Microphone lab
Contents
1. Measure the current at several different voltages say 1 10 volts in steps of 1 volt Note that the data sheet specifies a maximum voltage for the microphone Don t exceed it Record the voltages and currents in a table and try to fit the I vs V curve with various functions If your measurements are all for voltages over lv you should see fairly constant current DC characterization with Arduino Measuring with the multimeter and recording the data by hand in a lab notebook or by typing it into a file is tedious and so relatively few data points well be taken This makes it easy to miss important phenomena so let s automate the measurement The goal is to use the Data Logger from C ProgramFiles DataLogger or http bitbucket org abe_k arduino data logger get default tar bz2 to record numbers from which you can derive either an I vs V curve or an R vs V curve where the resistance here is the DC resistance R V I not the dynamic resistance 4 There are some limitations of the Arduino analog to digital converter that are important for this lab Figure 2 Schematic for testing the the DC characteristics of the microphone The voltage source is the bench power supply Schematic drawn using http www circuitlab com editor e There are only 6 channels that can be recorded on for Uno and other ATmega328 based boards some of the newer boards have more pins that can be used as analog inputs e The highest voltage allowed is 5v a2. for this lab we are mainly interested in looking at the electret microphone as a simple sensor but some of the FET behavior will appear in the DC characterization of the microphone Read http en wikipedia org wiki Electret and http en wikipedia org wiki Electret_microphone Oscilloscope An oscilloscope is a device for visualizing a time varying voltage There are two main types analog and digital http en wikipedia org wiki Oscilloscope The analog oscilloscopes use a cathode ray tube which shoots an electron beam at a phosphor coated screen to make a bright dot The electron beam is deflected horizontally by a sawtooth waveform so that the spot moves repeatedly across the screen from left to right at a constant speed The beam is deflected vertically by the input voltage so that what appears on the screen is the voltage as a function of time The digital oscilloscopes record the input voltage for a chunk of time then display it on the screen The recorded data can be manipulated in various ways that are not feasible with the analog scope and the data can be saved on a computer for further analysis It is now possible to get devices that serve as the front end of a digital oscilloscope using a laptop or tablet computer for the display These are currently less sophisticated than dedicated oscilloscopes but considerably cheaper for similar levels of capability Traditionally oscilloscopes have a display that is 10 divisions w
3. giving a stable image like looking at a rotating object with a stroboscope Finally there is usually an auto mode that starts a new trace without waiting for a trigger signal which allows you to see the input without having to set proper trigger conditions Generally you use the auto mode to look at the signal and choose appropriate trigger conditions For more information about triggering look at http hobbyprojects com oscilloscope_tutorial oscilloscope trigger _controls html 3 Pre lab assignment Read about electret microphones as cited above Read the data sheet for the CMA 4544PF W electret microphone we ll be using for this lab Read about oscilloscopes in general as cited above Read about the controls for the oscilloscopes in the lab Kikusui COS 5041 analog scope http www kikusui co jp kiku_ manuals C C0S5040_5041_E PDF Tektronix TDS3052 2 channel digital storage scope Somewhat surprisingly Tektronix hides their user manual behind a login requirement making it relatively inaccessible to students or purchasers of used equipment Given how extremely confusing their control panels are without a manual this is extremely short sighted of them if students have bad experiences learning to use their equipment who will specify the equipment later I found a copy at http www cs washington edu education courses cse466 07wi labs 12 Oscope TDS3000Manual pdf The analog scopes are easier to learn to use and will be
4. BME 194 Applied Circuits Lab 2 Electret Microphones Kevin Karplus January 12 2013 1 Design Goal For this lab you will e characterize the DC behavior of an electret microphone using the Arduino to automate the measurement e design a simple circuit to bias the microphone appropriately to have an output that is centered at lv e observe the waveform of the microphone on the oscilloscope and e design a very simple high pass DC blocking filter to convert the microphone output from being centered at 2 5v to being centered at Ov using just resistors and capacitors Much of this lab is familiarization with the oscilloscope DC and AC coupling input scaling X1 X10 probes time base triggering and with the Arduino data logger 2 Background Electret microphone An electret microphone consists of a permanently charged capacitor an electret connected to the gate of a field effect transistor One plate of the capacitor is a diaphragm moved by the changes in air pressure that make up sound Because the charge on the electret is constant but the capacitance changes with the separation between plates of the capacitor the voltage across the capacitor changes with the separation Vit Q C t The small change in voltage is converted to a change in the saturation current of the field effect transistor FET which can be measured externally We will look at field effect transistors more carefully in a later lab
5. ay to do this The bench power supply provides a known voltage that is used as the AREF reference voltage of the Arduino s analog to digital converter the 18 turn trimmer potentiometer trimpot allows us to present a lower voltage to the microphone plus 10kQ resistor that act as a voltage divider We can use two analog inputs of the Arduino to record the voltage across the resistor which we can convert to a current through the resistor since we know the resistance and the voltage across both the microphone and the resistor If we subtract V4 from Vo we get the voltage across the microphone Since you are connecting an external voltage reference to AREF be sure to specify the external option in the data logger configuration pane Your files will be much easier to work with if you have the data logger compute the voltages of AO and Al for you Also remember to document in the notes what AO and Al are connected to so that you can interpret the files later For two or three different settings of AREF say 0 55v 1 7v and 5v record a series of mea surements say every 100ms as you adjust the trimpot from one end of its range to the other Each different AREF setting will need a different output file for the data since the DataLogger puts metadata in the file about the settings used Note you can do this part of the lab at home without the bench power supply by using the 5v supply from the Arduino board but you need to measure what vo
6. ide and 8 divisions high The oscilloscopes have controls for setting the scaling of the inputs and of the time base in terms of how many volts or seconds there are per division The inputs usually also have a choice of DC coupled or AC coupled input http en wikipedia org wiki Capacitive_coupling http en wikipedia org wiki Direct_coupling Both analog and digital scopes have trigger conditions which determine when a sweep across the scope display starts These trigger conditions can be based on any of the inputs to the scope there is often an additional input to the scope available only for triggering On analog scopes the trigger is usually a voltage level and whether the input signal is rising or falling as it crosses that voltage level Digital scopes may have the ability to do more complicated triggering logic Because digital scopes have memory they allow you to see what happened before the trigger as well as afterward The triggers can be done once single triggering producing one sweep across the screen On digital scopes this gives you a record of a one time event that can be carefully analyzed but on an analog scope the fluorescence quickly fades unless the trace is captured photographically The triggers can also be done in normal mode where each occurrence of the trigger starts a new trace If you have a repetitive input signal this allows the analog display to show the same portion of the signal repeatedly
7. input signal One way to get that is to hook up the function generator to a loudspeaker to put out a sine wave of known frequency say between 50Hz and 5kHz Try using both AC coupling and DC coupling for the scope input and see what happens if you change the triggering conditions What is the amplitude of the AC output of the microphone The AC amplitude is half the peak to peak voltage that is it is the maximum voltage minus the minimum voltage divided by 2 Note that this amplitude will depend on how loud your sound input is Add a DC blocking capacitor to the microphone as shown in Figure 5 and connect scope probes to both Vout and Vac Use the oscilloscope to compare the output of the microphone at the two points at different frequencies of input Use the multimeter s to measure the RMS AC voltage at the two points for different frequencies say 5Hz 10Hz 20Hz 50Hz 100Hz 50kHz Plot the ratio RM S V4c RMS Vout as a function of frequency don t move your speaker or microphone around during these tests as you don t want to change the relationship between the two Your hands and body can act as important reflectors of sound also so keep them well away from the mic and speaker during this test Note the effect should be different depending on what size capacitor you use particularly at the lower frequencies Try a few different sizes from 4704F down to 100pF 6 Demo and writeup Demo the oscilloscope outputs to a TA or inst
8. ltage that is with a multimeter it depends on the power being delivered through the USB port which can fluctuate quite a bit and still be within the USB spec The bench supply is a much more reliable DC signal Getting lower AREF voltages than 5v at the input to the trimpot using just the Arduino may require using another trimpot as a voltage divider to divide down the 5V supply Write a gnuplot script to plot the data as I vs V points and as R vs V points I found it useful to plot each file separately so the points are color coded by which file they cam from but to make a larger file by concatenating all the separate files for fitting models to the data Plotting with log scales on both axes makes the data look simpler and easier to come up with models for Using a linear scale for voltage may hide the low voltage behavior Here are four models to try fitting e linear resistance model I V Rz You may want to fit Rz for just the lower voltages across the microphone e saturation current source model I Isat You may want to fit the saturation current Isat for just the higher voltages across the microphone e blended linear and saturation model R V 4 R V Isat or I V V 4 R V Tsat This makes a smooth transition from the linear model to the saturation model with the limy_ o R V Rz and limy R V Isat V This model is similar to a standard model for FET behavior e power law model The saturation current a
9. mi lanove and Leonardo versions of the Arduino board Arduino Mega boards not tested yet but should work on your own computer if the drivers are installed Arduino Due not supported by the Data Logger e USB cable for board Tools for this lab e wire cutters e wire strippers e small screwdriver for trimpot Equipment in lab e power supply for providing power to microphone Figure 1 Back of the electret microphone In this orientation the lower terminal is Terminal 2 which should be connected to ground and the upper terminal is Terminal 1 which should be connected via a series resistor to the positive voltage source or terminal 2 to the positive power rail and terminal 1 via a series resistor to ground e multimeter for measuring current and voltage e oscilloscope for observing signal e function generator for producing a sound signal for mic 5 Procedures DC characterization with multimeter The first part of the lab is to measure the DC current through the microphone for various DC voltages across it With the equipment in the lab this is most easily done by hooking up a multimeter configured as an ammeter in series with the mic and powering the pair from the bench power supply Connecting clip leads to the mic can be difficult but the mic plugs into the breadboard easily allowing wires or double ended header pins to be used as probe points Be sure to get the polarity right as shown in Figure 1
10. nd the lowest is Ov e The resolution is only 10 bits 1024 steps e The steps seem to be more uniformly spaced at the low end of the range than the high end so differences at the high end are less accurate than differences at the low end e The external reference voltage AREF must be at least 0 5v this is not in the data sheet but when I tried lower AREF voltages the reading was always 1023 and no more than 5v e Readings are not simultaneous but the analog to digital converter is quickly switched from one input to another It takes at least 104s for each conversion For slowly changing signals this skewing of the inputs in time doesn t matter much but for fast changing signals it can make a big difference e The Arduino has limited memory size for queuing recordings and will drop data points if its queue is full The serial line rate of 115200 baud means that you can t sample inputs more frequently than once per millisecond with one analog channel adding about 0 5msec for each additional analog channel The Leonardo using a different USB protocol may be able to get a little more speed out of the USB channel to the point where the Python program on the host computer becomes a more important bottleneck The Arduino can only record voltages as well as digital values and timestamps so we need to convert the current through the microphone into a voltage that we can measure with the Arduino The circuit in Figure 2 provides one w
11. ructor showing that you know how to adjust the time base the input scaling and the triggering On the Monday after the lab turn in a writeup describing what you did Remember that the instructors are not your audience a bioengineer who has not read the assignment is Provide at least e a purpose of the report the problem you are solving or the design information you are providing the I vs V or R vs V curves for the electret microphone which electret microphone e a schematic of the tested circuit a plot of the ratio of the RMS voltage after the DC blocking capacitor to the RMS voltage before the DC blocking capacitor as a function of frequency 7 Design Hints When trying to come up with a function for the I vs V curve look at the plot with different ways of scaling the x and y coordinates To choose a suitable initial DC blocking capacitor value for testing try to block frequencies below about 50 Hz much higher than one would normally want to use Use the pull up resistor you chose as the resistance and the RC time constant RC say for a cutoff frequency f to compute the capacitance If that works try a capacitor about 50 times larger to get a 1Hz cutoff We ll look at RC filtering in more detail later in the quarter
12. s I measured it was not constant but increased slightly as voltage increased I modeled this with a power law R V e VP Isat or I V vaia V Isat If p 1 this is the same as the previous model but with p lt 1 the current goes up with voltage even in the saturation region The current versus voltage plots for the four models are shown in Figures 3 and 4 When I tried this lab I got a very good fit with the last model except at very low voltages and currents where the quantization errors of the Arduino made measurement unreliable Microphone to oscilloscope You will be converting the current output of the electret microphone to a voltage output by putting the mic in series with a load resistor connected to a 5 v power supply like a voltage divider but with the microphone between the output and ground Using your measurements or the formula you found for the I vs V curve compute what resistance this load resistor should have to get a 1v Current vs voltage for electret mic T T 100 F T fe Ss E o 10 F E e D o 5 S 1E linear model saturation current blended model 0 1 L i power law in saturation region 0 001 0 01 a Voltage across mic V Figure 3 Current versus voltage plot for the electret mic The log scale for current makes the low voltage behavior clearer For this figure I ve deliberately suppressed the data and the parame
13. sufficient for this lab though the digital scopes offer considerably more capability if you can figure out the controls Despite the inventory claims on the BELS web pages there appear to be more digital scopes than analog scopes in the lab so you ll probably have to learn to use them at least minimally you can t be sure that the simpler analog scopes will be available Read how to use the function generators in the lab Agilent 33120A You won t need most of the capability of those function generators which are really overkill for a beginning circuits course just how to generate a sine wave of known frequency which is covered in pages 19 21 of the User s Guide http www home agilent com upload cmc_upload A11 6C0633120A_USERSGUIDE_ENGLISH pdf Prepare gnuplot scripts for plotting current versus voltage and equivalent resistance versus voltage including fitting different models You ll probably have to tweak these scripts once you get real data but you ll want to have a usable basis to start from rather than spending all your lab time reading gnuplot documentation 4 Parts tools and equipment needed Parts for this lab from kit e CUI inc CMA 4544PF W electret microphone http www digikey com product detail en CMA 4544PF W 102 1721 ND 1869981 e resistors e 10kQ trimpot e breadboard e loudspeaker Parts students need to provide on their own e Arduino board Drivers should be installed on lab computers for Uno Uno R3 Due
14. ters I got to give the flavor of the models Your plots should of course include the data and you should use sprintf in the titles of the plotted curves to print the parameters of the models Current vs voltage for electret mic 250 l 200 T e 5 150b 2 E D fe 100 z p 5 O 50 linear model saturation current blended model L 1 power law in saturation region 0 0 001 0 01 0 1 1 10 Voltage across mic V Figure 4 Current versus voltage plot for the electret mic The linear scale for current makes the high voltage behavior clearer Again I ve suppressed the data and the parameters Electret mic Figure 5 Schematic for the microphone the load resistor and the DC blocking capacitor You will need to fill in the values used I ve used a standard nFET symbol for the electret microphone rather than square box I used in Figure 2 because CircuitLab does not have a symbol for a microphone Schematic drawn using http www circuitlab com editor output voltage Choose the closest standard resistance value from your collection of resistors and measure the DC voltage you actually get Now hook up the oscilloscope probe to the output of the microphone and try to view the output of the microphone as a time varying signal You can whistle or talk into the mic or provide some other sound source It may be easiest to learn to use the scope if you have a known
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