable, Bluetooth or other methods.

(When used with traditional microcontroller tools instead of the Arduino IDE,

standard AVR ISP programming is used.)

The Arduino board exposes most of the microcontroller's I/O pins for use by

other circuits. The Diecimila, Duemilanove, and current Uno provide 14 digital I/O

pins, six of which can produce pulse-width modulated signals, and six analog inputs.

These pins are on the top of the board, via female 0.10-inch (2.5 mm) headers.

Several plug-in application shields are also commercially available.

The Arduino Nano, and Arduino-compatible Bare Bones Board and Boarduino

boards may provide male header pins on the underside of the board to be plugged into

solderless breadboards.

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There are many Arduino-compatible and Arduino-derived boards. Some are

functionally equivalent to an Arduino and may be used interchangeably. Many are the

basic Arduino with the addition of commonplace output drivers, often for use in

school-level education to simplify the construction of buggies and small robots.

Others are electrically equivalent but change the form factor, sometimes permitting

the continued use of Shields, sometimes not. Some variants use completely different

processors, with varying levels of compatibility.

2) Software: The Arduino integrated development environment (IDE) is a

cross-platform application written in Java, and is derived from the IDE for the

Processing programming language and the Wiring projects. It is designed to introduce

programming to artists and other newcomers unfamiliar with software development. It

includes a code editor with features such as syntax highlighting, brace matching, and

automatic indentation, and is also capable of compiling and uploading programs to the

board with a single click. A program or code written for Arduino is called a "sketch".

Figure 3.17: Arduino IDE

Arduino programs are written in C or C++. The Arduino IDE comes with a software

library called "Wiring" from the original Wiring project, which makes many common

input/output operations much easier.

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3.3.2 Arduino Mega 2560

The Arduino Mega 2560 is a microcontroller board based on the ATmega2560

(datasheet). It has 54 digital input/output pins (of which 15 can be used as PWM

outputs), 16 analog inputs, 4 UARTs (hardware serial ports), a 16 MHz crystal

oscillator, a USB connection, a power jack, an ICSP header, and a reset button. It

contains everything needed to support the microcontroller; simply connect it to a

computer with a USB cable or power it with a AC-to-DC adapter or battery to get

started. The Mega is compatible with most shields designed for the Arduino

Duemilanove or Diecimila.

Figure 3.18: Arduino Mega 2560

The Mega 2560 is an update to the Arduino Mega, which it replaces. The

Mega2560 differs from all preceding boards in that it does not use the FTDI

USB-to-serial driver chip. Instead, it features the ATmega16U2 (ATmega8U2 in the

revision 1 and revision 2 boards) programmed as a USB-to-serial converter.

Revision 2 of the Mega2560 board has a resistor pulling the 8U2 HWB line to

ground, making it easier to put into DFU mode. Revision 3 of the board has the

following new features:

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● pinout: added SDA and SCL pins that are near to the AREF pin and two other

new pins placed near to the RESET pin, the IOREF that allow the shields to

adapt to the voltage provided from the board. In future, shields will be

compatible both with the board that use the AVR, which operate with 5V and

with the Arduino Due that operate with 3.3V. The second one is a not

connected pin, that is reserved for future purposes.

● Stronger RESET circuit.

● Atmega 16U2 replace the 8U2.

3.3.2.1 Summary

Operating Voltage 5V

Input Voltage (recommended) 7-12V

Input Voltage (limits) 6-20V

Digital I/O Pins 54 (of which 15 provide PWM output)

Analog Input Pins 16

DC Current per I/O Pin 40 mA

DC Current for 3.3V Pin 50 mA

Flash Memory 256 KB of which 8 KB used by bootloader

SRAM 8 KB

EEPROM 4 KB

Clock Speed 16 MHz

3.3.2.2 Power

The Arduino Mega can be powered via the USB connection or with an

external power supply. The power source is selected automatically.

External (non-USB) power can come either from an AC-to-DC adapter

(wall-wart) or battery. The adapter can be connected by plugging a 2.1mm

center-positive plug into the board's power jack. Leads from a battery can be inserted

in the Gnd and Vin pin headers of the POWER connector.

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The board can operate on an external supply of 6 to 20 volts. If supplied with

less than 7V, however, the 5V pin may supply less than five volts and the board may

be unstable. If using more than 12V, the voltage regulator may overheat and damage

the board. The recommended range is 7 to 12 volts.

The power pins are as follows:

● VIN. The input voltage to the Arduino board when it's using an external power

source (as opposed to 5 volts from the USB connection or other regulated

power source). You can supply voltage through this pin, or, if supplying

voltage via the power jack, access it through this pin.

● 5V. This pin outputs a regulated 5V from the regulator on the board. The

board can be supplied with power either from the DC power jack (7 - 12V),

the USB connector (5V), or the VIN pin of the board (7-12V). Supplying

voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your

board. We don't advise it.

● 3V3. A 3.3 volt supply generated by the on-board regulator. Maximum current

draw is 50 mA.

● GND. Ground pins.

● IOREF. This pin on the Arduino board provides the voltage reference with

which the microcontroller operates. A properly configured shield can read the

IOREF pin voltage and select the appropriate power source or enable voltage

translators on the outputs for working with the 5V or 3.3V.

3.3.2.3 Memory

The ATmega2560 has 256 KB of flash memory for storing code (of which 8

KB is used for the bootloader), 8 KB of SRAM and 4 KB of EEPROM (which can be

read and written with the EEPROM library).

3.3.2.4 Input and Output

Each of the 54 digital pins on the Mega can be used as an input or output, using

pinMode(), digitalWrite(), and digitalRead() functions. They operate at 5 volts. Each

pin can provide or receive a maximum of 40 mA and has an internal pull-up resistor

26

(disconnected by default) of 20-50 kOhms. In addition, some pins have specialized

functions:

● Serial: 0 (RX) and 1 (TX); Serial 1: 19 (RX) and 18 (TX); Serial 2: 17 (RX)

and 16 (TX); Serial 3: 15 (RX) and 14 (TX). Used to receive (RX) and

transmit (TX) TTL serial data. Pins 0 and 1 are also connected to the

corresponding pins of the ATmega16U2 USB-to-TTL Serial chip.

● External Interrupts: 2 (interrupt 0), 3 (interrupt 1), 18 (interrupt 5), 19

(interrupt 4), 20 (interrupt 3), and 21 (interrupt 2). These pins can be

configured to trigger an interrupt on a low value, a rising or falling edge, or a

change in value. See the attachInterrupt() function for details.

● PWM: 2 to 13 and 44 to 46. Provide 8-bit PWM output with the analogWrite()

function.

● SPI: 50 (MISO), 51 (MOSI), 52 (SCK), 53 (SS). These pins support SPI

communication using the SPI library. The SPI pins are also broken out on the

ICSP header, which is physically compatible with the Uno, Duemilanove and

Diecimila.

● LED: 13. There is a built-in LED connected to digital pin 13. When the pin is

HIGH value, the LED is on, when the pin is LOW, it's off.

● TWI: 20 (SDA) and 21 (SCL). Support TWI communication using the Wire

library. Note that these pins are not in the same location as the TWI pins on

the Duemilanove or Diecimila.

The Mega2560 has 16 analog inputs, each of which provide 10 bits of resolution

(i.e. 1024 different values). By default they measure from ground to 5 volts, though is

it possible to change the upper end of their range using the AREF pin and

analogReference() function.

There are a couple of other pins on the board:

● AREF. Reference voltage for the analog inputs. Used with analogReference().

● Reset. Bring this line LOW to reset the microcontroller. Typically used to add

a reset button to shields which block the one on the board.

3.3.2.5 Communication

27

The Arduino Mega2560 has a number of facilities for communicating with a

computer, another Arduino, or other microcontrollers. The ATmega2560 provides

four hardware UARTs for TTL (5V) serial communication. An ATmega16U2

(ATmega 8U2 on the revision 1 and revision 2 boards) on the board channels one of

these over USB and provides a virtual com port to software on the computer

(Windows machines will need a .inf file, but OSX and Linux machines will recognize

the board as a COM port automatically. The Arduino software includes a serial

monitor which allows simple textual data to be sent to and from the board. The RX

and TX LEDs on the board will flash when data is being transmitted via the

ATmega8U2/ATmega16U2 chip and USB connection to the computer (but not for

serial communication on pins 0 and 1).

A SoftwareSerial library allows for serial communication on any of the

Mega2560's digital pins.

The ATmega2560 also supports TWI and SPI communication. The Arduino software

includes a Wire library to simplify use of the TWI bus; see the documentation for

details. For SPI communication, use the SPI library.

3.3.2.6 Programming

The Arduino Mega can be programmed with the Arduino software. The

ATmega2560 on the Arduino Mega comes preburned with a bootloader that allows

you to upload new code to it without the use of an external hardware programmer. It

communicates using the original STK500 protocol (reference, C header files).

You can also bypass the bootloader and program the microcontroller through

the ICSP (In-Circuit Serial Programming) header; see these instructions for details.

The ATmega16U2 (or 8U2 in the rev1 and rev2 boards) firmware source code is

available in the Arduino repository. The ATmega16U2/8U2 is loaded with a DFU

bootloader, which can be activated by:

● On Rev1 boards: connecting the solder jumper on the back of the board (near

the map of Italy) and then resetting the 8U2.

● On Rev2 or later boards: there is a resistor that pulling the 8U2/16U2 HWB

line to ground, making it easier to put into DFU mode. You can then use

28

Atmel's FLIP software (Windows) or the DFU programmer (Mac OS X and

Linux) to load a new firmware. Or you can use the ISP header with an external

programmer (overwriting the DFU bootloader). See this user-contributed

tutorial for more information.

3.3.2.7 Automatic (Software) Reset

Rather than requiring a physical press of the reset button before an upload, the

Arduino Mega2560 is designed in a way that allows it to be reset by software running

on a connected computer. One of the hardware flow control lines (DTR) of the

ATmega8U2 is connected to the reset line of the ATmega2560 via a 100 nanofarad

capacitor. When this line is asserted (taken low), the reset line drops long enough to

reset the chip. The Arduino software uses this capability to allow you to upload code

by simply pressing the upload button in the Arduino environment. This means that the

bootloader can have a shorter timeout, as the lowering of DTR can be

well-coordinated with the start of the upload.

This setup has other implications. When the Mega2560 is connected to either a

computer running Mac OS X or Linux, it resets each time a connection is made to it

from software (via USB). For the following half-second or so, the bootloader is

running on the Mega2560. While it is programmed to ignore malformed data (i.e.

anything besides an upload of new code), it will intercept the first few bytes of data

sent to the board after a connection is opened. If a sketch running on the board

receives one-time configuration or other data when it first starts, make sure that the

software with which it communicates waits a second after opening the connection and

before sending this data.

The Mega2560 contains a trace that can be cut to disable the auto-reset. The

pads on either side of the trace can be soldered together to re-enable it. It's labeled

"RESET-EN". You may also be able to disable the auto-reset by connecting a 110

ohm resistor from 5V to the reset line; see this forum thread for details.

3.3.2.8 USB Overcurrent Protection

The Arduino Mega2560 has a resettable polyfuse that protects your computer's

USB ports from shorts and overcurrent. Although most computers provide their own

29

internal protection, the fuse provides an extra layer of protection. If more than 500

mA is applied to the USB port, the fuse will automatically break the connection until

the short or overload is removed.

3.3.2.9 Physical Characteristics and Shield Compatibility

The maximum length and width of the Mega2560 PCB are 4 and 2.1 inches

respectively, with the USB connector and power jack extending beyond the former

dimension. Three screw holes allow the board to be attached to a surface or case. Note

that the distance between digital pins 7 and 8 is 160 mil (0.16"), not an even multiple

of the 100 mil spacing of the other pins.

The Mega2560 is designed to be compatible with most shields designed for the Uno,

Diecimila or Duemilanove. Digital pins 0 to 13 (and the adjacent AREF and GND

pins), analog inputs 0 to 5, the power header, and ICSP header are all in equivalent

locations. Further the main UART (serial port) is located on the same pins (0 and 1),

as are external interrupts 0 and 1 (pins 2 and 3 respectively). SPI is available through

the ICSP header on both the Mega2560 and Duemilanove / Diecimila.

30

CHAPTER 4

METHODOLOGY

4.1 Block Diagram

Figure 4.1: Block Diagram of the System

The above block diagram represents the working of the system. Although each

component or section of the system was explained in the previous chapter, here is a

brief description of every block in the system and how each component is

synchronized with each other:

● Picking: Picking is nothing but plucking of string using a finger or a plectrum

for the pickup to sense the note played and sending it to the sensor system for

analog processing. This provides feedback to controller. Picking may have to

be done twice or thrice, so that the tuning process is done is a number of

iterations and the final result is accurate.

● Sensor system: The sensor system is provided to capture the vibration of the

string and compute the fundamental frequency of vibration using an analog

circuit. The fundamental frequency is calculated and a corresponding voltage

is given at the output of the entire circuit. The reason for computing

fundamental frequency is that the representation of musical notes is done by a

single frequency. However, the signal from guitar or any other instrument for

that matter consists of overtones, which are responsible for the different timbre

or in other words tone colour.

31

● Microcontroller: A microcontroller, Arduino Mega 2560 in this case, is the

controller in the control system. It is to be programmed to receive current

input, compare it with the set point of the system and initiate the actuator to

compensate for the error.

● Servomotor: This is the actuator used to move the tuning key and change the

tension in the string. It provides precise control of angular position, velocity

and acceleration, and hence is suitable for this application.

4.2 Microphone Frequency Sensor

The very first task that was needed to be addressed was finding out a suitable

method to find frequency of the string that was being played. A suitable sensor was to

be found out that would capture the frequency of the input signal and to convert the

signal into its corresponding voltage. Initially it was suggested that a microphone was

to be used to detect the sound frequency because of its easy availability in the market.

Initially a sensor was needed to be found out that would catch the frequency of

the signal. It was suggested that we use a microphone to catch the signal from guitar.

A suitable circuit which converted frequency of the input signal to a corresponding

voltage was found out.

Figure 4.2: Microphone frequency sensor

First stage of the process is the amplification which takes the microphone

output and amplifies it by a gain of several tens or so. Then comes the Frequency to

voltage converter stage using the LM2917. This chip is divided to a tachometer

section and an op amp section. The chip used here is a ground-referenced tachometer

32

input and an internal connection between the tachometer output and the op amp

non-inverting input.

This circuit was rejected because the microphone is more susceptible to

noises. These noises cause disturbances in the actual signal whose frequency is to be

measured and because of this the frequency increases or decreases which in turn leads

to fluctuation in the voltage. As this happens it leads to the voltage going above 5

Volts which is not desired as tightening and loosening of the string cannot be done

using servo motor. As Arduino mega does not operate above 5 Volts. In this circuit

for every 1000Hz, voltage changes by 1 Volt which is not useful as our entire

operation operates in frequencies below 1000Hz.

4.3 String Mechanics

A steel guitar string with scale length L will vibrate at the fundamental

frequency and multiples of the fundamental, called harmonics. The fundamental

wavelength is directly proportional to the scale length and the harmonics are multiples

thereafter, as depicted in Figure 4.3.

Figure 4.3: Fundamental and harmonics output from a guitar string

Adjusting the tension on the string changes the fundamental frequency the

string will vibrate at. The tension required to vibrate at a specific frequency is

determined with the equation below:

33

The tension on the string has a non-linear relationship with the frequency

generated and the frequencies expected require very slight modifications of the

tension. For very slight adjustments to the tension, a stepper motor in combination

with a guitar tuning peg was selected. To make the guitar tuner fully automated, a

servo motor was included in the design. The servo motor, with attached guitar pick,

plucks the string and creates vibrations for the inductive pick-up coil.

4.4 Low Pass Filter

A Low pass filter was designed to pass the frequency below the cut off

frequency (400Hz) and eliminate the frequencies above the cut off frequency. The

circuit which was designed in the chapter 2 was tested using a Cathode Ray

Oscilloscope and a function generator. At first a signal was generated using the

function generator which is similar to the signal generated by the guitar while the

string is played. After the signal is detected the low pass filter eliminates the

frequencies but it doesn’t remove the 2nd overtone so a 4th order low pass filter was

made which didn’t create much of a difference in removing the 2nd overtone, so a

3rd order low pass filter was finally made for the process as the filter eliminated all the

upper harmonics above second overtone. Even the magnitude of the second overtone

was reduced to almost one-fifth of the input magnitude. By providing sufficient pass

band gain in the circuit, the effect of second overtone in the signal can be neglected.

34

Figure 4.4: Simulation of Low Pass Filter on Multisim 13.0

The output of the low pass filter was sent to the frequency to voltage converter. As we

are using an Arduino mega microcontroller which works when voltage is given as an

input between 0 to 5 Volts then it turns the knob of the key and thus tunes the guitar.

4.5 Frequency to Voltage Conversion

Initially 555 Timer was used to convert frequency into voltage as the Arduino

Mega requires an input in voltage.

Figure 4.5: Frequency to Voltage converter using 555 Timer

Here is a very simple circuit diagram of a frequency to voltage (F to V) converter.

Such a circuit finds numerous applications in projects like digital frequency meters,

tachometers etc. The circuit is mainly based on a LM555 timer IC. The IC is wired in

mono shot mode to convert the input frequency into a fixed pulse width, variable

frequency PWM signal. Resistors R4 and capacitor C2 provides the necessary timing

for the circuit. The transistor T1 forms a discharge path parallel to C2 which is

necessary for re triggering the IC. Capacitor C1 acts as an input DC decoupler. The

output of the circuit is not a pure DC but a PWM waveform. Additional circuitry is

required to convert this PWM waveform to pure DC.

This circuit was rejected because the frequency which is converted to voltage

doesn’t lie between 0 to 5 Volts. It was either above 5 Volts or below 0 Volts and

35

hence was not suitable for the process. So instead the LM2917 was selected because it

gave us the required output.

The LM 2917 series are monolithic frequency to voltage converter with a high

gain op amp/comparator design to operate a relay, lamp or other load when the input

frequency reaches or exceeds selected rate. The tachometer uses a charge pump

technique and offers frequency doubling for low ripple, full input protection and its

output swings to ground for a zero frequency input.

The op amp/comparator is fully compatible with the tachometer and has a

floating transistor as its output. This feature allows either a ground or supply referred

load of upto 50 mA. The collector may be taken above Vcc upto a maximum Vce of

28 Volts. The basic configuration offered includes an 8 pin device with a ground

referenced tachometr input and an internal connection between the tachometer output

and the op amp non inverting input. This version is well suited for single speed or

frequency switching or fully buffered frequency to voltage conversion applications.

The more versatile configurations provide differential tachometer input and

uncommitted op amp inputs. With this version a tachometer input may be floated and

op amp becomes suitable for active filter conditioning of the tachometer output.

36

Figure 4.6: Zener Regulated Frequency to Voltage Converter

4.6 Program

After the signal is processed and the frequency is represented in the form of

voltage, the microcontroller needs to be programmed such that it reads the input from

the circuit, compares it with the set point and initiates the actuator to compensate for

the error. The following flow chart illustrates the algorithm of the program.

37

Start

38

CHAPTER 5

CONCLUSIONS AND SCOPE FOR FUTURE WORK

The scope of the project lied within tuning the E4 string of the guitar. It may

later be expanded to include all the six string of the guitar. Also, the current setup is

merely a prototype to establish the concept of self-tuning; the implementation of the

system can be done on a real guitar, where micro servos will be coupled to the shaft of

warm gear. Guitar players using a movable bridge will find this system very useful as

using the tremolo arm often disturbs the tuning of the guitar.

The current system was built and calibrated to provide fine tuning. Coarse

tuning cannot be done using this system due to restriction in movement of servo

motors to 180 or 360 degrees, though a stepper motor can be used for the purpose of

course training. In that situation, the tuning keys on headstock can be fixed with

stepper motors to provide coarse tuning and knobs at the movable bridges can be

coupled with servo motors to provide fine tuning.

The whole system can also be implemented for different tunings used in

contemporary music like DADGAD, Dropped D, D Standard, DGDGBD, etc. In that

case a second input would have to be taken from the user and set values for each

tuning will have to be stored in the microcontroller and called when the corresponding

tuning is selected.

Although the auto-tuning process is achieved, the design of the system has

some minor flaws.

Firstly, the third order low pass Butterworth filter, set at a cut-off frequency of

400 Hz, completely removes third overtone (1318.51 Hz), only half of the amplitude

of second overtone (659.25 H) is removed. Since the element of second overtone is

very less in the signal, it is neglected after filtering.

One way to overcome this is selective placement of pickup across the length

of the string. This is based on the principle of nodes and antinodes, which is explained

below:

39

The picture shows three situations. The top diagram shows the first harmonic

of the string's pitch, called the fundamental.

The diagram labeled "2nd" is the second harmonic. As we, know, the string

can only vibrate at whole number multiples of the fundamental pitch (1x, 2x, 3x, etc.).

One cannot see it very well in a vibrating guitar string, but there are fat and skinny

spots. The point of minimum movement is called a node. The point of maximum

movement is called an antinode. The number of the harmonic is found by counting the

number of antinodes on the string.

Figure 5.1: Node and Antinode

The next picture shows the same three harmonics with the pickup positioned

midway along the string. The pickup only detects string vibration when the string is

vibrating near the pickup. In the top and bottom diagrams of Figure 5.2, the

fundamental and third harmonic, the string is moving a lot in the pickup's vicinity. But

for the second harmonic, the pickup is situated over a node, meaning that the pickup

does not hear the second harmonic.

40

Figure 5.2 Pickup placed midway across the length of the string

Now as interesting as that sounds, such a setup is not possible in a guitar as the

pickups are mounted near the bridge and not on the centre of the bridge. So this

design cannot really be implemented. Hence, the problem of removing the second

overtone is not addressed by this method.

41

REFERENCES

This project took a lot of research and in depth reading of the concepts

required to build the system. Here is a list of all the books and websites referred to

successfully complete the project.

Books:

● Ramakant A.Gayakward, Op-amps and Linear Integrated Circuits, IV edition,

Pearson Education, 2003 / PHI

● D.Roy Choudhary, Sheil B.Jani, Linear Integrated Circuits, II edition, New

Age, 2003

● John G. Proakis and Dimitris C. Manolakis, Digital Signal Processing

Principles, Algorithms and Applications, Prentice Hall of India, 3rd edition,

1996

● Hugh Young and Roger Freedman, University Physics, Vol. 1, Pearson

Education, 13th edition

Websites:

● http://www.performing-musician.com/pm/jan08/articles/gibsonrobotguita

r.htm

● http://www2.gibson.com/Products/Min-ETune.aspx

● http://www.gizmag.com/review-peavey-at200-auto-tune-guitar/26205/

● http://engin1000.pbworks.com/w/page/18942682/Automatic%20Guitar%

20Tuner

● http://brunopinho.com/seng466_spring_2013/?p=5

● http://www.convict.lu/Jeunes/Spielberg/Spielberg.htm

● http://www.seattlerobotics.org/encoder/200112/elik.htm

● http://www.instructables.com/id/Arduino-Audio-Input/?ALLSTEPS

● http://www.instructables.com/id/Reliable-Frequency-Detection-Using-DS

P-Techniques/?ALLSTEPS

● http://forum.arduino.cc/index.php?PHPSESSID=kl67agklaa1nu8h0ao08j

921j1&topic=191267.04

42

● http://stackoverflow.com/questions/5044289/how-to-find-the-fundamental

-frequency-of-a-guitar-string-sound

● http://www.circuitstoday.com/f-to-v-converter

● http://www.aqdi.com/pickups.htm

● http://www.phy.mtu.edu/~suits/notefreqs.html

● http://www.electronics-tutorials.ws/filter/filter_8.html

43

APPENDIX A

The program code written on Arduino IDE is:

#include <Servo.h>

Servo myservo;

int inputfromguitar = A1;

int pos = 0;

int targetvoltage= 114; //analoginputvalue

int inputvoltage;

void setup()

{

myservo.attach(9);

}

void loop()

{

inputvoltage = analogRead(A1);

if(inputvoltage>51)

{

myservo.write(0);

while(inputvoltage<=targetvoltage)

{

inputvoltage = analogRead(A1);

myservo.write(pos); // tell servo to go to position in variable 'pos'

44

delay(15);

pos=pos+1; } } }

45