MG811 code


I find this Arduino sample code for using MG811 sensor (CO2 level).

Author:  Tiequan Shao:
         Peng Wei:
Lisence: Attribution-NonCommercial-ShareAlike 3.0 Unported (CC BY-NC-SA 3.0)
Note:    This piece of source code is supposed to be used as a demostration ONLY. More
         sophisticated calibration is required for industrial field application. 
                                                    Sandbox Electronics    2012-05-31
/************************Hardware Related Macros************************************/
#define         MG_PIN                       (0)     //define which analog input channel you are going to use
#define         BOOL_PIN                     (2)
#define         DC_GAIN                      (8.5)   //define the DC gain of amplifier
/***********************Software Related Macros************************************/
#define         READ_SAMPLE_INTERVAL         (50)    //define how many samples you are going to take in normal operation
#define         READ_SAMPLE_TIMES            (5)     //define the time interval(in milisecond) between each samples in 
                                                     //normal operation
/**********************Application Related Macros**********************************/
//These two values differ from sensor to sensor. user should derermine this value.
#define         ZERO_POINT_VOLTAGE           (0.220) //define the output of the sensor in volts when the concentration of CO2 is 400PPM
#define         REACTION_VOLTGAE             (0.020) //define the voltage drop of the sensor when move the sensor from air into 1000ppm CO2
float           CO2Curve[3]  =  {2.602,ZERO_POINT_VOLTAGE,(REACTION_VOLTGAE/(2.602-3))};   
                                                     //two points are taken from the curve. 
                                                     //with these two points, a line is formed which is
                                                     //"approximately equivalent" to the original curve.
                                                     //data format:{ x, y, slope}; point1: (lg400, 0.324), point2: (lg4000, 0.280) 
                                                     //slope = ( reaction voltage ) / (log400 –log1000) 
void setup()
    Serial.begin(9600);                              //UART setup, baudrate = 9600bps
    pinMode(BOOL_PIN, INPUT);                        //set pin to input
    digitalWrite(BOOL_PIN, HIGH);                    //turn on pullup resistors
   Serial.print("MG-811 Demostration\n");                
void loop()
    int percentage;
    float volts;
    volts = MGRead(MG_PIN);
    Serial.print( "SEN-00007:" );
    Serial.print( "V           " );
    percentage = MGGetPercentage(volts,CO2Curve);
    if (percentage == -1) {
        Serial.print( "<400" );
    } else {
    Serial.print( "ppm" );  
    if (digitalRead(BOOL_PIN) ){
        Serial.print( "=====BOOL is HIGH======" );
    } else {
        Serial.print( "=====BOOL is LOW======" );
/*****************************  MGRead *********************************************
Input:   mg_pin - analog channel
Output:  output of SEN-000007
Remarks: This function reads the output of SEN-000007
float MGRead(int mg_pin)
    int i;
    float v=0;
    for (i=0;i<READ_SAMPLE_TIMES;i++) {
        v += analogRead(mg_pin);
    v = (v/READ_SAMPLE_TIMES) *5/1024 ;
    return v;  
/*****************************  MQGetPercentage **********************************
Input:   volts   - SEN-000007 output measured in volts
         pcurve  - pointer to the curve of the target gas
Output:  ppm of the target gas
Remarks: By using the slope and a point of the line. The x(logarithmic value of ppm) 
         of the line could be derived if y(MG-811 output) is provided. As it is a 
         logarithmic coordinate, power of 10 is used to convert the result to non-logarithmic 
int  MGGetPercentage(float volts, float *pcurve)
   if ((volts/DC_GAIN )>=ZERO_POINT_VOLTAGE) {
      return -1;
   } else { 
      return pow(10, ((volts/DC_GAIN)-pcurve[1])/pcurve[2]+pcurve[0]);

I have a question about these variables : ZERO_POINT_VOLTAGE and REACTION_VOLTGAE
It’s written that"These two values differ from sensor to sensor. user should derermine this value.". My question is how can I do that ? And is it really necessary ? Because I found a lot of examples where people always use these values.

Thank you !

There are 2 email addresses in the code, have you tried to contact them? They should know...

No I first try here because there is a lot of "quick" answer ! but I will ask also by email thanks !

does anyone tried with two sensors and have the code and he's prepare to paste it here?