Major update (18/06/2019): added the new SMD version of OpenEMG.
Context
Electromyography (EMG) is an electrodiagnostic medicine technique for evaluating and recording the electrical activity produced by skeletal muscles. An electromyograph detects the electric potential generated by muscle cells when these cells are electrically or neurologically activated.
It can be a useful and intriguing sensor technology to use with Arduino or any other micro-controller. Therefore, I designed the openEMG project, an open-hardware, easy to use and very reliable EMG sensor.
OpenEMG PCB (SMD version), fully populated.
With its compact form factor, this sensor board outputs an analog voltage (0 to 5V), depending of the muscular signal.
This video showcases the project:
Features & usage
This table gives the electrical caracteristics of the board.
| Description | Value |
|---|---|
| Input Voltage | 5V |
| Output Voltage | 0 to 5V (proportional to muscle contraction) |
| Maximum input current | < 10 mA |
OpenEMG should be wired like this:
Both muscle electrodes must be placed on the same muscle, about 2cm apart; the ground electrode should be place on a neutral zone (bone) about 5cm from the other electrodes.
On the board, there is a potentiometer to tune the gain of the output amplifier. This gain is strongly dependent on the muscle that is measured and the position of the electrodes. If it is set too high, OpenEMG will be very sensitive and output either 0 or 5V. If it is set too low, muscle contractions may not be properly picked up.
For instance, this video shows a basic sketch that turns a servomotor based on the strength of the contraction (old THT version of the board).
The graph in the top right corner displays the output waveform of the board as it is read from the Arduino nano.
The corresponding Arduino code is the following:
#include <Servo.h>
Servo myServo = Servo(2);
void setup(){
myServo.attach();
}
void loop(){
myServo.write(map(analogRead(A0),0,1023,0,180));
delay(50);
}The circuit
OpenEMG's circuit is designed to be as simple to make as possible. This is the complete circuit diagram (click on image to view in full-screen):
There are 5 parts to this circuit:
- The ICL7660 generates a -5V rail for the operationnal amplifiers;
- The first OP-amp (U1B) is a classic differential amplifier configuration;
- The next two OP-amps (U1A and U1D) are a second order bandpass filter with a gain of 2.5 and a frequency range of 20 Hz to 500 Hz;
- The last OP-amp (U1C) is an amplifier with a tunable gain from 50 to 150;
- Finally, D1 rectifies and C9 smoothes the signal to make it readable by a micro-controller.
The LM324 chip can be replaced with any pin-compatible quad OP-amp, such as the TL084.
Note: the C9 capacitor makes the output more micro-controller friendly. It can be removed to obtain the raw signal.
The PCB files
In these archives, you will find:
- The KiCad circuit design files;
- The bill of material (BOM);
- The Gerber files for manufacturing.
I designed two versions of the PCB:
- A standard through-hole version;
- A more compact surface mount version.
Both version are functionnally identical.
Surface-mount (SMD) version
The surface mount (SMD) version is compact and suitable for professionnal PCB manufacturing.
It is 32mm×26mm, using 0805 components (fairly easy to hand-solder). I ordered panels of it on JLCPCB.
Through-hole (THT) version
As an alternative, the through-hole (THT) version is designed to be very easy to make with DIY tools: it is a single layer board with large clearances and wide tracks.
It is 47mm×35mm. See my article on homemade PCB manufacturing.
Safety warning: It is recommanded that the power supply of the Arduino is not connected to the wall outlet. Although it is extremely unlikely, a faulty power adapter could technically have its output connected to the live wire!
Bonus - ECG with OpenEMG
Although OpenEMG is designed for electromyography, I tried placing the electrodes near the heart (positions V3 and V4). I removed the C9 smoothing cap to get the raw capture. This graph is the result:
We can clearly see 3 pulses per cycle, separated by 850ms (70 BPM).
Author: Charles Grassin
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