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Optical Theremin Design & Implementation
Critical Design Review
Team Lucky Sevens
Corbin Reeder
Vladimir Bakirov
Nikolaus Fritz
5 March 2014
Abstract
One of the more obscure musical instruments ever invented is an electronic device called a
theremin. This instrument can be played with no physical contact and is controlled by the
position of the musician’s hands, one of which controls the amplitude while the other controls
the frequency. This project tackles creating a theremin device which senses hand position via
the light intensity incident on a photodiode. The first sub-system of our design transforms the
light intensities into voltage signals using analog circuitry. After that, the voltage signals are
sampled using the analog inputs on an NI myDAQ along with a LabVIEW Virtual Instrument.
The LabVIEW coding scales and coerces the signals to acceptable values, then uses them to
generate a sine wave. This sine wave is then written to the myDAQ and outputted through its
3.5mm TRS connector. Our design offers a selectable auto-tune feature which enables the
device to only play musical notes. The final product of this project yielded a fully functioning
optical theremin that automatically adjusts to be played in different ambient light levels.
Introduction
This project is aimed at taking us through the design process and teaching us how to develop a
design review document. Team Lucky Sevens undertook the design and implementation of an
optical theremin device to meet the requirements of Lab 2 for EE 300W. The device must be
capable of generating a user-controllable audio tone. This audio tone will have amplitude and
frequency that can be adjusted through light intensity in a range of different ambient settings.
Our implementation of this device includes an analog photodetector circuit, a LabVIEW virtual
instrument, and an NI myDAQ. The light intensities incident on the amplitude and frequency
sensors are transformed into voltage signals then sampled using the myDAQ. Using LabVIEW
code, the signals are normalized then used to generate an output sine waveform through the
myDAQ’s 3.5mm audio port.
Rationale
A block diagram showing the high level system design can be found under Figure 1 in the
appendix. LabVIEW programming along with the MyDAQ was chosen to read inputs, generate,
and output an audio signal because of its usability and flexibility in design. The photodector
circuits were designed and implemented first in order to provide appropriate inputs for the
MyDAQ to sample. Photodiodes produce a leakage current in proportion to the light intensity.
In order to be used as an input to the MyDAQ, this current must first be transformed into a
voltage range between ±10 volts. The best way to achieve this is through a transimpedance
amplifier. The TL074 op-amp was used for this because it features low input bias and offset
currents as well as high input impedance. Current limiting resistors and zener diodes were used
to protect the circuit devices from high current and voltage levels.
The LabVIEW code auto-adjusted the light intensity to ambient light levels to improve usability,
accuracy, and precision as opposed to a user estimating light intensity range. The DAQ assistant
read in a set number of samples for the amplitude and frequency light intensities. This set
number of samples was used in order to prevent a large backup of data that would occur with
continuous sampling, which crashes the program. Next, normalization occurred in order to
prepare the data for any auto tuning and to be sent to generate the signal. Signal generation
occurred at high enough frequency to comply with the Nyquist sampling theorem. These
samples were continuously outputted in order to create a smooth sounding audio tone.
Implementation
Figure 1: Front Panel
Figure 2: Theremin VI Block Diagram
Photodector circuit:
The photodetector circuit has a photodiode that acts as a variable current source based on light
intensity. At ambient light levels in the demo room, the photodiode produced a current of
300nA with a 1kohm limiting resistor. The transimpedance amplifier included a TL074CN op
amp whose input-output relationship is described by the formula Vout=-Iin*Rb. Rb was chosen as
10Mohm to produce a Vout of 3 volts. In order to protect the MyDAQ from an input greater than
10 volts in high light levels, a 10 volt rated zener diode in series with a 1kohm resistor was put
at the output of the op amp. MyDAQ inputs were read from across the zener diode. This circuit
was constructed twice to provide two distinct light levels for frequency and amplitude. Testing
revealed that with non-ideal effects, ambient light level inputs were at 6.2 volts for the
amplitude circuit and 7.8 volts for the frequency circuit. This was within the requirements for
the MyDAQ input.
Figure 3: Photodetector Circuit Schematic
Ambient Light Adjuster:
On program start up, 100 samples for both the frequency and amplitude light intensities are
read in from the MyDAQ. These samples are averaged to find the maximum ambient light
levels. These values and an error signal will be passed out and used for the normalizer and to
ensure the program runs in the designated order, respectively.
Figure 4: Calibrator Block Diagram
Input Sampler:
After the maximum light levels have been
acquired, the program begins to take samples
through the DAQ Assistant. 10 samples are
read in at 10kHz for both analog in channels.
This signal is split into the separate frequency
and amplitude values, which get averaged by
the MEAN function. These values are
displayed to fill a design requirement and are
sent to the normalizer.
Normalizer:
The normalizer takes in 6 inputs. These are the
user-controlled frequency minimum and
maximum values, the two average sampled
values, and the two ambient level values. The
Figure 5: Input Sampler
amplitude sample is converted into a value between 0 and 1 by dividing the input sample by the
maximum possible amplitude value (the ambient). The frequency sample is ranged between the
user’s frequency minimum and maximum by this formula: fmin+(fmax-fmin)*Lsamp/Lambient (L being
light intensity in volts). The normalized values are passed out, displayed, and sent to the
optional auto-turner.
Figure 6: Normalizer Block Diagram
Auto-Tuner:
If active, the auto-tuner will coerce the normalized frequency sample to the nearest note. An
array is created that stores the frequency values of the 12 notes in an octave for the first 8
octaves. The frequency sample is compared to the values in the array and thresholded to the
nearest note. A check is made to ensure the note’s frequency is above the user-set minimum
and below the user-set maximum. If it does not fall in this range, the next highest or lowest
note, respectively, is chosen. If the auto-tuner is not active, the normalized values are sent
directly to the signal generator.
Figure 7: Autotuner Block Diagram
Signal Creator:
The Simulate Signal function is used to create a virtual sine wave. To prepare the sine wave for
output, the frequency of the wave is set by the auto-tuner (if active) or the normalizer (inactive
auto-tuner). The amplitude is set as the normalized sample value. This value, between 0 and 1,
complies with the MyDAQ’s maximum output range of ±2 volts. 50k samples per second are
generated to comply with the nyquist theorem, as you must sample at least 2x as fast as your
highest frequency (20kHz). 5k samples are then displayed and sent to the DAQ Assistant for
output.
Audio Output:
The DAQ Assistant receives the data generated by
the Simulate Signal function and generates the
output wave. The 5k samples read in are all
written out to the audio out terminal of the
MyDAQ at the same speed that the samples are
generated. This ensures no data loss or backup,
allowing instantaneous changing of the frequency
and amplitude values. Continuous generation of
these samples creates a smooth sounding audio
tone to be heard. Speakers or headphones are
plugged into the audio out jack to listen to the
generated tone.
Figure 8: Output Generation
Conclusion
Analog circuitry, an NI myDAQ, and a LabVIEW virtual instrument came together in this project
to create a simple and easy to use optical theremin. An analysis of our initial block diagram
compared to our final project is located under Figure 1 in the appendix. A bill of materials is
located under Figure 2 in the appendix. Our team learned from breaking the design process
into stages and decomposing the system into sub-systems. This project was a useful and
enjoyable exercise for team-building and writing technical design documents. We also greatly
increased our understanding of data sampling and generation within LabVIEW. The outcome of
this project yielded a fully functioning optical theremin that is able to output audio tones
according to the user’s hand position.
Appendix
Figure 1: System Block Diagram
Analysis: Our final product shows that our initial block diagram was an accurate representation
of the system. However, the “calibrate light levels” block was incorrectly placed under the
“light detection circuits” block instead of the “signal generation” block.
Figure 2: Bill of Materials
Bill of Materials
Optical Theremin
Item
Manufacturer
Model
Supplier
Unit Price Quantity
Op-Amp
Texas
Instruments
Advanced
Photonix Inc.
NXP
Semiconductors
Stackpole
Electronics Inc.
Stackpole
Electronics Inc.
National
Instruments
National
Instruments
TL074CN
Digi-Key
Corp.
Digi-Key
Corp.
Digi-Key
Corp.
Digi-Key
Corp.
Digi-Key
Corp.
National
Instruments
National
Instruments
$0.62
1
Extended
Price
$0.62
$2.98
1
$2.98
$0.21
2
$0.42
$0.08
4
$0.32
$0.09
2
$0.18
$999.00
1
$999.00
$233.00
1
$233.00
Photodiodes
Zener Diodes
1kΩ Resistors
10MΩ
Resistors
LabVIEW
2011
myDAQ
PDB-C142
NZX10B,133
CF14JT1K00
CF18JT10M0
LabVIEW
Base
myDAQ
Total $1,236.52