How To Use MQ-135 Gas Sensor

A Gas sensor serves to detect and measure the concentration of gas in the environment. Nowadays you can find many different types of easy to use gas sensors, especially in MQ series. In this article, I’m covering the MQ-135 gas sensor which is in fact a very popular and inexpensive air quality sensor.

With the increasing air and environmental pollution, monitoring air quality is getting more and more important. Did you know? Even a small raise in the concentration of carbon dioxide can cause increased fatigue, headaches or nausea. The MQ135 air quality sensor will be a good assistant here as it can provide useful information about the surrounding air. The MQ-135 gas sensor is available as a prewired module or as just the gas sensor part alone. If you’re trying to detect the presence of a gas( not measuring PPM) only then you can buy a cheap module since it comes with onboard electronics to provide a digital output signal.

The MQ-135 gas sensor module has four pins (standard male headers) as shown above. The air quality information is readable via the AO pin. Since AO is a varying voltage, this pin should be connected to one analog input of a microcontroller. The DO pin is an open-collector digital output (but with a pull-up resistor onboard) that becomes low when the detection level exceeds a predefined level. The detection threshold can be tailored through the small trimpot soldered on the bottom of the module. Following is the generic schematic diagram of the MQ-135 module (forged by me).

Initial preparation of the module shouldn’t present any undue difficulties if you strictly follows the procedure lined here. Well, first of all power up the module alone with a regulated 5VDC power supply. The heater of the MQ-135 sensor requires 5V and have 33Ω±5% resistance, so your power supply must render a minimum 200mA of current for the sensor part. Remember that at first the MQ-135 gas sensor have to be kept on continuously for its preheating time (over 24 hour) before you can actually play with it. Thereafter, introduce the MQ-135 sensor to the gas you want to detect and slowly adjust the trimpot until DO gets low. I tested my module with isopropyl alcohol, cigar lighter gas, perfumed body spray, and the breath of mine.

Now every time your sensor gets introduced to that gas at predefined concentration, DO will go low (0V) else will remain high (5V). Note that the green LED (OUT_LED) should also light up when gas is observed. Also note that when the sensor is powered up for gas detection, it needs around 60-120 seconds to settle, because the heater will need that much time to heat the sensor up. Remember, the MQ-135 gas sensor is suitable for detecting (or measuring) of NH3, NOx, alcohol, Benzene, smoke, CO2, etc.

If you’ve made it to this point, well done! Now you can add an annunciator of your choice at the output of the module through a suitable driver circuitry. A driver circuit is very crucial here because sink current (Isink) of the LM393 IC is just about 20mA. According to its datasheet, the maximum output current may be as high as 20mA, independent of the magnitude of Vcc, output short circuits to Vcc results in excessive heating and eventual destruction of the chip. So, an electromagnetic relay (also known as electromechanical relay) based driver is recommended here because such a driver setup can not only provide best electrical isolation but also drive other ‘beefy’ electric loads apart from a minuscule annunciator. You can either use a self-made driver circuitry for that or a readymade 5VDC (active-low input type) electromagnetic relay/solid-state relay module available for cheap.

As might be expected you can take the AO output to achieve the same thing rather in a slightly different way. Try to read the AO output (0-5V) through a microcontroller so that you’ll get an output voltage directly proportional to the concentration of the gas to which the sensor is keyed to detect. This lets you experiment more with the analog output values and develop your project accordingly. Give it a good try!

This is a very bare code to read values from the MQ-135 module with its AO connected to A0 of an Arduino Uno. The values read from the sensor is proportional to the air quality measured by the sensor, and it’s displayed on the serial monitor. Remember to use a separate 5VDC power supply for the MQ-135 module (as mentioned earlier) but firmly join their ground rails (GND) together so that they can work with a common ground connection.

int sensorValue;
void setup()
{
Serial.begin(9600); // Baud rate 9600
}
void loop()
{
sensorValue = analogRead(0); // Read analog input pin A0
Serial.println(sensorValue, DEC); // Print the value read
delay(1000); 
}

Analog way (ppm)? It’s not an easy walk!

Are you getting unreasonable analog readouts? Don’t worry, we’re now going to take a close look at the MQ-135 gas sensor in this session.

The MQ-135 gas sensor has an inbuilt variable resistor (sense resistor) that changes its resistance value according to the concentration of gas. If the gas concentration is high, the resistance decreases, and if the gas concentration is low, the resistance increases. The MQ-135 gas sensor basically needs only one key component as the external component – just a load resistor. The load resistor serves to adjust the sensor’s sensitivity and accuracy. The value can range anywhere from 10KΩ to 47KΩ (the higher the resistance, the more sensitive the sensor becomes). Since sense resistance (Rs) value of MQ-135 gas sensor is different for various kinds and various concentration of gases, sensitivity adjustment becomes very necessary.

The datasheet recommends to calibrate the MQ-135 sensor for 100ppm NH3 or 50ppm Alcohol concentration in air and use value of the load resistor (RL) about 20KΩ (10KΩ to 47KΩ). But in the MQ-135 Chinese module introduced here, we can see a 1KΩ (102) load resistor. Not a good practice of course!

So if you want to build a microcontroller-based trusty air quality monitor yourself with MQ-135, you should replace the 1KΩ load resistor with one 10-22KΩ load resistor to get appropriate readings. By the way, it’s worthy to note down the acceptable CO2 ppm values:

• 400ppm – 750ppm: Good for health
• 750 ppm – 1200 ppm: Take care
• 1200 ppm (and above): Harmful to health

Next figure depicts the typical sensitivity characteristics of MQ135 gas sensor for several gases. That means the given plot tells us the concentration of a gas in part per million (ppm) according to the resistance ratio of the sensor (Rs/Ro). Here, Rs is the resistance of the sensor that changes depending on the concentration of gas, whereas Ro is the resistance of the sensor at a known concentration without the presence of other gases, or in the fresh air.

In this sensitivity characteristic curve (borrowed from the datasheet), Ro is the sensor resistance at 100ppm of NH3 in clean air, and Rs is the sensor resistance at various concentrations of gases.

If you want to do anything serious with MQ-135 gas sensor, then you should learn to analyze this plot pretty well. However that needs some skill and patience as the scale is log-log i.e. in a linear scale, the behavior of the gas concentration with respect to the resistance ratio is exponential (not linear). For determining Rs/Ro, you can use the fact that the Rs/Ro ratio is a constant in clean air. Also, by definition, only the Rs changes when a gas is present while Ro remains constant. Therefore, if you can determine the value of Rs in clean air, you can also determine Ro and ultimately Rs/Ro. A bit indigestible for a novice, but I’ll try to shed some light on it later. Meanwhile simply note that in science and engineering, a log–log plot is a two-dimensional graph/plot of numerical data that uses logarithmic scales on both the horizontal and vertical axes (https://en.wikipedia.org/wiki/Log–log_plot).

As discussed in the beginning, if you only want to detect the presence of a gas in your surroundings then you can simply use the cheap MQ-135 module and use its DO output as explained. On the other hand, you’re in need of a precise ppm count then it’d be better to use the MQ-135 gas sensor alone as shown below, the analog route (especially the sensor calibration and associated maths) is somewhat tough though (but there’s no shortcut)!

In principle, MQ-135’s internal sensor resistor R_S and external load resistor R_L forms a voltage divider. In the datasheet we can see that the sensor resistor has a value of 30KΩ-200KΩ (100ppm NH3). And, as pointed on the log–log plot, R_S in fresh air (under certain temperature and humidity) will be a constant value.. The main part of the calibration is to calculate the R_O by sampling and averaging the readings when the gas sensor is placed in fresh air. Once the R_O is derived, Rs value can be calculated.

Naturally, an Arduino microcontroller can be used to read analog values outputted by this setup, and it’s not very hard because there’s one simple library (from G.Krocker) prepared for Arduino platform (https://github.com/GeorgK/MQ135). With its help, you can do some experiments along with the Arduino code snippet posted here https://blog.robberg.net/mq-135-arduino/. You’re free to use the AO output of your MH-135 Chinese module as well, but you might need to change its load resistor to 10KΩ (or much better 22KΩ).

While using the library-supported Arduino setup, MQ-135 gas sensor calibration is done at first by finding the value of Ro in fresh air, and then using that value to find Rs through the formula: Rs = (Vcc/VRL-1)x RL, that means Rs = (5V/(sensorValue * (5.0/1023.0))-1)*RL. Anyway, I’m not feeling comfortable with the MQ-135 gas sensor library/math/code and still got a few things for my next homework.

But a moment, I welcome your follow-through on this:

• Download the MQ135 Library and include it in your Arduino IDE
• Prepare the hardware setup with MQ-135 gas sensor and 10KΩ Load Resistor (RL)

After the 24 hours preheat process, perform the following procedures in the environment of 20 degree C/35% air temperature, and read the value of RZERO

#include "MQ135.h"
const int ANALOGPIN=0;
MQ135 gasSensor = MQ135(ANALOGPIN);
void setup(){
Serial.begin(9600);
}
void loop(){
float rzero = gasSensor.getRZero();
Serial.println(rzero);
delay(1000);
}

• Insert the resultant value in the “MQ135.h” file in the MQ135 Arduino library file

• Start monitoring air quality

#include "MQ135.h"
const int ANALOGPIN=0;
MQ135 gasSensor = MQ135(ANALOGPIN);
void setup(){
Serial.begin(9600); 
}
void loop(){
float ppm = gasSensor.getPPM();
Serial.println(ppm);
delay(1000);
}

Take note, my basic Arduino Uno setup was tested with a modified Chinese MQ-135 module (10KΩ load resistor) and ofcourse it’s supported by the MQ135 Arduino library (RZERO value updated accordingly). In the MQ135 Arduino library, the atmospheric CO2 level for calibration purpose is defined as 397.13 ppm but I think nowadays that reference value needs an uptick (https://www.sciencedaily.com/releases/2019/06/190604140109.htm).

Thanks for reading with the utmost patience!