MQ gas sensor
Where a project needs to detect the presence of gases in the air, the MQ family offers a cost-effective solution. The sensors all have a common interface and so the description below is representative of the whole series.
Figure 1: MQ gas sensor module
The name of the sensor indicates the type of gas that it can detect. The table below lists the most common models and you can click the sensor name for further information. Please note that this list is not exclusive - use the links at the bottom of the page for further details about models that target other gases.
| Sensor | Gas(es) |
|---|---|
| MQ-2 | Flammable gases including LPG and propane |
| MQ-3 | Ethanol |
| MQ-4 | Methane and natural gas |
| MQ-6 | Methane and LPG |
| MQ-7 | Carbon monoxide and hydrogen |
| MQ-8 | Hydrogen |
| MQ-9 | Carbon monoxide and flammable gases |
| MQ-131 | Ozone |
| MQ-135 | Ammonia, benzene, ethyl alcohol, carbon dioxide |
| MQ-136 | Hydrogen sulphide |
Operation
The sensors use an electrochemical probe to detect the target gases. Different chemical treatments are used for the different target gases, A common feature is that the chemical process requires a specific temperature and the sensors are therefore equipped with a heating element. For this reason, each one needs a power supply with a specific voltage. You should check the datasheet to find out the requirements of the sensor you are using. Another consequence of using a heating element is that when first powered, the readings may be inaccurate until the sensor has reached its target temperature.
Most MQ sensors require 5V, which is no problem if your microprocessor works at 5V (like and Arduino or Raspberry Pi); however, if your board can only provide 3.3V (like a Particle Argon), you will need to introduce a second power supply. This is illustrated in Fig. 2. The figure also illustrates the use of a voltage divider to step the output signal from the sensor down from 5V to 3.3V to protect the microprocessor.
Figure 2: 5V gas sensor and 3.3V microprocessor
Hardware interface
Fig. 1 shows the sensor packaged as a module. The sensor itself typically has six pins, and that is how it will be shown on the datasheet. The module reduces this to four pins which simplifies its use in a breadboard layout. The pin assignments are described in the table below.
| Pin label | Explanation |
|---|---|
| VCC | Power input - see datasheet for the required voltage |
| GND | Ground |
| DO | Digital output |
| AO | Analogue output |
The DO pin can be used to provide a binary output - high when the gas is detected and low otherwise. The sensitivity can be changed by adjusting the on-board potentiometer labelled (1) in Fig. 3.
Figure 3: MQ sensor potentiometer
The AO pin will give an analogue output where the voltage ranges between 0 and the input voltage, and is proportional to the concentration of the target gases.
Introduction to gas sensors