Hello,
I guess I need some help with MQ-8. So this is my circuit:
And I have tried to calibrate it using this code (taken from MQ-8 Hydrogen/H2 Sensor Module | Sandbox Electronics):
/*******************Demo for MQ-8 Gas Sensor Module V1.0*****************************
Contact: support[at]sandboxelectronics.com
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 2014-02-03
************************************************************************************/
/************************Hardware Related Macros************************************/
#define MQ_PIN (0) //define which analog input channel you are going to use
#define RL_VALUE (10) //define the load resistance on the board, in kilo ohms
#define RO_CLEAN_AIR_FACTOR (9.21) //RO_CLEAR_AIR_FACTOR=(Sensor resistance in clean air)/RO,
//which is derived from the chart in datasheet
/***********************Software Related Macros************************************/
#define CALIBARAION_SAMPLE_TIMES (20) //define how many samples you are going to take in the calibration phase
#define CALIBRATION_SAMPLE_INTERVAL (200) //define the time interal(in milisecond) between each samples in the
//cablibration phase
#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 interal(in milisecond) between each samples in
//normal operation
/**********************Application Related Macros**********************************/
#define GAS_H2 (0)
/*****************************Globals***********************************************/
float H2Curve[3] = {2.3, 0.93,-1.44}; //two points are taken from the curve in datasheet.
//with these two points, a line is formed which is "approximately equivalent"
//to the original curve.
//data format:{ x, y, slope}; point1: (lg200, lg8.5), point2: (lg10000, lg0.03)
float Ro = 10; //Ro is initialized to 10 kilo ohms
void setup()
{
Serial.begin(9600); //UART setup, baudrate = 9600bps
Serial.print("Calibrating...\n");
Ro = MQCalibration(MQ_PIN); //Calibrating the sensor. Please make sure the sensor is in clean air
//when you perform the calibration
Serial.print("Calibration is done...\n");
Serial.print("Ro=");
Serial.print(Ro);
Serial.print("kohm");
Serial.print("\n");
}
void loop()
{
Serial.print("H2:");
Serial.print(MQGetGasPercentage(MQRead(MQ_PIN)/Ro,GAS_H2) ); //H2 in ppm
Serial.print( "ppm" );
Serial.print("\n");
delay(200);
}
/****************** MQResistanceCalculation ****************************************
Input: raw_adc - raw value read from adc, which represents the voltage
Output: the calculated sensor resistance
Remarks: The sensor and the load resistor forms a voltage divider. Given the voltage
across the load resistor and its resistance, the resistance of the sensor
could be derived.
************************************************************************************/
float MQResistanceCalculation(int raw_adc)
{
return ( ((float)RL_VALUE*(1023-raw_adc)/raw_adc));
}
/***************************** MQCalibration ****************************************
Input: mq_pin - analog channel
Output: Ro of the sensor
Remarks: This function assumes that the sensor is in clean air. It use
MQResistanceCalculation to calculates the sensor resistance in clean air
and then divides it with RO_CLEAN_AIR_FACTOR. RO_CLEAN_AIR_FACTOR is about
10, which differs slightly between different sensors.
************************************************************************************/
float MQCalibration(int mq_pin)
{
int i;
float val=0;
for (i=0;i<CALIBARAION_SAMPLE_TIMES;i++) { //take multiple samples
val += MQResistanceCalculation(analogRead(mq_pin));
delay(CALIBRATION_SAMPLE_INTERVAL);
}
val = val/CALIBARAION_SAMPLE_TIMES; //calculate the average value
val = val/RO_CLEAN_AIR_FACTOR; //divided by RO_CLEAN_AIR_FACTOR yields the Ro
//according to the chart in the datasheet
return val;
}
/***************************** MQRead *********************************************
Input: mq_pin - analog channel
Output: Rs of the sensor
Remarks: This function use MQResistanceCalculation to caculate the sensor resistenc (Rs).
The Rs changes as the sensor is in the different consentration of the target
gas. The sample times and the time interval between samples could be configured
by changing the definition of the macros.
************************************************************************************/
float MQRead(int mq_pin)
{
int i;
float rs=0;
for (i=0;i<READ_SAMPLE_TIMES;i++) {
rs += MQResistanceCalculation(analogRead(mq_pin));
delay(READ_SAMPLE_INTERVAL);
}
rs = rs/READ_SAMPLE_TIMES;
return rs;
}
/***************************** MQGetGasPercentage **********************************
Input: rs_ro_ratio - Rs divided by Ro
gas_id - target gas type
Output: ppm of the target gas
Remarks: This function passes different curves to the MQGetPercentage function which
calculates the ppm (parts per million) of the target gas.
************************************************************************************/
int MQGetGasPercentage(float rs_ro_ratio, int gas_id)
{
if ( gas_id == GAS_H2) {
return MQGetPercentage(rs_ro_ratio,H2Curve);
}
return 0;
}
/***************************** MQGetPercentage **********************************
Input: rs_ro_ratio - Rs divided by Ro
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(rs_ro_ratio) is provided. As it is a
logarithmic coordinate, power of 10 is used to convert the result to non-logarithmic
value.
************************************************************************************/
int MQGetPercentage(float rs_ro_ratio, float *pcurve)
{
return (pow(10,( ((log(rs_ro_ratio)-pcurve[1])/pcurve[2]) + pcurve[0])));
}
I'm getting weird output, like:
Calibrating...
Calibration is done...
Ro=1.66kohm //<-- This should be 10k
H2:25ppm
H2:25ppm
H2:25ppm
H2:25ppm
*continues to print 25ppm, value drops to ~8ppm after few minutes*
or:
Calibrating...
Calibration is done...
Ro=3.36kohm //<-- Again, this should be 10k
H2:25ppm
H2:25ppm
H2:25ppm
H2:25ppm
*continues to print 25ppm, value drops to ~8ppm after few minutes*
Seems like it doesn't read resistance properly (random value each time I launch serial window) and I don't understand why. Is there something wrong with my circuit?
