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Arduino MQ135
connect the sensor as follows for raw sensor :
A H A >>> 5V
B >>> A0
H >>> GND
B >>> 10K ohm >>> GND
connect the sensor as follows if on board :
Vcc >>> 5V
Gnd >>> Gnd
Aout >>> A0
Contribution: epierre
Based on David Gironi
#include <SPI.h>
#include <MySensor.h>
#include <Wire.h>
#define CHILD_ID_AIQ 0
#define MQ135_DEFAULTPPM 399 //default ppm of CO2 for calibration
#define MQ135_DEFAULTRO 68550 //default Ro for MQ135_DEFAULTPPM ppm of CO2
#define MQ135_SCALINGFACTOR 116.6020682 //CO2 gas value
#define MQ135_EXPONENT -2.769034857 //CO2 gas value
#define MQ135_MAXRSRO 2.428 //for CO2
#define MQ135_MINRSRO 0.358 //for CO2
unsigned long SLEEP_TIME = 30000; // Sleep time between reads (in seconds)
double mq135_ro = 10000; // this has to be tuned 10K Ohm
double val = 0; // variable to store the value coming from the sensor
double valAIQ =0;
double lastAIQ =0;
MySensor gw;
MyMessage msg(CHILD_ID_AIQ, V_LEVEL);
void setup()
// Send the sketch version information to the gateway and Controller
gw.sendSketchInfo("AIQ Sensor MQ135", "1.0");
// Register all sensors to gateway (they will be created as child devices)
* get the calibrated ro based upon read resistance, and a know ppm
long mq135_getro(long resvalue, double ppm) {
return (long)(resvalue * exp( log(MQ135_SCALINGFACTOR/ppm) / MQ135_EXPONENT ));
* get the ppm concentration
double mq135_getppm(long resvalue, long ro) {
double ret = 0;
double validinterval = 0;
validinterval = resvalue/(double)ro;
if(validinterval<MQ135_MAXRSRO && validinterval>MQ135_MINRSRO) {
ret = (double)((double)MQ135_SCALINGFACTOR * pow( ((double)resvalue/ro), MQ135_EXPONENT));
return ret;
void loop()
double valr = analogRead(AIQ_SENSOR_ANALOG_PIN);// Get AIQ value
double val = ((float)22000*(1023-valr)/valr);
//during clean air calibration, read the Ro value and replace MQ135_DEFAULTRO value with it, you can even deactivate following function call.
mq135_ro = mq135_getro(val, MQ135_DEFAULTPPM);
//convert to ppm (using default ro)
valAIQ = mq135_getppm(val, MQ135_DEFAULTRO);
Serial.print ( "Val / Ro / value:");
Serial.print ( val);
Serial.print ( " / ");
Serial.print ( mq135_ro);
Serial.print ( " / ");
Serial.print ( valAIQ);
if (valAIQ != lastAIQ) {
lastAIQ = ceil(valAIQ);
// Power down the radio. Note that the radio will get powered back up
// on the next write() call.
gw.sleep(SLEEP_TIME); //sleep for: sleepTime
/***************************** 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
int MQGetPercentage(float rs_ro_ratio, float ro, float *pcurve)
return (double)(pcurve[0] * pow(((double)rs_ro_ratio/ro), pcurve[1]));
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