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Thomas_Portable_Air_Quality.pdf
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Thomas_Portable_Air_Quality.pdf
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savvy.ino
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savvy.ino
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/*
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Arduino Sensors & Components
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Arduino + MQ-7 CO gas sensor
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The MQ-7 gas sensor measures concentrations of carbon monoxide (CO).
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Background
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See link at http://www.savvymicrocontrollersolutions.com/index.php?sensor=mq-7-gas-sensors for information on calculations and equations within the Arduino sketch file.
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See link at http://www.savvymicrocontrollersolutions.com/index.php?sensor=mq-gas-sensors for general information about the MQ line of gas sensors and how to make connections to them.
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The Code
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*/
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/*
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MQ-7 CO (carbon monoxide) Gas Sensor
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This sketch includes the timer function and the sensor
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reading functions.
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Sensor must be one time 'burned-in' for 24 hours prior to
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general use. Burn-in consists of 5.0V for 60 sec followed
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by 1.4V for 90 sec. You can run this sketch for 24 hours
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(ignoring the readings / alarms), to burn-in the sensor.
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After burn-in, regular use of the sensor consists of
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calibration in clean air. Therafter, the sensor needs
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to be subjected to a heating cycle of 5.0V for 60 sec,
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followed by 1.4V for 90 sec. After two heating cycles (5.0 min
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total), a reading may be taken at the end of the high 5.0V
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heating cycle, just before transitioning to the low 1.4V
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heating cycle.
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This sketch checks for the presense of CO every 20 minutes.
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It performs a 2 cycle (5.0 min) warm up (blinking the
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green LED), then it takes a sensor reading (blinking the red LED).
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If the COG threshold of 50 ppm is exceeded, an alarm condition is
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set and it persists until the device is reset.
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In the United States, OSHA limits long-term workplace exposure
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levels above 50 ppm. The average level in homes is 0.5-5ppm.
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The level near properly adjusted gas stoves in homes and from
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modern vehicle exhaust emissions is 5-15ppm. The exhaust from
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automobiles in Mexico City central area is 100-200ppm.
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The amount of CO that can be created from the exhaust from
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a home wood fire is 5000ppm. Concentrations as low as 667ppm
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may cause up to 50% of the body's hemoglobin to convert to
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carboxyhemoglobin (a level that may result in seizure,
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coma, and death).
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Startup & calibration:
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Green LED = fast blink. Heating up MQ-7 sensor.
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Red LED = fast blink. Taking MQ-7 sensor reading.
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Red LED = on 5 sec + buzzer chirp. DHT11 sensor error.
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Red LED = steady on + buzzer = CO concentration exceeded !
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Normal operation:
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Green LED = steady on. Power on
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Red LED = on ~3 sec. Taking MQ-7 sensor reading.
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Red LED = on 5 sec + buzzer chirp. DHT11 sensor error.
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Red LED = steady on + buzzer = CO concentration exceeded !
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Resources:
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A0 gas sensor signal
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DIO2 green LED.
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DIO3 buzzer
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DIO4 red LED.
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DIO5 DHT11 temperature / humidity sensor.
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DIO7 NPN transistor for 12VDC relay
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Written by: Mark Kiehl
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*/
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byte pinGreenLED = 2;
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byte pinRedLED = 4;
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byte pinBuzzer = 3;
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boolean heaterHigh = true;
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byte heat_cycles = 0;
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// 60 sec high heat (5.0V)
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unsigned long timerA = 60000;
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unsigned long timerAlap = millis(); // timer
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// 90 sec low heat (1.4V)
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unsigned long timerB = 90000;
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unsigned long timerBlap = millis(); // timer
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// The difference between timerB and timerRead is
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// how long a measurement will be made for MQ-7.
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unsigned long timerRead = 60000;
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byte pinNPN = 7;
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byte pinMQ = A0;
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boolean alarmCO = false;
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// The USA OSHA exposure limit for CO is 50 ppm.
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// The average level in a home is 0.5 to 5 ppm.
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// The level in a home with a proper adj gas stove is 5 to 15 ppm.
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// A CO of 667 ppm may result in seizure, coma, and death.
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const unsigned int CO_threshold = 50;
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// Adjust vRef to be the true supply voltage in mV.
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float vRef = 5000.0;
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float RL = 10.0; // load resistor value in k ohms
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float Ro = 10.0; // default value 10 k ohms. Revised during calibration.
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const float Ro_clean_air_factor = 10.0;
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float mV = 0.0;
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unsigned long samples = 0;
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////////////////////////////////////////////////////////////////////////
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// DHT11 humidity/temperature sensor
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#include "DHT.h"
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const unsigned int pinDHT = 5;
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boolean errDHT11 = false;
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// Uncomment whatever type you're using!
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//#define DHTTYPE DHT11 // DHT 11
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#define DHTTYPE DHT22 // DHT 22 (AM2302)
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//#define DHTTYPE DHT21 // DHT 21 (AM2301)
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DHT dht(pinDHT, DHTTYPE);
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////////////////////////////////////////////////////////////////////////
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// With sensor holes facing you:
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// Connect pin 1 (on the left) of the sensor to +5V
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// Connect pin 2 of the sensor to whatever your DHTPIN is
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// Connect pin 4 (on the right) of the sensor to GROUND
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// Connect a 10K resistor from pin 2 (data) to pin 1 (power) of the sensor
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void setup() {
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pinMode(pinGreenLED, OUTPUT);
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delay(1);
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pinMode(pinRedLED, OUTPUT);
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delay(1);
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pinMode(pinMQ, INPUT);
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delay(1);
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pinMode(pinBuzzer, OUTPUT);
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delay(1);
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pinMode(pinNPN, OUTPUT);
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delay(1);
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Serial.begin(9600);
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while (!Serial) {
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; // wait for serial port to connect
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}
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// Delay to give the Arduino Micro time to connect the Serial Monitor.
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delay(35000);
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Serial.print("DHT");
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Serial.print(DHTTYPE);
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Serial.println(" setup");
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dht.begin();
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// Get the ambient conditions (deg C & relative humidity) from DHT11
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float ambRH = dht.readHumidity();
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float ambTemp = dht.readTemperature();
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Serial.println(" ");
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// check if returns are valid, if they are NaN (not a number) then something went wrong!
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if (isnan(ambTemp) || isnan(ambRH)) {
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Serial.println("Failed to read from DHT");
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Serial.println(" ");
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errDHT11 = true;
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digitalWrite(pinRedLED, HIGH);
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digitalWrite(pinBuzzer, HIGH);
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delay(100);
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digitalWrite(pinBuzzer, LOW);
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delay(5000);
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digitalWrite(pinRedLED, LOW);
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} else {
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// DHT11 ok, .. proceed.
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Serial.print("ambTemp = ");
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Serial.print(ambTemp);
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Serial.println(" deg C");
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Serial.print("ambRH = ");
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Serial.print(ambRH);
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Serial.println("% ");
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}
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Serial.println(" ");
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Serial.println("Calibrating MQ-7 CO sensor in clean air..");
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Serial.println(" 60 sec high heat cycle..");
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digitalWrite(pinNPN, HIGH);
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// set = 200
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for(int i = 200; i > 0; i--){
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blinkLED(pinGreenLED);
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}
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digitalWrite(pinNPN, LOW);
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Serial.println(" 60 sec warmup complete");
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Serial.println(" 90 sec heat cycle..");
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// set = 300
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for(int i = 300; i>0; i--){
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blinkLED(pinGreenLED);
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}
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Serial.println(" 90 sec warmup complete. Reading MQ-7..");
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// If mV > 3000, then repeat warm-up..
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// take a reading..
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// set = 300
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for(int i = 300; i>0; i--){
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blinkLED(pinGreenLED);
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mV += Get_mVfromADC(pinMQ);
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samples += 1;
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}
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mV = mV / (float) samples;
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Serial.print(" avg A");
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Serial.print(pinMQ);
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Serial.print(" for ");
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Serial.print(samples);
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Serial.print(" samples = ");
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Serial.print(mV);
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Serial.println(" mV");
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Serial.print(" Rs = ");
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Serial.println(CalcRsFromVo(mV));
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// Conv output to Ro
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// Ro = calibration factor for measurement in clean air.
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// Ro = ((vRef - mV) * RL) / (mV * Ro_clean_air_factor);
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// Hereafter, measure the sensor output, convert to Rs, and
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// then calculate Rs/Ro using: Rs = ((Vc-Vo)*RL) / Vo
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Ro = CalcRsFromVo(mV) / Ro_clean_air_factor;
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Serial.print(" Ro = ");
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Serial.println(Ro);
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// Values in clean air are:
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// Rs = 6.99
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// Ro = 0.70
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Serial.println("Sensor calibration in clean air complete");
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Serial.println("Setup complete. Monitoring for CO..");
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Serial.println(" ");
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digitalWrite(pinNPN, LOW);
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mV = 0.0;
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samples = 0;
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// Start with heater on high
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heaterHigh = true;
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timerAlap = millis(); // reset the timer
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blinkLED(pinGreenLED);
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blinkLED(pinRedLED);
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// Chirp the buzzer
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digitalWrite(pinBuzzer, HIGH);
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delay(1);
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digitalWrite(pinBuzzer, LOW);
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delay(1);
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//digitalWrite(pinGreenLED, HIGH);
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//delay(1);
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}
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void loop() {
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// if millis() or timer wraps around, we'll just reset it
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if (timerAlap > millis()) timerAlap = millis();
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if (timerBlap > millis()) timerBlap = millis();
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if (heaterHigh == false && heat_cycles == 2 && (millis() - timerBlap > timerRead)) {
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// take reading of MQ sensor..
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digitalWrite(pinGreenLED, HIGH);
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mV += Get_mVfromADC(pinMQ);
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samples += 1;
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} else {
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digitalWrite(pinGreenLED, LOW);
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}
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if (heaterHigh == true) {
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// High heat applied for 60 sec
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digitalWrite(pinNPN, HIGH);
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// Timer A
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if (millis() - timerAlap > timerA) {
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timerAlap = millis(); // reset the timer
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timerBlap = millis(); // reset the timer
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heaterHigh = false;
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}
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} else {
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// heaterHigh = false
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// Low heat applied for 90 sec
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digitalWrite(pinNPN, LOW);
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// Timer B
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if (millis() - timerBlap > timerB) {
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timerAlap = millis(); // reset the timer
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timerBlap = millis(); // reset the timer
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heaterHigh = true;
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heat_cycles += 1;
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Serial.print("end of heat_cycle = ");
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Serial.println(heat_cycles);
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// Report on MQ-7 measurement at end of
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// the low phase of the 3rd heat cycle.
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if (heat_cycles == 3) {
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mV = mV / float (samples);
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Serial.print("samples = ");
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Serial.println(samples);
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Serial.print("A");
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Serial.print(pinMQ);
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Serial.print(" = ");
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Serial.print(mV);
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Serial.println(" mV");
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Serial.print("Rs = ");
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Serial.println(CalcRsFromVo(mV));
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Serial.println(" ");
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mV = 0.0;
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samples = 0;
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}
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}
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}
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if (heat_cycles >= 3) {
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heat_cycles = 0;
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}
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}
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float RsRoAtAmbientTo20C65RH(float RsRo_atAmb, float ambTemp, float ambRH) {
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// Using the datasheet for MQ-7 sensor, derive Rs/Ro values
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// from - 10 to 50 C and 33, 65, and 85 % relative humidity.
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// For the measured Rs/Ro, use linear interpolation to calculate the
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// standard Rs/Ro values for the measured ambient temperature and RH.
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// Next, calculate a correction factor from the standard Rs/Ro at ambient
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// temp and RH relative to standard Rs/Ro at 20C and 65 RH.
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// Apply this correction factor to the measured Rs/Ro value and return the
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// corrected value. This corrected value may then be used against the Rs/Ro
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// Rs/Ro vs CO concentration (ppm) chart to estimate the concentration of CO.
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// Calc RsRo values at ambTemp & 33% RH, 65% and 85% RH
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float RsRo_at_ambTemp_33RH = -0.00000593 * pow(ambTemp, 3) + 0.000533 * pow(ambTemp, 2) - 0.0182 * ambTemp + 1.20;
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float RsRo_at_ambTemp_85RH = -0.0000000741 * pow(ambTemp, 3) + 0.000114 * pow(ambTemp, 2) - 0.0114 * ambTemp + 1.03;
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//float RsRo_at_65RH = ((65.0-33.0)/(85.0-65.0));
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float RsRo_at_ambTemp_65RH = ((65.0-33.0)/(85.0-33.0)*(RsRo_at_ambTemp_85RH-RsRo_at_ambTemp_33RH)+RsRo_at_ambTemp_33RH)*1.102;
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// Linear interpolate to get the RsRo at the ambient RH value (ambRH).
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float RsRo_at_ambTemp_ambRH;
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if (ambRH < 65.0) {
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RsRo_at_ambTemp_ambRH = (ambRH - 33.0)/(65.0 - 33.0)*(RsRo_at_ambTemp_65RH - RsRo_at_ambTemp_33RH) + RsRo_at_ambTemp_33RH;
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} else {
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// ambRH > 65.0
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RsRo_at_ambTemp_ambRH = (ambRH - 65.0)/(85.0 - 65.0)*(RsRo_at_ambTemp_85RH - RsRo_at_ambTemp_65RH) + RsRo_at_ambTemp_65RH;
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}
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// Calc the correction factor to bring RsRo at ambient temp & RH to 20 C and 65% RH.
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const float refRsRo_at_20C65RH = 1.00;
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float RsRoCorrPct = 1 + (refRsRo_at_20C65RH - RsRo_at_ambTemp_ambRH)/refRsRo_at_20C65RH;
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// Calculate what the measured RsRo at ambient conditions would be corrected to the
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// conditions for 20 C and 65% RH.
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float measured_RsRo_at_20C65RH = RsRoCorrPct * RsRo_atAmb;
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return measured_RsRo_at_20C65RH;
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}
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float CalcRsFromVo(float Vo) {
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// Vo = sensor output voltage in mV.
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// VRef = supply voltage, 5000 mV
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// RL = load resistor in k ohms
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// The equation Rs = (Vc - Vo)*(RL/Vo)
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// is derived from the voltage divider
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// principle: Vo = RL * Vc (Rs + RL)
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//
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// Note. Alternatively you could calc
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// Rs from ADC value using
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// Rs = RL * (1024 - ADC) / ADC
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float Rs = (vRef - Vo) * (RL / Vo);
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return Rs;
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}
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unsigned int GetCOPpmForRatioRsRo(float RsRo_ratio) {
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// If you extract the data points from the CO concentration
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// versus Rs/Ro chart in the datasheet, plot the points,
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// fit a polynomial curve to the points, you come up with the equation
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// for the curve of: Rs/Ro = 22.073 * (CO ppm) ^ -0.66659
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// This equation is valid for ambient conditions of 20 C and 65% RH.
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// Solving for the concentration of CO you get:
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// CO ppm = [(Rs/Ro)/22.073]^(1/-0.66666)
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float ppm;
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ppm = pow((RsRo_ratio/22.073), (1/-0.66659));
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return (unsigned int) ppm;
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}
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float Get_mVfromADC(byte AnalogPin) {
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// read the value from the sensor:
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int ADCval = analogRead(AnalogPin);
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// It takes about 100 microseconds (0.0001 s) to read an analog input
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delay(1);
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// Voltage at pin in milliVolts = (reading from ADC) * (5000/1024)
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float mV = ADCval * (vRef / 1024.0);
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return mV;
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}
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void blinkLED(byte ledPIN){
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// consumes 300 ms.
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for(int i = 5; i>0; i--){
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digitalWrite(ledPIN, HIGH);
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delay(30);
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digitalWrite(ledPIN, LOW);
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delay(30);
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}
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}
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