Measure Muscle Sounds! Part 1: Electronic Board

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!

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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-Key BU-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

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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.

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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.

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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).

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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!

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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.

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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.

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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.

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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.

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

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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.

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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.

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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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