Build an EMG Audio Amplifier! (Electromyography)

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Introduction: Build an EMG Audio Amplifier! (Electromyography)

About: Hi! While I'm busy, I think of projects I'd like to work on. Sometimes, when I have free time, I work on those projects.

Whenever your body needs to make a movement, the nervous system sends out tiny electrical signals that control your muscles. The technique of electromyography (EMG) allows us to amplify and measure these electrical signals. In addition to being a useful clinical tool for diagnosing various neurological disorders, EMG recordings can be used for engineering projects such as controlling prosthetic devices or playing music. If you've watched the White Rabbit Project, you know that EMG signals can even be used to control someone else's arm!

This Instructable will walk you through the process of building an EMG amplifier that allows you to translate the tiny electrical signals produced by motoneurons into an audio signal played through speakers. These instructions are for making a two channel amplifier (i.e. listen to the activity of two muscles at once), but it's simple to make a single channel amplifier to save time and/or parts.

Sure, you can spend more money on a fancy pre-made MyoWare sensor, but why not save the money, learn a new skill, and build one yourself? Let's get started!

Background Knowledge

This Instructable includes breadboarding a circuit and soldering a few wires. If you don't have some of this necessary background, I recommend checking out the Electronics Instructables class.

Safety Note

This project does involve connecting oneself to an electrical circuit. We'll be powering our circuit off of two 9V batteries; at no point should your circuit (especially when you're connected to it) be connected to AC power from the wall. This paper from Delsys has a section on electrical safety as well as a useful description of EMG techniques in general.

Step 1: Parts and Tools

A particularly cool part of this project is that it's actually fairly cheap and easy to put together! The most important part of the system, the LT1167 instrumentation amplifier, only costs $7.32 on DigiKey. The type of surface electrodes aren't all that important and can be found for around $10 on Amazon. Instead of an expensive power supply, the whole system is powered off of just two 9V batteries wired in series. And if you're lucky, you'll have a lot of the other electrical components scattered around your work space!

Parts

(2x) LT1167 (instrumentation amplifier) (DigiKey)

(2x) LT1112 (or any dual op-amp chip) (DigiKey)

(1x) LM386N (audio amplifier) (DigiKey)

(1x) Audio jack (SparkFun)

(5x) Surface EMG electrodes (two per muscle and one for a reference) (Amazon)

Breadboard (Amazon)

Various resistors, capacitors, and jumper wires

(2x) 9V battery

Tools

Speaker with an auxiliary input + an 1/8" audio cable

Soldering iron

Wire strippers

Heat shrink tubing

Electrical tape

Step 2: Amplifier Concepts & Design

EMG signals are measured differentially, meaning the signal we amplify and analyze is actually the difference in electrical potential between two points on the muscle. I recommend the Wikipedia article on differential amplifiers for a more in-depth review of the topic, but I'll give a brief explanation here.

Our main goal with this EMG amplifier is to amplify the signal we're interested in (muscle activity) without increasing the noise (often from external electrical interference). A differential amplifier operates on the assumption that any noise signal interfering with our EMG recording will uniformly affect the local region of muscle we're recording from. In other words, a large source of interference across the room will introduce the same amount of electrical noise in our recordings whether we record from a muscle in the middle of your forearm or a few centimeters away from that same position.

Making use of this assumption, we're able to achieve our goal of maximizing the signal to noise ratio by amplifying the difference between two recording sites close together on a muscle. The components of the signal that are common between the recording sites (the electrical noise) will be removed when we take the difference (i.e. subtraction) between the two recordings. The ability of an amplifier to reject the common signal is referred to as the common-mode rejection ratio (CMRR). An ideal differential amplifier would have an infinite CMRR, rejecting all the noise that is the same at either input.

Instrumentation amp circuit diagram

The main chip we'll be using in our amplifier circuit is an LT1167 instrumentation amplifier. An instrumentation amplifier is a special type of differential amplifier that includes an electrical buffer on each input to provide power when amplifying small signals. Without these buffers, the input voltage source (from the electrodes) would attempt to drive current through the R2 resistors on the inputs to the third op-amp, corrupting the signal. The diagram above shows the most common arrangement for an instrumentation amplifier.

With this basic understanding of how muscle activity can be recorded using a simple differential amplifier, let's build an amp!

Step 3: Circuit Diagrams

EMG Amplifier

For the EMG amplification, we'll follow the schematic for the "nerve impulse amplifier" described in the LT1167 instrumentation amplifier datasheet (shown below).

We'll assemble one of these nerve impulse amplifier circuits for each muscle we'd like to record from, which in our case is two. You only need to assemble one of these circuits if you're building a single channel amplifier. If you study the pictures of my assembled circuit in the next step closely, you'll notice a few extra resistors and capacitors than are shown in the schematics here. That's because I've added passive, first-order low-pass filters on the outputs of the instrumentation amps to further reduce noise. These filters aren't completely necessary but should help clean up your signal if you're interested.

Audio Amplifier

We'll also need to build a simple audio amplifier circuit to enable the output of the EMG amplifier to drive external speakers. We'll follow a circuit from the LM386 datasheet (shown below).

Keep these diagrams on hand as you're assembling the circuits. I don't know how many times I've fixed a problem with my circuit just by going back and making sure I've followed the diagram correctly. You'd think I'd know how to follow instructions by now...

Step 4: Power Supply

Before we can assemble our circuit, we need to prepare our power supply. If you look at the datasheet for the LT1167 instrumentation amp, you'll notice that it requires both a positive (+S) and negative (-S) power supply along with a ground. This need for a ±S arises from our desire to amplify muscle activity, which is inherently a bipolar signal. In other words, the electrical activity generated by muscles can either be positive or negative with respect to zero volts. The amplifier must therefore have both a negative and positive supply in order to accommodate and amplify these bipolar muscle signals. Without a negative supply, the amplifier would be unable to produce any outputs less than zero volts.

Two 9 volt batteries are both a convenient and safe way to power our EMG amplifier. Because we'll be attaching ourselves to the circuit, we don't want our circuit connected in any way to AC power from the wall. The diagram below illustrates how to connect the batteries in a ±9 volt arrangement.


Step 5: Prepare the Electrodes

As I mentioned above, we'll need a total of five electrodes for this project. EMG recordings are conducted with a differential amplifier, meaning we are amplifying the difference between two points of reference on the muscle. This means we'll need two electrodes per muscle. Additionally, we need a single reference for the muscle activity to be measured with respect to. Here is a link to some surface EMG electrodes sold on Amazon. The exact type of electrode isn't too important for our purposes. If you want to be extra fancy, you can attach solid-core wire to the ends of the electrodes like I've shown in the picture above to make them easier to plug into the breadboard.

I'll give more details for placing the electrodes after we've assembled the circuit.

Twisted wire pairs

You'll also want to twist the wires together for each electrode pair as shown above. The wire from the ground electrode does not need to be twisted to anything. In addition to cleaning up the mess around your circuit, twisted wire pairs help reduce electrical noise picked up by the electrodes. By alternating the position of the wires back and forth, any external electromagnetic interference (e.g. 60 Hz from mains) will affect the wires an equal amount. The differential amplifier will then remove this common noise signal.

Step 6: Assemble the Circuit

Follow the circuit diagrams shown in Step 3 to assemble the instrumentation and audio amplifier circuits. The final assembled circuit is shown below.

Assembled EMG amplifier circuit

Step 7: Test It Out!

With the circuit assembled you'll be able to slap some electrodes on your arms and start listening to your muscle activity in real time. Like I've shown in the picture below, place two electrodes on the inside edge of each forearm, parallel to the length of the muscle and separated by about 2 cm. Place the reference electrode on the bony part of one of your elbows, away from the electrodes on the muscles. Of course, if you'd like to record from a different muscle, just place the electrode pair in the right position, taking care to align the electrodes parallel to the length of the muscle fibers.

Image of electrodes on forearm

Once the electrodes are in place and you've plugged into the speaker, power on the circuit with the two 9 volt batteries. Play around with the variable resistor to adjust the gain of the signal going into your speaker. You'll want to turn the gain up as much as you can until you notice clipping in the audio, after which you'll control volume via the speaker.

The audio in my video is clipping a bit, but it's a pretty decent example of what a clean EMG signal should sound like.

Step 8: Applications & Ideas

Building an EMG amplifier opens up a number of possibilities for interesting projects. As I mentioned in the intro, in addition to being a useful clinic tool, EMG recordings have become more common in basic science and engineering research as a control signal for various devices. Below are a few examples of EMG recordings being used in novel settings.

EMG recordings also play an important role in motor control research, a field of neuroscience devoted to studying how the nervous system initiates and coordinates movement. If you're interested in a more in depth look into that research, here are some cool papers from the field:

I'd love to see what projects you come up with on your own! Let me know if you have any questions about this Instructable, I'd be happy to help.

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    30 Comments

    0
    jjl3
    jjl3

    Question 1 day ago

    Hi, I'm building the same circuit for a school project. I need to derive the limits of the two gains in the instrumentation amplifier, however I am struggling to correlate the instrumentation amplifier in step 2 and the EMG amplifier circuit in step 3. Would you be able to explain how the second (figure8) is related to the instrumentation amplifier circuit?

    0
    Sea_Foam
    Sea_Foam

    Question 3 months ago

    Hello! Just found your project and I was wondering if you have an Arduino code for this to make it work and test it out? I am new to this field and I would love to try this out!

    0
    ghadamalkawi07
    ghadamalkawi07

    Question 4 months ago on Step 3

    Hi,I have some issues with number of component used in the circuit diagram we have one instrumentation amplifier but in bread bored there is two of it ?

    The bread bored in the figure is it built for single arm or both???
    Thanks.

    0
    johnwmillr
    johnwmillr

    Answer 4 months ago

    Hello ghadamalkawi07, you're right—the bread board picture shows two circuits (one for each arm). Sorry for not making that clear!

    0
    cdwilliamson
    cdwilliamson

    10 months ago

    I am a little new to electronics and I am currently working on this for a school project. I have the circuit assembled, However, I am having trouble understanding exactly how the +-power supply connects to the circuit. Also, in the Amplifier with gain diagram, it shows Vin going into the potentiometer. Do I just leave that open like shown in the assembled circuit?

    0
    johnwmillr
    johnwmillr

    Reply 9 months ago

    Hi there,
    Sorry if this response is too late, I only got an email notification today. Are you using a breadboard? The +/- connections from the power supply should just connect to the long rails at either edge of the board (marked with the red + and blue - in the Step 6 images). The battery diagram in Step 4 of this tutorial is as clear an explanation as I can give for how to hook power up to the circuit.

    In the amplifier with gain diagram, Vin is the input signal coming out of the "nerve impulse amplifier" designed in Step 3 (labeled "OUTPUT").

    Does that help answer your questions?

    0
    zevinton
    zevinton

    1 year ago

    Hello, I am trying to build this circuit for a school project but having difficulties make it work. Do you actually have a schematic or anything showing connections and what type of resistors/ capacitors you used? that would be really helpfull.

    0
    johnwmillr
    johnwmillr

    Reply 1 year ago

    Hi zevinton,

    What are the particular difficulties you're running into with this project?

    I'm afraid I don't have any schematics beyond what's shown in this tutorial. My advice would be to start simple—try to assemble just the "Nerve Impulse Amplifier" shown in Figure 8. Don't worry too much about the resistor and capacitor values until you've got a working prototype put together. As long you have the right order of magnitude on those values, you'll be fine. If you get the nerve impulse amplifier working, send its output into Vin of the audio amplifier.

    Good luck!
    John

    0
    jjhui
    jjhui

    Question 2 years ago

    Hi. May i know what type of the filter you used in the circuit? Is it high pass filter or band pass filter or both? Thanks

    0
    johnwmillr
    johnwmillr

    Reply 2 years ago

    I think I combined a low pass and high pass to make a band pass filter, but I'm not sure. You mostly don't want frequencies over approximately 1,000 Hz.

    0
    DeepthiR8
    DeepthiR8

    Question 2 years ago on Step 6

    As per the circuit diagram we only need 4-op amps altogether. But in the circuit you have connected 5-op amps.. Can you please explain

    0
    johnwmillr
    johnwmillr

    Answer 2 years ago

    Hi DeepthiR8,
    Sorry for not getting back to you sooner. I realize the pictures of the circuit I assembled don't exactly match the diagrams.

    From left to right:
    - Op-Amp (A)
    - Instrumentation amp (A)
    - Op-Amp (B)
    - Op-Amp (B)
    - Audio amp (LM386)

    The first four chips (A & B) are actually two copies of the same circuit. For both A and B, the electrode input goes into the Instrumentation Amp and from there into the Op-Amp. The Op-Amps of A and B both output into the audio amplifier. I forget now, but I *think* I added the Op-Amp chips to A and B either as bandpass filters or extra buffers. You don't have to include them.

    So the most simple circuit to amplify just a single muscle would only use two chips:
    1.) Instrumentation amp (gets input from the electrode)
    2.) Audio amp (gets input from instrumentation amp)

    Let me know if that answers your question!

    John


    0
    SuraA2
    SuraA2

    Question 2 years ago on Step 1

    Hi, thank you for posting this project, its really helpful.
    I have couple of questions:
    1) can I use AD620 instead of LT1117?
    2) Can I connect it to microcontroller pic 32 ?

    0
    johnwmillr
    johnwmillr

    Reply 2 years ago

    Hi there,
    I'm glad you've found the project helpful!
    1) Yes, you should be able to swap in the AD620 fairly easily, since it's an instrumentation amplifier just like the LT1117.
    2) I don't know the specifics of the PIC 32, but you should be able to still use the EMG signal as an input. The only possible concern is if the PIC 32 sampling rate is less than approximately 500 Hz. Usually you want a sampling rate of at least 2 kHz when sampling EMG, but you can get by with about 500 Hz.

    I hope that helps! Let me know if you have any more questions.

    0
    KirahakiTDH
    KirahakiTDH

    Question 2 years ago

    Is there any way to replace that LT1117 with INA12x?

    0
    johnwmillr
    johnwmillr

    Answer 2 years ago

    It should be fine to replace the LT1167 with the INA12x. Have you tried it and ran into any issues? Any instrumentation amplifier will work.

    0
    Luis099
    Luis099

    Question 3 years ago on Step 7

    I'm attempting to build this project for school but I'm a little lost on finding the rating for one of the electrolytic capacitors. I only see the 250µf

    0
    johnwmillr
    johnwmillr

    Answer 3 years ago

    Do you mean the electrolytic capacitor in the top left going from power to ground? The value of that one isn't too important, it's just to smooth any fluctuations in the power source. 250 uF should be fine.

    0
    Luis099
    Luis099

    Reply 2 years ago

    Thank you for the reply, and I was assuming it wouldn't hurt to use a different value for it but I just needed to be sure. It also helps with the documentation I'm having to make for class. Thanks again! You'll probably hear from me again as I build and experiment with this project.

    0
    johnwmillr
    johnwmillr

    Reply 2 years ago

    Good luck on your project! Let me know if you have more questions and be sure to post a picture here once you've finished!