Lab 2: Designing an Optical Theremin. EE 300W Section 5 Team #3: Penn Power United Gregory Hodgkiss, Nasser Aljadeed 10/23/15

Transcription

1 Lab 2: Designing an Optical Theremin EE 300W Section 5 Team #3: Penn Power United Gregory Hodgkiss, Nasser Aljadeed 10/23/15

2 Abstract The purpose of this lab is to design an optical theremin, a musical instrument that can be played without physical contact. The design requires two OP906 photodiodes, a TL074 op amp, two 10 MΩ resistors, NI mydaq, and LabVIEW programming. The photodiodes function as light detectors that detect light reaching the circuit, allowing the user to control the frequency pitch and volume that the theremin produces based on light intensity. Functioning as a transimpedance amplifier, the TL074 op amp converts the diode current into a negative signal voltage. The NI mydaq is used for signal acquisition and generation. The optical theremin has an auto tune and equalizer feature that can be adjusted by the user. The auto tune feature alters the pitch produced and tunes the pitch to the nearest half tone while the equalizer feature adjusts the frequency response of an audio system using linear filters. Introduction The objective of part one of lab 2 was to design an optical theremin that could be controlled with two sensors, one controlling pitch and the other controlling volume. Our optical theremin will be constructed using a circuit with two photodiodes to provide inputs to our LabVIEW code. We will use the inputs from our circuit to generate a sinusoidal audio tone that can be played with our NI mydaq. Light intensity, maximum/minimum frequency can be manipulated by the user on the front panel of the LabVIEW code to adjust the tone. The second part of this lab was to implement an auto tune function. This will be another option that can be toggled on and off by the user on the front panel. The auto tune function will be able to round the frequencies to the closest note in one of the standard 8 octaves. The final part of lab 2 is an audio equalizer. The audio equalizer will be able to adjust the overall volume and the bass, treble and mid tone of the input signal. The signal will either be the theremin or a.wav file that can be selected on the front panel.

3 Rational Constructing the optical theremin involves the interface of physical circuity and LabVIEW programming. Penn Power United designed a circuit that uses a TL074 op amp functioning as a transimpedance amplifier, which converts the leakage current produced from two photodiodes into two voltage signals, independently. These signals are the inputs that is filtered through our LabVIEW code. The LabVIEW code converts the analog output voltage of a transimpedance amplifier into a digital value that corresponds to frequency or amplitude. We scaled the inputs to the needed range of voltage and frequency for our preferred output wave by using multiple sub VI s, signal generation functions, user controlled values and the output voltage of the circuit. Implementation The circuit uses two OPTEK OP906 photodiodes. The photodiodes are connected to a transimpedance amplifier using the TL074 OP Amp see Figure (1). The photodiode will produce a leakage current when exposed to light. This current will pass through the R f resistor producing a voltage at the output. This output voltage will be read by the mydaq. The mydaq accepts voltages from 10 to +10 volts. After some testing, we selected a 10MΩ resistor to provide 8V when the photodiodes is exposed to maximum light. The LabVIEW code will receive the input voltages from mydaq using a DAQ assistant. The DAQ assistant will provide dynamic data to our labview code. The data will be converted into a double before we normalize it. To normalize both the frequency and amplitude signal we used two sub VI s ( Figure (2) and Figure (3) ). In the frequency VI the user can also set the maximum and minimum frequency value via the front panel. Once both signals are normalized the user has the option to turn on the auto tune function before they are passed onto the Simulate Signal function. The Simulate Signal function generates a sinusoid that can then be passed to another DAQ assistant. This DAQ assistant is setup to produce max/min of 2 to 2 volts to be in the range of the audio out port on the mydaq. A second DAQ assistant is used to handle the

4 audio equalizer. The audio equalizer will either play a.wav file or play the tone from the theremin allowing the user to control the volume, bass, treble, and mid tone. Both the frequency VI and amplitude VI use the same logic to normalize the signal. The following equation was implemented using labview functions in Figures (2 & 3). sin Lmin ( Lmax Lmin)(f max f min )+ f min. This Equation will take in the frequency from the circuit and normalize it with the light intensities specified by the user on the front panel. That value is then altered to be bounded between the frequency max/min specified by the user. The amplitude VI works the same way as the frequency VI other than restricting the value between a min and max amplitude. To create the auto tune effect we created a separate VI to manipulate the signal Figure (7). The VI first step was to build a large array of the specific frequencies corresponding to notes in the 8 octaves. The array is made by using the first octave of notes and then taking each one of those and multiplying the frequency by a factor of 2 n with n being the number of octaves. This will run inside of a for loop for 8 (total number of octaves) iterations. Each time the loop runs the frequencies will be added into an array by using a shift register. Then our normalized frequency will be compared to the array and rounded to the nearest value. This value will outputted from the VI back to the main theremin VI for creating the sound wave. In order to create the audio equalizer [Figure (6)] we had to separate the input into 3 different signals bass, treble, and mid tone. To split the signal into these 3 sub signals we used linear filters. A highpass filter was used to filter the the treble tones from 1500 Hz and up. For the mid tones a bandpass filter was used to filter tones from 100 Hz to 1500 Hz. The final tones, bass tones, will be filtered by using a lowpass filter from 100 Hz and lower. Once the filters have separated the tone into the three components we can scale each individual signal. The factor that we scale the signals by can be controlled by the user. Once the individual signals have been scaled we can add them back together to recreate the sound wave and similarly scale the combined wave to control the overall volume. Finally the equalized sound wave is sent back to the main VI to be outputted. Combining all three parts we can complete the optical theremin VI (Figures 4 & 5).

5 Conclusion To build an optical theremin we had to create a system that combined allowed hardware and software to work together. Our optical theremin was fully controllable by the front panel and photodiodes allow the user to manipulate the pitch and volume by blocking and adding light. The autotune feature rounds the frequencies, in real time, to specific notes over an 8 octave range. Finally the user can control the bass, treble, and midtone levels via the front panel. This is done by running the signal through an audio equalizer. Appendices Figure 1. Transimpedance Amplifier Figure 2. Amplitude Sub VI

6 Lab 2 Deliverables Figure 3. Frequency Sub VI Part No. Part Name Quantity Price / Unit Price 1 TI TL074 Low Noise JFET Input Op Amp 1 $0.61 $ MΩ 2 $0.50 $ OPTEK OP906 Photodiode 2 $0.61 $ NI mydaq 1 $ $ Elenco 9438 Breadboard 1 $31.64 $31.64 TOTAL $ Table 1. Bill of Materials Design modifications made during testing Comparing our final design to our initial design, there were not a lot of changes. We had to alter some specifications in the design to meet the requirements. In the frequency and amplitude VI, our initial calculations were out of range. The output voltage signal from the transimpedance amplifier was negative, giving us the incorrect pitch for a

7 note. We added an absolute value function in LabVIEW to return the absolute value of the voltage output signal, giving us the correct values. Another problem we ran into during testing was regarding the equalizer. The equalizer processes (in real time) the sound output of the theremin with a LabVIEW to LED interface. The equalizer alters the frequency response of an audio system using three linear filter; lowpass (bass filter), bandpass (midtone filter) and highpass (treble filter) that are each user adjustable from the front panel. Our equalizer was working but it was not processing the sound output, in real time, of our theremin so we were not able to add the three indicator lights to the physical circuitry that illuminate on the front panel corresponding to each frequency band. Initial Block Diagram Figure 8. Initial Block Diagram Block Diagram Analysis At the N=0 level, only the input (light), transformation (Optical Theremin) and output (music) are labeled. The input of the system is the amount of light detected by two photodiodes. The range of light intensity and output frequency are user controlled. A musical tone is the output of the optical theremin by which the user changes the pitch and volume by altering the amount of light reaching each photodiode.

8 At the N=1 level, the transformation was divided into three major aspects; circuit, LabVIEW and MyDAQ. The data is provided through the physical circuity segment and inputted into to the LabVIEW program. Then the desired output is obtained by manipulating the information received from the MyDAQ. The three major components of the design are broken up at the N=2 level. The circuitry portion of the design is subdivided into three systems; photodiode, op amp and power source segment. The photodiode detects the amount of light reaching the circuit. The op amp is functioning as a transimpedance amplifier and converts the leakage current of the photodiode into a negative signal voltage. The agilent triple output power supply supplies the op amp with ±15 V. The LabVIEW portion is divided into four systems; volume control, sinusoid generation signal, autotune and equalizer. The output voltage of the circuit is normalized and placed into an array when entered into LabVIEW. The volume and pitch of the tone produced is proportional to the amount of light detected by each photodiode, independently. The autotune feature tunes the pitch to tones in the equal-tempered scale within the human hearing range and can be turned on and off by the user. Users are able to read in a wav file to be processed by the audio equalization VI and adjust volume, bass, midtone and treble ranges from the front panel. And finally we used the mydaq to generate a sine wave to be played as an audio tone. DAQ Assistant Setting Parameters and Observations We had a lot of difficulty setting the parameters for the DAQ assistant, especially when choosing the amount of input samples to read. The amount of samples had to be chosen to get maximum data with great quality. By setting the sample rates high and samples low, we got smooth quality sound from the theremin.

9 Figure 4. Optical Theremin Front Panel Figure 5. Optical Theremin Block Diagram

10 Figure 6. Equalizer Block Diagram Figure 7. Auto tune Block Diagram