Measure Muscle Sounds! Part 1: Electronic Board


Introduction: Measure Muscle Sounds! Part 1: Electronic Board

About: Experienced technical lead with a passion for design, usability and accessibility. Creator of tecla, the best smart hub for switch accessibility.
This is a step-by-step guide on how to make your very own prototype acoustic myography sensor. That means a sensor you can use to measure muscle sounds and other physiological vibrations. What is this good for? well... it is a more affordable alternative to the electrodes used in powered prostheses (specially if you make your own). To see what else you can do with these sensors, check out these videos.

The short paper below describes briefly how this sensor works and the diagram illustrates how it is built. For more details, check my MASc thesis. There are also some videos of the sensor in that page that you can check out.

Have fun!

Step 1: Bill of Materials

Here is a detailed bill of materials with suppliers and part numbers. The circuit schematics are also included at the bottom.

Part: printed circuit board (PCB)
Supplier: Any PCB manufacturer... or make your own!
Qty: 1 per sensor

Part: Knowles Acoustics BU-27135 Accelerometer
Supplier: Digi-KeyBU-7135-ND ($39.85 for 1 - minimum order 500)
Qty: 1 per sensor

Part: Panasonic WM-63PRT Omnidirectional Microphone
Supplier: Digi-Key P11961-ND ($3.47 for 1)
Qty: 1 per sensor

Part: SMD Resistor 33.0k 1/10 Watt 5% 0805
Supplier: Digi-Key RR12P33.0KDCT-ND ($0.14 for 10)
Qty: 2 per sensor

Part: SMD Resistor 5.6k 1/10 Watt 5% 0805 SMD Resistor
Supplier: Digi-Key RR12P5.6KDCT-ND ($0.14 for 10)
1 per sensor

Part: SMD Resistor 2.2k 1/10 Watt 5% 0805 SMD Resistor
Supplier: Digi-Key RR12P2.2KDCT-ND ($0.14 for 10)
1 per sensor

Part: Ceramic capacitor 1uF 16 Volt 0805
Supplier: Digi-Key PCC2249CT-ND ($0.13 for 10)
2 per sensor

Part: Ribbon Cable
Supplier: Any surplus electronics store
Qty: 2.5 Feet

Step 2: Get the Board Printed

The first thing you need to do is print the sensor's printed circuit board (PCB). You can either send the gerber and NC drill files (attached below) to a manufacturer or try doing it yourself at home by following another instructable such as (Mostly) easy PCB manufacture. Do not use the images below, they are not to scale and are only included as a reference, use the gerber files provided instead. Also, if you get the boards printed professionally, make sure you get a fair amount of them done (e.g. 50+) to reduce the cost since the board is very small and it may be way too expensive to make just one.

Once your boards are printed, they should look something like the third figure below.

Step 3: Trim the Board

Trim the PCB along the thin straight lines printed on the accelerometer side (shown as red dotted lines) using an edge cutter or some other cutting device. Be careful not to damage the trail on the bottom side.

Step 4: File the Corners

After trimming, the corners of the board should be rounded with a file. This will prevent your board from cutting into the silicone once it is embedded (Part 2 of the instructable).

Step 5: Pin the Mike

Soldering the microphone in place is one of the most difficult steps of the whole process, a lot of things can go wrong, so be careful!

If you are lucky, you will be able to find a microphone with leads... if, like me, you are in fact NOT lucky, you will have to attach the leads to a surface mount microphone yourself. With the current design, the leads are necessary because the microphone must be soldered from the accelerometer side. Besides, there is a track that can't touch the microphone's case so it must be covered with paper, which makes reflow soldering impossible (see next step). However, if you come up with a better idea, please let me know. I am not that good at solving this kind of things.

You can also try this alternative: Soldering underneath chips. I have not tried it myself, but it looks like it may work.

Note: For this and the following steps, you may find the following instructable very useful: hand-soldering teeny tiny chips!

Step 6: Prepare the Board (microphone)

The red circle shows where the microphone should sit and indicates the polarity of the leads.

Before mounting the microphone, a piece of paper should be glued on the microphone side of the board to ensure that the grounded case of the microphone does not touch the line connecting its positive lead. Two holes should be made on the paper to allow the microphone leads to penetrate through the paper.

Step 7: Solder the Mike

The leads on the microphone should be trimmed and filed to avoid protrusion to the accelerometer side. Press the microphone firmly to insure minimal spacing between the microphone and the PCB. Apply solder to the through holes from the accelerometer side to secure the microphone in place.

The resulting board should now have the microphone secure.

Note: Spacing between microphone and board should be minimal and the microphone should be fairly stable.

Step 8: Complete the Microphone Side

Solder on the appropriate components corresponding to the circuit schematic for the microphone. Use the proper traces on the PCB. The completed microphone side should appear as shown.

Step 9: Prepare the Board (accelerometer)

Turn the board around and glue a piece of paper (red square) on the accelerometer side. This will cover the positive lead end of the microphone and prevent it from shorting on the accelerometer case when the latter is attached.

Step 10: Solder Accelerometer Circuit

Solder the appropriate circuit components into the spaces given following the circuit schematic for the accelerometer shown.

Step 11: Solder the Accelerometer

Arrange the BU-7135 accelerometer on the board as shown ensuring the ground and the positive leads are appropriately positioned over their respective connections. Apply enough solder to join the accelerometer leads to the board and to the middle lead to help secure the accelerometer.

Step 12: Solder the Cable

Prepare 2.5-feet of a four-wire ribbon cable. Strip and apply solder to the ends of the wire to connect.

Insert the ribbon cable from the microphone side connecting the red marked wire to the power line hole. The rest of the wires in the ribbon cable should be inserted in order in the remaining three wire holes in the PCB. The ribbon cable tips should protrude on the accelerometer side, not on the microphone side.

Solder the ribbon cable on to the board from the accelerometer side and then trim the protruding ends of the ribbon cable. The accelerometer side of the board is shown below after soldering and trimming.

Step 13: Test Your Sensor

Congratulations! you have finished the first part of the Measure Muscle Sounds! instructable.

All you need to do next is to connect your sensor to a 5V power source and see if you get signals on a scope. Depending on your scope, you may need to use an amplifier (specially for the accelerometer).

You should be able to see some mike signals if you blow some air to the sensor on the microphone side. To test the accelerometer, grab the sensor from the cable and shake it up and down. The signal will be very small but you should be able to see the sinusoidal vibration.

Check out the second part of this tutotial: Silicone Embedding



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    first i want to thank you as this demonstration helped me allot during my work ..but i need to know what kind of signal processing is used in order to view the signal ..or where can i get that kind of information.

    What kind of sounds would you get if you ran it through an amp?  Could you make your own precussion backup to music by dancing or moving with the sensors on?

    Would it make sense to use a digital accelerometer and subtract the waves in software? Is the amplitude of the audio signal much smaller than the amplitude of the acceleration signal? Maybe something like the DRL+ diff amp circuit in an EEG would be useful, to cancel out the acceleration electronically even if the person is moving around a lot. Were you able to find any recordings?

    maybe you can hack a Wii joystick it has a three axis acc. the heart sounds are fantastically clear! It is better than the commercial electronic stethoscopes.

    nice instructions. where do you buy an Accelerometer?

    The accelerometer does not appear to be accessible to the kinds of hobbyists who are going to follow these directions. What properties are important for the accelerometer and microphone to have, so we can suggest substitutes? Low-frequency response seems to be the major factor. What else? Sensitivity? Noise?

    7 replies
    The ADXL32x series is cheaper ($8.60) and more obtainable (Newark and Digi-Key sell them in single quantities), and go from DC to 2.5 kHz. They are smaller, too.

    The three models have different noise, sensitivity and range, though. Which would be best for this application?

    Part#   Range Sensitivity Noise Density (ug/rtHz)ADXL320 ±5g   174 mV/g    250ADXL321 ±18   57 mV/g     320ADXL322 ±2g   420 mV/g    220

    In principle, none of these accelerometers are appropriate for the application, unless you are willing to place the sensor perpendicular to the board which would make the whole thing bulkier. The direction of measurement for the ADXLXXX sensors is along their long sides rather than the short one as it is the case with the Knowles accelerometer. If you surface mount these flush to the board, they will give you the movement parallel to the skin rather than perpendicular to it. You may still want to try it, since it may be easier to at least detect "noise" this way, but I doubt you will be able to filter it out.

    Yes, but if they were mounted perpendicular to the microphone, in the correct orientation, which would be the most correct for this application? I'm not an expert on accelerometers and their specs, in other words.

    Just guess at the maximum acceleration that would ever be seen on the surface of a wildly moving arm and get one that can measure that much? Go for the one with the greatest sensitivity? I don't know what's important for this.

    I don't imagine an arm could normally generate much more than 1g of acceleration. I would go for sensitivity and noise. The ADXL322 seems to be best.

    I just found another page with a bunch of acc that may be useful too. You could try to get the price on those:

    I would go for sensitivity and noise. The ADXL322 seems to be best.

    Excellent. Thanks.

    There's also the ADXL330 for an application that requires the chip to be mounted parallel to a PCB (though the Analog chips are only 4 mm wide, so the height in perpendicular orientation isn't a big deal):

    Part#   Range Sensitivity Noise Density (ug/rtHz)ADXL320 ±5g   174 mV/g    250ADXL321 ±18   57 mV/g     320ADXL322 ±2g   420 mV/g    220ADXL330 ±3.6g 300 mV/g    280

    It has three axes (with Z perpendicular to the package surface) but is less than a third of the price of the Knowles (one for $9.99 Newark or $11.58 Digi-Key), so the extra axes aren't costing anything. I think I'll get some of these and see what I can do with them.

    Might also find it helpful to combine vibration information from the other axes anyway? I'm not sure if there's a direct relationship between the vibration signal and the microphone signal, or if you're just using the time-averaged amplitude of vibration to ignore microphone signals during that period. I guess I should read through the thesis more carefully. :-)

    I just found another page with a bunch of acc that may be useful too.

    Ok. They look kind of big. Here are some other companies that make ones that can be bought through Digi-Key:

    Analog Devices

    I should point out that the signal-to-noise ratio of the ADXL sensors is worst than the Knowles sensors... we tested and abandoned them because of this. If you actually fill the form in the page I posted, you will get the smaller sensors, including some that are surface mount. Try 0-10g, single axes, amplified.

    And "cheaper" is an understatement, since the minimum order for the Knowles accelerometers is $19,925. :-)

    Do you really need a circuit board to attach three SMT components? Why not just solder them directly to the component leads? It would be cheaper and smaller. If the board is needed for some kind of mechanical structure or acoustic baffling or something, why not just a piece of standard prototyping board? That would be cheaper and easier than creating a custom board for such a simple circuit.

    2 replies

    Well, I wanted to have a bit more control over the final size of the sensor (including thickness of the board), but you are right, the printed board is not essential and there may be better/easier ways to do it. If you come up with something better you can include it here... I have added you as a collaborator.

    These "surfboards" (not a very useful term for search engines) are like perfboards but for surface mount components. I'm sure there are even more appropriate prototyping boards for surface mount components, but I can't find them right now.


    It would be good if you could list typical prices here, and acceptable part substitutes.


    Operon - Thanks for putting this together. We've been thinking for a while that MMG might be a low cost alternative for UE prosthetics. We'll have to add building some of these and trying them out to our list of stuff to do. Jon Kuniholm Shared Design Alliance/Open Prosthetics Project

    1 reply

    Not a problem at all... let me know if there is anything else I can do. I will be posting the second (and trickiest) part soon.

    mind explaining just what the accelorometer does?