OpenEMG Arduino Sensor


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Electronics - 24-02-2018

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 (THT version)

With its compact form factor, this sensor board outputs an analog voltage (0 to 5V), depending of the muscular signal.

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:

OpenEMG wiring diagram

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.


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:

Full OpenEMG circuit diagram

This circuit is divided into 5 parts:

  1. The ICL7660 generates a -5V rail for the operationnal amplifiers;
  2. The first OP-amp (U1B) is a classic differential amplifier configuration;
  3. 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;
  4. The last OP-amp (U1C) is an amplifier with a tunable gain from 50 to 150;
  5. Finally, D1 rectifies and C9 smooths the signal to make it readable by a micro-controller.

The TL084 chip can be replaced with any pin-compatible quad OP-amp, such as the LM324.

Note: the C9 capacitor makes the output more micro-controller friendly. It can be removed to obtain the raw signal.

Downloads

In this archive, you will find:

  • The circuit diagram, in PDF;
  • The PCB negative, in PDF;
  • The Kicad design files.

Download archive

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.

Author: Charles Grassin


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