These sensors are electrochemical and vary their resistance when exposed to certain gases, internally it has a heater in charge of increasing the internal temperature and with this the sensor can react with the gases causing a change in the value of the resistance. The heater depending on the model may need a voltage between 5 and 2 volts, the sensor behaves like a resistor and needs a load resistance (RL) to close the circuit and with this make a voltage divider and be able to read it from a microcontroller.

Due to the heater it is necessary to wait for a warm-up time for the output to be stable and have the characteristics that the manufacturer shows in its datasheet, said time * depending on the model *can be between 12 and 48 hours.

In the market, MQ sensors are usually found in modules, which simplifies the connection part and facilitates its use, it is enough to feed the module and start reading the sensor, these modules also have a digital output which internally works with a comparator and with the help of a potentiometer we can calibrate the threshold and thus be able to interpret the digital output as presence or absence of gas.

**NOTE: THIS SENSOR WILL HEAT A LITTLE**

**Credit: **https://www.jayconsystems.com/tutorials/gas-sensor...

Calculations

This is a Work in progress, update Soon

Take a look and suscribe:

Promo Video:

### Supplies:

## Step 1: Materials

Materials:

1 Arduino Uno

1 Oled Display 128 x 32 i2c Version

2 2.2k Ohm Resistors

1 Trimpot 100K Ohm

1 MQ-137 Sensor

1 RGB LED

Softwares:

https://www.arduino.cc/en/Main/Software

Librarys:

## Step 2: Calculations

We can derive a formula to find RS using Ohm's Law:

V = I x R

Which in our circuit is equal

to: I = VC / (RS+RL) RL=47kΩ

Ro: sensor resistance in the clean air.

Rs: sensor resistance at various concentrations of gases.

Then RS = [(VC x RL) / VRL] - RL

This formula will help us find the values of the sensor resistance for different gases. From the graph, we can see that the resistance ratio in fresh air is a constant:

RS / R0 = 2.6 ppm Fig. 3.

To calculate R0 we will need to find the value of the RS in fresh air.

This will be done by taking the analog average readings from the sensor and converting it to voltage.

Then we will use the RS formula to find R0. We will show you how to do this in the Arduino IDE after the wiring setup.

The scale of the graph is log-log. This means that in a linear scale, the behavior of the gas concentration with respect to the resistance ratio is exponential. The data for gas concentration only ranges from 10 ppm to 1000 ppm. Even though the relation between resistance ratio and gas concentration may seem linear, in reality, it is not. First of all, we will treat the lines as if they were linear. This way we can use one formula that linearly relates the ratio and the concentration. By doing so, we can find the concentration of a gas at any ratio value even outside of the graph’s boundaries.

The formula we will be using is the equation for a line, but for a log-log scale. The formula for a line is:

y = mx + b Where:

y: X value

x: X value m: Slope of the line

b: Y intercept

For a log-log scale, the formula looks like this:

log(y) = m*log(x) + b

Note: the log is base 10.

Okay, let’s find the slope. To do so, we need to choose 2 points from the graph.

The formula to calculate m is the following:

y = mx + b where:

value of x: X

value of m: Slope of line

b: Y intercepcion Point Graph (20,1.2) y (40,1)

m = [log(y) - log(y0)] / [log(x) - log(x0)]

m = log(1/1.2) / log(40/20)

m = -0.26303440583

Now for (30,1.1) log(y) = m*log(x) + b b = log(y) - m*log(x) b = log(1.1) - (-0.26303440583)*log(30)

b = 0.42992639673

Where: on the air RS / R0 = 3.6 R0 = 2.19

If you are not sure how logarithms work, you can refer to this link:http://www.rapidtables.com/math/algebra/Logarithm...

All credit:

## Step 3: Mounting

If You dont have the means to build a PCB Here i left the Fritzing Sketch

you want to reach the 100% of this project please visit and watch my tutorials(Soon):

www.youtube.com/user/josexers

## Step 4: Programming: First Steps

The first step that must be done to calculate R0, how we achieve this with a Sub Program called CAlc_R0 which is attached.

At first we uploas this sketch, and we wil get a serial responce from it.

We take note of this Value, IT'S VERY IMPORTANT!!

it's NOT A FIXED VALUE! it depends of the ambient

Now... we update some values in our pmain program:

int gas_sensor = A0; **//Sensor pin **

float m = -0.263; **//Slope **

float b = 0.42; **//Y-Intercept **

float R0 = 2.19; **//Sensor Resistance in fresh air from previous code**

## Step 5: 3D Printing

Here i left the files you need to print the box of our Project, it was made in Autosesk 123D Design.

Here is my Thingiverse Page:

https://www.thingiverse.com/thing:2637529

## Step 6: Extra Content

Notice that you can Add some more features to this sensor, such as RS-485 Modbus already mounted Compatible Library.

Also Notice that there is a RELAY OUTPUT, that we can use as a safety switch.

Also a JAva made Software its Compatible, comming soon.

## Step 7: ANDROID APP

This is an extra.

You can also Monitor how the sensor is reading Ammonia.

I used the MIT APP INVENTOR2

## 10 Discussions

2 months ago

Hello,

I'm reading the Ro and I'm getting negative values, I made the cable change, the mq137 sensor, the mega, I left 24 hours reading and I still get negative values.

8 months ago

Hats off

Question 1 year ago on Step 4

hey i'm still confused how to find R0 value

is the R0 value is the converted analog average readings to the voltage?

Answer 1 year ago

yes, it is a calculated value using the analog reading.

1 year ago

Thanks for the tutorial!! Where did you get your sensor from? And on Step 6: Extra Content on the "Y" axis says 0-100ppm, but the values go from 1000 to 8000. Is it because of accuracy of the sensor?

Reply 1 year ago

yes it is, but you must be careful, read the sensor specs first

1 year ago

how do we take Ro value from graph

Reply 1 year ago

Ro is measured by anoter sketch, Step 4

1 year ago

Nide done dude!

1 year ago

Looks good. You should enter this into the Arduino contest that is currently running.