How to detect carbon monoxide with a MQ-7 sensor module
How gas sensors work
I found an excellent thesis paper on how Tin Dioxide (SnO2) gas sensors work here. It also goes into the details of it's temperature dependency. (See details further down)
Video
Watch my video on the tests here.The module from ICStation
You can get this module here. (Use code andyics for 15% off your order)The intended mode of operation is to apply 5V to the module and either read analog values from AOUT or set the threshold of the comparator to the desired value and read from the DOUT pin if it has tripped.
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| The terminals |
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| Comparator and trimmer |
It seems to me that ICStation treats all MQ-series sensors the same way. But the MQ-7 is different from the rest. According to the data sheet, it gives the best results on the following cycle:
- Pre-heat sensor for 48h
- Heat heater with 5V for 60 seconds
- Heat at 1.4V for 90 seconds
- Read the sensor near the end of the 90 seconds
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| You can see me breathing at the sensor |
I am quite sure there is no siginificant quantity of CO in my breath. And I not a smoker. The sensor reacts to a wide range of gases, as well as moisture and ambient temperature,
Tricking the module into datasheet-like conditions
To build this, you need the following components:- Leonardo (or clone thereof), an UNO works just as well
- ICStation's MQ-7 module
- 10k resistor (optional)
- IRLZ34N N-Channel Mosfet
The IRLZ34N is a very common N-Channel MosFET what already has a very low (0.046 Ohm) source-drain resistance with 5V at the gate. It can handle currents way beyond our reqirements for the flimsy heater on the module.
The heater can run on DC or AC, so PWM should be ok. I can then set the duty cycle of the PWM so that it is the equivalent of 1.4V. (See code below.)
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| The 10k resistor is optional |
The setup with the "switching" mosfet.
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| Setup with Mosfet |
The Arduino code
The code for the "proper" usage cycle: /*
MQ-7 cheater
Uses PWM and an N-Channel MosFET to trick an ICSTATION MQ-7 CO detector
into measuring CO according to the datasheet of the manufaturer.
*/
int sensorPin = A0; // select the input pin for the CO sensor
int sensorValue = 0; // variable to store the value coming from the sensor
// Initial setup
void setup() {
// initialize digital pin LED_BUILTIN as an output
pinMode(LED_BUILTIN, OUTPUT);
// initialize the serial port
Serial.begin(9600);
}
// the loop function runs over and over again forever
void loop() {
analogWrite(LED_BUILTIN, 255); // turn the heater fully on
delay(60000); // heat for 60 second
// now reduce the heating power
analogWrite(LED_BUILTIN, 72); // turn the heater to approx 1,4V
delay(90000); // wait for 90 seconds
// we need to read the sensor at 5V, but must not let it heat up. So hurry!
digitalWrite(LED_BUILTIN, HIGH);
delay (50); //don't know how long to wait without heating up too much. Getting an analog read apparently takes 100uSec
// read the value from the sensor:
sensorValue = analogRead(sensorPin);
Serial.println(sensorValue);
}
Increased sensitivity
Under the same conditions (candle suffocated under jar), the FET-Pulsed version showed a significantly higher peak. |
| FET-Pulsed heater |
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| Heater on 5v constantly |
