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MQ-137 - Ammonia Gas Sensor

MQ-137 Pin Configuration

Pin No:

Pin Name:


For Module



This pin powers the module, typically the operating voltage is +5V



Used to connect the module to system ground


Digital Out

You can also use this sensor to get digital output from this pin, by setting a threshold value using the potentiometer


Analog Out

This pin outputs 0-5V analog voltage based on the intensity of the gas 

For Sensor


H -Pins

Out of the two H pins, one pin is connected to supply and the other to ground



The A pins and B pins are interchangeable. These pins will be tied to Supply voltage.



The A pins and B pins are interchangeable.   One pin will act as output while the other will be pulled to ground.


MQ-137 Sensor Features

  • Operating Voltage is +5V
  • Can be used to Measure Ammonia, Carbon monoxide
  • Analog output voltage: 0V to 5V
  • Digital Output Voltage: 0V or 5V (TTL Logic)
  • Preheat duration over 24 hours
  • Can be used as a Digital or analog sensor
  • The Sensitivity of Digital pin can be varied using the potentiometer


Alternative MQ Gas Sensors

Sensor Name

Gas to measure


Methane, Butane, LPG, Smoke


Alcohol, Ethanol, Smoke


Methane, CNG Gas


Natural gas, LPG


LPG, butane


Carbon Monoxide


Hydrogen Gas


Carbon Monoxide, flammable gasses




Air Quality


Hydrogen Sulphide gas




Benzene, Toluene, Alcohol, Propane, Formaldehyde gas, Hydrogen


Methane, Natural Gas


Natural gas, Coal Gas


Alcohol, Ethanol, smoke


LPG, butane


Carbon Monoxide


Carbon Monoxide, flammable gas


MQ-137 Equivalent Sensors

MQ-135, CJMCU-4541


Selecting between Sensor type and module type

When it comes to measuring or detecting a particular Gas, the MQ series Gas sensors are the most commonly used ones. These sensors can either be purchased as a module or as just the sensor alone. If you are trying to only detect (not measuring ppm) the presence of a gas then you can buy it as a module since it comes with an op-amp comparator and a digital out pin. But if you planning to measure the ppm of a gas it is recommend to buy the sensor alone (without module).


Where to Use MQ-137 Gas sensor?

The MQ-137 Gas sensor can detect or measure gasses like Ammonia (NH3) and Carbon Mono-oxide (CO). The module version of this sensor comes with a Digital Pin which makes this sensor to operate even without a microcontroller and that comes in handy when you are only trying to detect one particular gas. When it comes to measuring the gas in ppm the analog pin has to be used, the analog pin is also TTL driven and works on 5V and hence can be used with most common microcontrollers.

So if you are looking for a sensor to detect or measure Ammonia or CO with or without a microcontroller, then this sensor might be the right choice for you.


How to use MQ-137 Sensors to detect gas

Using a MQ sensor to detect a gas is very easy. You can either use the digital pin or the analog pin to accomplish this. Simply power the module with 5V and you should notice the power LED on the module to glow and when no gas it detected the output LED will remain turned off meaning the digital output pin will be 0V. Remember that these sensors have to be kept on for pre-heating time (mentioned in features above) before you can actually work with it. Now, introduce the sensor to the gas you want to detect and you should see the output LED to go high along with the digital pin, if not use the potentiometer until the output gets high. Now every time your sensor gets introduced to this gas at this particular concentration the digital pin will go high (5V) else will remain low (0V).

You can also use the analog pin to achieve the same thing. Read the analog values (0-5V) using a microcontroller, this value will be directly proportional to the concentration of the gas to which the sensor detects. You can experiment with this values and check how the sensor reacts to different concentration of gas and develop your program accordingly.


How to use MQ-137 sensor to measure PPM

If you are looking for some accuracy with your readings then measuring the PPM would be the best way to go with it. It can also help you to distinguish one gas from another. So to measure PPM you can directly use a module. A basic wiring for the sensor from datasheet is shown below.

MQ-137 Wiring Diagram

The procedure to measure PPM using MQ sensor is the same but few constant values will vary based on the type of MQ sensor used. Basically we need to look into the (Rs/Ro) VS PPM graph given in the datasheet (also shown below).

MQ-137 Sensor Sensitivity Characteristics

The value of Ro is the value of resistance in fresh air and the value of Rs is the value of resistance in Gas concentration. First you should calibrate the sensor by finding the values of Ro in fresh air and then use that value to find Rs using the formula:

MQ-137 Resistance

Once we calculate Rs and Ro we can find the ratio and then using the graph shown above we can calculate the equivalent value of PPM for that particular gas.



  • Detect or measure Gases like NH3 or CO
  • Air quality monitor
  • Gas leak alarm
  • Safety standard maintenance
  • Maintaining environment standards  in hospitals 


2D model of MQ-137 Gas sensor

If you purchased as sensor then you can use the following dimensions to create your own PCB for your application.

MQ-137 Gas Sensor Dimensions

Component Datasheet

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