lab!2:!optical!theremin! - pennsylvania state...
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Lab 2: Optical Theremin Team 2 Flyback
By Brian Pugh, Andrew Baker, and Michael Betts
Table of Contents
Abstract ....................................................................................................................................... 3
Introduction .............................................................................................................................. 3 Rationale ..................................................................................................................................... 4
Implementation ....................................................................................................................... 5 Hardware ............................................................................................................................................. 5 Software ............................................................................................................................................... 5
Conclusion .................................................................................................................................. 6 Appendix A ................................................................................................................................. 6
Bibliography ........................................................................................................................... 13
Abstract A simple optical Theremin is designed to produce sound over a user-‐defined scale and with optional distortion sound effects. The hardware consists of two photodiodes sensing light input. The signal from the photodiodes get amplified and then read into a computer by a Texas Instruments myDAQ. A LabVIEW virtual instrument handles all calibrating, auto-‐tuning, noise generation, and user parameter input.
Introduction The Theremin is an electronic musical device invented by Leon Theremin in the year 1928. The Theremin operates by having two metal antennas that sense the relative positioning of the operator’s hands. Bringing a hand closer to one antenna changes the instrument’s pitch, and bringing a hand closer to the other antenna changes the volume. The antennas detect the hand’s relative location by precisely measuring capacitance; the hand acts as the ground plate of the capacitor and as the hand gets closer to an antenna, capacitance increases. [1] A conventional Theremin is relatively complex to design and construct. Our simple optical Theremin uses changes in ambient light to measure relative hand location instead of capacitance. A transimpedance amplifier circuit converts a photodiode’s small reverse bias current into a useable and measureable voltage signal. It is assumed that the optical Theremin will be used indoors under moderate lighting conditions; this assumption is made so that the amplification circuit can be tuned for the correct lighting conditions. Direct sunlight is orders of magnitude brighter than indoor lighting and can saturate the amplification circuit. When a hand is brought close to a photodiode, some ambient light is blocked and consequently changes and the output signal. Unlike a conventional Theremin, much of the heavy lifting is done in easy-‐to-‐configure, inexpensive software instead of complicated and expensive analog circuits. This amplified signal gets converted into a digital signal by the myDAQ and is processed in LabVIEW to produce musical tones. There are two light sensing photodiodes, one control pitch and the other controls volume. Other parameters such as pitch range, volume range, and distortion options can be changed by the user-‐friendly software interface. The output music can be listened to through the myDAQ’s 3.5mm TRS jack. With a bit of practice, real songs can be played on this implementation of a Theremin.
Rationale The optical Theremin consists of two distinct parts: hardware and software. The hardware needs to detect changes in light and transform that information into information the computer and software can understand. The software (LabVIEW) then handles the calculations and sound generation. These two subsystems are further broken up into the high-‐level block diagram in Figure 1. We are using two OP906 photodiodes to detect light due to availability. These photodiodes generate a current that is directly proportional to the amount of ambient light; the max current output from these photodiodes is 35µA under very bright light.[2] Unfortunately, the myDAQ’s cannot directly measure small currents, so a transimpedance amplifier needs to be designed to amplify the signal and convert it into a measureable voltage. The actual output currents of the photodiodes are unknown, but we know the max output current is 35µA. If we want the output voltage to range between 0 and 5 volts, then we can calculate the feedback resistor with Equation 1. A 100kΩ feedback resistor was calculated, but there was no measureable output voltage signal under ambient lighting. Through experimentation, we determined that the photodiode output current is much lower than 35µA under our operating conditions. A feedback resistor of 5.1MΩ gave a satisfactory 3~5V range under ambient lighting.
𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 1: 𝑉!"# = −𝐼!𝑅! Transimpedance amplifier gain We used the commonly available TL074CN quad-‐opamp for the transimpedance amplifier circuit. This opamp works well in this amplification because it has very high (1012Ω) input resistance and very low input offset voltage (~3mV). These two parameters make the TL074CN very attractive for amplifying a relatively slow, small amplitude signal.[3] Other, more obscure opamps such as the NJM4580 would offer better performance in a smaller package at a lower price, but were unavailable. Two of the four opamps in the TL074CN are currently not used. With no additional components they could be used as a buffer for both channels. But, the myDAQ has a very large (>10GΩ[4]) input impedance, making any loading effects completely insignificant. The two extra opamps will not be used as buffers because they increase circuit complexity, points of failure, and add no benefit for this application. We used two separate loops for the audio generation and configured it so that the inputs and outputs could occur simultaneously. Sub-‐VIs were used to accomplish modular functions that were separate from the original task of generating an audio signal from the input data. For loops with case structures were used to allow quick changes between states and configurations. The inputs were normalized to account for high and low light levels. These were then scaled to match user inputs; in order to provide a linear relationship between light levels and the “note”, the output
frequency was generated using a logarithmic scale to account for the frequency doubling that occurs between octaves. Both the software and hardware can be designed and created independently. The Gantt chart in Figure 3 illustrates how our team divided up the work into as many parallel tasks as possible to reduce project time. The hardware is considerably less complex than the software, so while the software was still being programmed and debugged, this report was written.
Implementation
Hardware The finalized schematic used is available in Figure 2. The myDAQ supplies the ±15V rails to the TL074CN opamp. Both A0 and A1 are used to measure the transimpedance amplifiers’ outputs; the negative inputs for both channels need to be grounded to get meaningful results. For brighter environments, a smaller feedback resistor should be used for both channels. Similarly, in a dimmer environment a larger feedback resistor should be used to give the myDAQ the greatest voltage range possible.
Software The LabVIEW code consists primarily of two while loops on the main block diagram (one for the input from the photodiodes and the associated signal processing and one for controlling and outputting a waveform to the myDAQ) as well as several sub vis. Each of the while loops in the main vi have nested loops and case structures. When the code is initialized, it begins by setting minimum and maximum values for volume and frequency. It then moves to the signal processing loop within which it creates a waveform chart for both volume and frequency as well as contains the option for calibrating the entire system for both high and low light levels. It then proceeds to read from channels A0 and A1 on the myDAQ which are then normalized before plotting and calculating the corresponding volume and frequency in such a way as to distribute it across the entire selected range. Once this has been accomplished and if auto-‐tune is enabled, it is passed to the auto-‐tune sub vi where the frequency is then coerced into a key of F#, G, C majors or the chromatic scale depending on the users preference before passing this information to the output waveform loop. Once there, the output volume can be adjusted manually and both soft and hard clipping can be turned on and off by a Boolean controlling the clipping sub-‐VI. This waveform is then converted into a format readable by the myDAQ and is output on the audio out channel. It should also be noted that whatever controls on the front panel are not affecting operation are removed until they are again required.
Conclusion The end goal of this project was to design and construct an optical Theremin. The optical Theremin was required to generate user-‐controllable tones and volume through varying light intensity. The software was required to have a friendly front panel user interface, user configurable volume and pitch ranges, and to display both graphically and numerically the measured light intensities from the photodiodes. The realized final optical Theremin design met all design requirements and can be easily replicated by others through this final documentation. Further research can be done to improve responsiveness, increase user sensitivity, and add additional effects.
Appendix A
Figure 1: High Level N=1 Block Diagram
Figure 2: Transimpedance Amplifier Schematic
Week 1 2 3 4 Circuit Design and Construction
Circuit Debugging LabVIEW Coding
LabVIEW Debugging Design Verification/Validation Design Review Document
Figure 3: Gantt Chart
Component Quantity Unit Price ($) Supplier NI myDAQ 1 179.00 Studica
OP906 Photodiode 2 0.59 Mouser TL074CN PDIP-‐14 1 0.61 Mouser
5.1MΩ, 5% Metal Film Resistor 2 0.09 Mouser 170 Tie Point Breadboard 1 1.93 Deal Extreme Breadboard Solid Wire -‐ -‐ -‐
TOTAL: 182.90
Figure 4: Bill of Materials
Figure 5: Front Panel
Figure 6: Clipping Block Diagram
Figure 7: Autotune Block Diagram
Figure 8: Main Block Diagram
Figure 8a: Inputs to loops
Figure 8b: Bottom Loop (Sound Generation and Output)
Figure 8c: Setup of Inputs in Top Loop
Figure 8d: Outer Case Structure False (Top Loop)
Figure 8e: Outer Case Structure True (Top Loop)
Bibliography [1] Glinsky, Albert. Theremin: Ether Music and Espionage. Urbana: U of
Illinois, 2000. Print. [2] Optek Technology. PIN Silicon Photo Diode OP906. June 1996. [3] Texas Instruments. TL07x Low-Noise JFET-Input Operational
Amplifiers. Feb. 2014. [4] National Instruments. NI MyDAQ Specifications. Aug. 2014.