P13323: Opto-Electric Guitar Pickup
Problem Description:
Design and build a guitar pickup using optical methods, as opposed to
traditional magnetic or piezo-electric pickups.
Objectives:
• Must not modify the guitar or impede natural playing.
• Whole system should be removable.
• Have the ability to process the signal using either analog or digital
means.
• Must be able to handle environmental noise.
• Power for the unit must last at least a couple songs.
System Overview:
Mechanical:
• Individual sensor mounts for each emitter-detector pair to
allow many points of adjustment.
• Allows for emitter and detector offset from the string to
change the amount of light received.
• Whole sensor mount slides on a guide to zero the string in
the sensor’s line of sight.
• Application-specific guide
bed allows for reliable reinstallation of the unit.
• Guide is strapped to guitar
with soft nylon web strap
for easy installation and
removal.
Analog
Processing
Sensors
(x6)
Analog/Digital
Input Switch
• Each string has an emitter-detector
pair consisting of an infrared LED
phototransistor.
• Signals are generated by the string
oscillations creating variations in the
amount of light detected by the
phototransistor.
The signals obtained by each of the six sensor channels are sent through a
gain amplifier to allow for proper voltage ranges required by the
microprocessor.
After the gain stage, the signals are processed using either analog or digital
means, depending on the status of the Analog/Digital switch.
•
Analog:
•
•
•
Analog/Digital
Output Switch
Sensors:
Processing:
•
Digital
Processing
Output to
Amplifier
After the gain stage, the signal from each channel will go through a
second-order Butterworth high-pass and low-pass filter set to minimize
frequency components due to noise sources, such as the 60 Hz noise
from electrical outlets, and allow the first two harmonics of each note to
pass through un-attenuated.
Once filtered, the six channels
are then summed into one
signal using a summing
amplifier and sent to the ¼”
jack as the output.
This processing path uses two
matching PCBs, as shown.
Each PCB contains the
Analog Processing Hardware
hardware for three string channels.
Phototransistor
Infrared LED
Guitar String
Digital:
•
•
The processor uses 6 ADC channels to
read each of the signals from the gain
stage.
These signals are then sent through digital
band-pass filters, summed together, and
sent to an external DAC via SPI.
Digital Processing Flowchart
Microprocessor Evaluation Board
Results:
•
•
•
From left to right:
Rachel Arquette (EE), Chris Perry (EE),
Joe Mauger (EE), R. Paul Hoops (EE),
Chris Steele (ME), Ye Kuang (EE),
Caroline Lichtenberger (EE)
•
Each component of the system was tested individually with a high rate of success.
Both the analog and digital processing paths have been successful in recreating the
original signal and outputting
the result to an amplifier for
auditory confirmation.
Every objective presented
by the customer was met.
Overall, this project proved
the functionality of an
optically-driven guitar pickup.
Digital: Test Signal
Special Thanks to
Dr. Dorin Patru (Tech. Advisor, Customer)
Mr. Les Moore (Guide)
Professor Mark Indovina (Tech. Advisor)
Dr. Carlos Barrios (Tech. Advisor)
Dr. Ivan Puchades (Tech. Advisor)
Mr. Ken Snyder (Guitar Expert, Purchasing)
Analog: Guitar Output
of Summing Amplifier
EME Department
Output of DAC