Download Lab 2: Optical Theremin Critical Design Review

PENNSYLVANIA STATE UNIVERSITY
Lab 2: Optical Theremin
Critical Design Review
Team UHV
Authors: Jinwen Zhu
James Kreibick
Brian Wengert
Matt Zenaldin
Date: February 22, 2013
Abstract
A traditional Theremin produces sound by manipulating the electric fields
produced by two antennae to control the frequency and amplitude of a sound wave.
To modernize this instrument, the antennae can be replaced with photodiodes
which allows the instrument to be played by determining the amount of light sensed
by each photodiode. The output current from a photodiode is very small meaning
that an amplification circuit is needed to boost the output. By using an inverting opamp circuit, the number of necessary components can be reduced and the hardware
component of the solution can be kept as simple as possible. Using a MyDAQ to
process the amplified signals, a program can use the signals to adjust the
parameters of an output wave that can be played through a speaker. The final result
is an instrument that is very similar to a Theremin but is controlled by manipulating
light. This allows many features of this instrument to be adjusted that would
optimize its efficiency in various environments.
Introduction
The Theremin is the first electronic musical instrument patented by a
Russian inventor named Leon Theremin. The player can control the frequency and
amplitude by placing his or her hands above two antennae to disturb the electric
field instead of physically touching the instrument. In this project, two antennae are
replaced with two photodiodes. The photodiodes generate a certain amount of
current proportional to light intensity. But since photodiodes can only generate a
small amount of current, the voltage needs to be amplified to get a noticeable result.
Other than a power source, LabVIEW will be used to normalize and set limits on
amplitude and frequency. With all parts implemented correctly, the player would be
able to get a range of pitches with volume control. For better performance, an autotune control will be included in the program. The auto-tune control takes the
frequency and sets the output signal to a specific frequency of an audible musical
note in various octaves. With the auto-tone enabled, the Theremin produces notes
similar to other instruments commonly found today.
Rationale
Using our initial block diagram as a template, we determined that the first
stage of solving this problem would be to amplify the output of the photodiode so
that the signal would be readable in LabVIEW. We determined that we should do
this via a hardware solution, specifically with an op-amp circuit. Once these signals
were at an acceptable level, we would use the MyDAQ to interpret the data and
generate the necessary output signal. This would help to keep hardware at a
minimum, reducing complications that may arise from unnecessarily complex
circuits. By relying on software for the majority of this solution, it also allowed
changes to be made easily to the project throughout the design process.
Implementation
For the hardware solution, we chose to use the simplest implementation
possible. By using an inverting op-amp that used the diode as a current source, we
were able to reduce the number of circuit components to one op amp, one resistor,
and one diode for each of the two circuits that we needed. Due to this, the circuit
was very easy to build and test as well as being very low cost. The outputs of this
circuit were read into the MyDAQ as two separate analog inputs. After inverting
these signals again so that their value was positive, they were normalized so that the
data would be easier to work with within the rest of the program. In order to make
the output sound musical, the signal used to determine the frequency of the output
was run through an “auto-tune” sub VI which rounded the input to specific
predetermined frequencies corresponding to musical notes. Both the frequency
signal and the signal used to determine the amplitude of the output were then used
to modulate a sine wave. The modulated sine wave was then sent as an output to
the audio jack on the MyDAQ allowing it to be played through a speaker.
Adjustments in the output of the two diodes change the inputs to the MyDAQ which
in turn modify the frequency and amplitude of the output sine wave.
Value Statement
This design of a Theremin instrument will enhance those musically inclined
to own and operate an instrument without touching any components while
producing audible tunes in multiple octaves. A design such as this produced in lab is
held to the quality of components available to us. However despite this limitation
owners will hear clear notes that will bring enjoyment to the ears.
Conclusions
An optical Theremin in the original design by Leon Theremin used antennas to
produce audible tones without touching anything, but manipulating the electric field. To
simplify the design we used photo diodes in an inverting op-amp circuit that would
produce a signal read through a myDAQ and thus produce musical notes as an output.
With this updated design people today can purchase a modified optical Theremin, and
still get the same results as before. Also, this design allows for future modifications and
improvements in the future since its largely software based.
Appendix
Gantt Chart:
Week 1
Week 2
Week 3
Week 4
Block Diagram
Design Concept
Hardware Construction
Hardware Testing
Labview Code
Auto-tune implementation
Critical Design Review
High Level Block Diagram
Photo diode current source
Amplitude
Signal
Transimpedance Amplifier
Frequency
Signal
Scale
Amplitude
Scale
Frequency
Mix of Signals
Audio Output
Figure 2: Amplitude Circuit Diagram
Figure 3: Optical Theremin Block Diagram
Figure 1: Frequency Circuit Diagram
Figure 4: Optical Theremin Front Panel
Financial Page
Cost of Parts (Quantity)
NI MyDAQ (1)
Power supply (1)
TL 074 J-FET op-amp (1)
PIN silicon photodiode (2)
1 MΩ resistor (2)
Misc. electronic parts
Other parts
Total Parts Est. ($)
Cost of Labor
Est. Labor ($) Engineering rate: $35/hr
Fringe ($) 15% of Est. Labor
Overhead ($) 40% of (Labor + Fringe)
Total Labor Est. ($)
$200.00
55.00
$0.45
1.10
0.14
50.00
50.00
$356.70
315.00
47.25
144.90
$507.15
Contingency ($) 10% of (Adjusted Parts + Total Labor):
$549.00
Grand Total
$1412.90