Automatic Guitar Tuner Group #10 Dariusz Prokopczak & Stephan Erickson ECE 445 Sr. Design December 9, 2014

Overview -Microphone listens to a plucked string -Frequency is measured, converted and stored as an 8-bit number -Motor turns the tuning peg to achieve desired note

Features -Manual wind/unwind buttons -Sharp, Flat, & in tune LEDs -Hex displays showing measured Frequency -Compatible with acoustic & electric guitars

System Level Block Diagram Key: Blue = signal Dotted = power supply

Waveform Conversion -Uses a Schmitt Trigger to convert sinusoid from microphone into a square wave -Square wave output has a frequency equal to the fundamental of the input -Each string is filtered separately to remove harmonic content

Waveform Conversion Requirements - Generates a square wave with a frequency equal to the fundamental ± 0.1Hz of the input wave - The 2nd harmonic frequency of each string is attenuated by at least 10dB at output of the filter -Output square wave must have a low value of 0 to 0.8 volts and high value of 2 to 5 volts

Filter Array -Original plan: use low pass RC filter to remove noise -Realized second harmonic from each string had a significant effect on measurements -Decided to use dedicated filters with a shorter transition band to eliminate fluctuations in frequency detection due to 2 nd harmonic

Low E (82 Hz) Filter

Filtering Out 2 nd Harmonic Channel 1 (top) = Filtered Microphone Signal Channel 2 (bottom) = Raw Microphone Signal (2 nd Harmonic Present)

Waveform Conversion In Action Channel 1 (top) = Raw Microphone Signal (2 nd Harmonic Present) Channel 2 (middle) = Filtered Microphone Signal Channel 3 (bottom) = Schmitt Trigger Output

Waveform Conversion Challenges - Modular testing of this circuit was successful (R&V were met) -Upon connecting this PCB to the frequency detection/control PCB we experienced random fluctuations due to lack of proper DECOUPLING!!!

Waveform Conversion Decoupling -When connecting all PCBs together; however, an increase in inductance occurred due to the addition of long wires -We suspected decoupling was the problem as we encountered a similar issue when connecting the motor power supply to the control board -Upon moving the waveform circuit to a breadboard, the system resumed normal functionality

Frequency Detection & Control -Window square wave output of waveform conversion with an oscillator (555 timer) -Accomplished by ANDing these two signals -Count the peaks of the windowed signal to output an 8-bit representation of the frequency -Compare this 8-bit value to the 8-bit ideal tuning frequency of the selected string outputted from the user interface -Turn motor in appropriate direction or stop when string is in tune

Frequency Detection & Control Requirements -A square wave input of a given frequency causes the output of this unit to represent the frequency of the input as a string of 8 bits within a tolerance of 4 Hz -A measured frequency different from the tuning frequency causes the motor to turn in the corresponding direction; a measured frequency equal to the tuning frequency causes the motor to stop -Stop motor if measured frequency is outside of range: -Minimum limit = 48Hz -Maximum limit = 360Hz

Input to Counter (previous configuration)

555 Timer Operation To double how quickly the frequency is updated (therefore improving accuracy in the time domain), we modified the duty cycle of the timer to minimize the low time of the square wave -Target Operation = 4 ± 0.1Hz; 99% duty cycle square wave (250 ±5ms high time) -Duty cycle = (R 1 + R 2 ) / (R 1 + 2R 2 ) = 0.99 or 99% -To achieve near 100% duty cycle we set R 1 much larger than R 2 -frequency = 1.44 / ((R 1 + 2R 2 )*C) = 3.96 Hz -Choose C = 1.0uF±5%; R 2 =3,900 Ohms±5%; R 1 =356 kOhms±0.1%

Updated Timer: 99% Duty Cycle

Trade-off Between Time/Frequency Precision -↑ Precision in Frequency Domain ⇔ ↓ Precision in Time Domain -This means that in order to achieve a more accurate frequency measurement, you have to update the frequency less often -Analogous to the uncertainty principle -The binary frequency at the output is updated at a rate of 4±0.1 Hz (250ms ± 5ms high time output of 555 timer)

Control/Frequency Detection Test -Motor control & frequency detection presented little challenge overall once prototype was properly implemented on PCB -Decoupling of the 12V supply’s input to the H-bridge -Accomplished using a 20uF capacitor 60 hex = 96Hz, tuning to 110Hz (A) flat LED indicates measured<target

User Interface Requirements -Flipping a switch corresponding to a given guitar string note causes an output to be given that corresponds to the chosen frequency (8 bit representation) -Holding a manual wind or unwind key causes the appropriate signal to be sent to the motor control circuit -The hex display shows correct frequency values stored in the frequency detection register. -LEDs properly indicate whether string is in tune/#/ ♭

User Interface Test -Bus switches contain 8 bit target frequency of each note -When the user selects the note they are tuning, control signals turn on the corresponding bus switch -Bus switch passes the 8 bit value to the control circuit -Due to the nature of the UI, debugging this PCB was easy

Final Demo Product Final Demo was Successful! - Goal to make an affordable commercial product which is easy to use -Targets one person tasked with tuning many instruments Future improvements: -Base 10 decimal display -Adjustable adaptor for a variety of instruments

Why Didn’t We Use a Microcontroller? -By using only analog & TTL chips we can now implement the functional prototype onto a single IC -The final product would consist only of the handheld motor with user interface components & microphone integrated

See it in action here: or on the course webpage