Lesson 22 Gas Sensor

Introduction

Gas Sensor MQ-2 is a sensor for flammable gas and smoke by detecting the concentration of combustible gas in the air. They are used in gas detecting equipment for smoke and flammable gasses in household, industry or automobile.

Required Components

– 1 * Raspberry Pi

– 1 * Breadboard

– 1 * Active Buzzer module

– 1 * PCF8591

– 1 * Gas sensor module

– 1 * 3-Pin anti-reverse cable

– 1 * 4-Pin anti-reverse cable

– Several Jumper wires

Experimental Principle

MQ-2 gas sensor is a kind of surface ion type and N-type semiconductors, which uses tin oxide semiconductor gas sensitive material. When ambient temperature is in 200 ~ 300℃, tin oxide will adsorb oxygen in the air and form oxygen anion adsorption to decrease electron density in semiconductor so as to increase its resistance. When in contact with the smoke, if grain boundary barrier is modulated by the smoke and changed, it could cause surface conductivity change. So you can gain the information of the smoke existence, The higher the smoke concentration is, the more conductive the material becomes, thus the lower the output resistance is.

In this experiment, if harmful gases reach a certain concentration, the buzzer will beep to warn. 

The schematic diagram of the module is as shown below:

Experimental Procedures

Step 1: Build the circuit. 

Raspberry Pi	GPIO Extension Board	PCF8591 Module
SDA	SDA1	SDA
SCL	SCL1	SCL
3.3V	3V3	VCC
GND	GND	GND

Gas Sensor Module	GPIO Extension Board	PCF8591 Module
DO	GPIO17	*
AO	*	AIN0
VCC	3V3	*
GND	GND	GND

Raspberry Pi	GPIO Extension Board	Active Buzzer Module
GPIO1	GPIO18	SIG
3.3V	3V3	VCC
GND	GND	GND

For C Users:

Step 2: Change directory. 

    cd /home/pi/SunFounder_SensorKit_for_RPi2/C/22_gas_sensor/

Step 3: Compile.

    gcc gas_sensor.c -lwiringPi

Step 4: Run.

    sudo ./a.out

For Python Users:

Step 2: Change directory.

    cd /home/pi/SunFounder_SensorKit_for_RPi2/Python/

Step 3: Run.

    sudo python3 22_gas_sensor.py

Place a lighter close to the MQ-2 gas sensor, and press the switch to release gasses. A value between 0 and 255 will be displayed on the screen. If harmful gases reach a certain concentration, the buzzer will beep, and “Danger Gas!” will be printed on the screen.

You can also turn the shaft of the potentiometer on the module to raise or reduce the concentration threshold.

The MQ-2 gas sensor needs to be heated up for a while. Wait until the value printed on screen stays steady and the sensor gets warm, which means it can work normally and sensitively at that time.

Note: It is normal that the gas sensor generates heat. Actually, the higher the temperature is, the sensor is more sensitive.