MQ-3 Alcohol Sensor Module Ethanol Breathalyzer

MQ-6 Gas Methane Sensor Module for Arduino features:
- Main chip: MQ-6 methane sensor and comparator LM393
- Working voltage: 5V
- Current: 150mA
- Digital output: TTL 0/1
- Analog output: analog voltage sensing value
- Quick response features:
- Detection concentration:
- 200ppm- 10000ppm LPG, LNG
- Adjustable sensitivity with the potentiometer
- Size: 32mm x 20mmx 18mm

MQ-8 Hydrogen Gas Sensor Module features:
- Main chip: MQ-8 methane sensor and comparator LM393
- Working voltage: 5V
- Current: 150mA
- Digital output: TTL 0/1
- Analog output: analog voltage sensing value
- Quick response features:
- Detection concentration:
  100ppm- 10000ppm hydrogen H2
- Adjustable sensitivity with the potentiometer
- Size: 32mm x 20mmx 18mm

MQ-4 Gas Methane Sensor Module for Arduino features:
- Main chip: MQ-4 methane sensor and comparator LM393
- Working voltage: 5V
- Current: 150mA
- Digital output: TTL 0/1
- Analog output: analog voltage sensing value
- Quick response features:
- Detection concentration:
200ppm- 10000ppm CH4, natural gas
- Adjustable sensitivity with the potentiometer
- Size: 32mm x 20mmx 18mm

High sensitivity to Ammonia (NH3), Nitrogen oxides (NOx), Alcohol, Sulphides, Benzene (C6H6), CO2, smoke and other harmful gases.  The MQ-135 sensor is to sensible for other gasses in the air so that is not possible to get good values for CO2 from it.
MQ-135 Air Quality Sensor features:
- Main chip: MQ-135 natural gas sensor and comparator LM393
- Working voltage: 5V
- Current: 150mA
- Digital output: TTL 0/1
- Analog output: analog voltage sensing value
- Quick response features:
- Detection concentration:
   10ppm - 300ppm NH3
   10ppm - 1000ppm Benzene
   10ppm - 300ppm Alcohol
- Adjustable sensitivity with the potentiometer
- Size: 32mm x 20mmx 18mm

Air quality sensor with high precision whose SDS011 uses laser dispersion to detect particle concentration between 0.3 and 10 um in the air.

With digital output by USB (UART) and built-in fan. It is a powerful sensor, fast (response less than 10s), accurate and reliable.

The Carbon Monoxide sensor is a low-power digital gas sensor which has a CCS811 sensor and an 8-bit microcontroller, contains an analog-digital converter so that it can detect indoor air quality which can detect monoxide carbon (CO) and a wide range of volatile organic compounds (VOCs). It is a sensor designed to monitor air quality inside personal devices such as mobile devices and smart watches, with low power consumption that can be used with battery

MQ-9 Carbon Monoxide Sensor Module features:
- Main chip: MQ-9 methane sensor and comparator LM393
- Working voltage: 5V
- Current: 150mA
- Digital output: TTL 0/1
- Analog output: analog voltage sensing value
- Quick response features:
- Detection concentration:
   20ppm- 2000ppm carbon monoxide
   500ppm- 10000ppm methane CH4
   500ppm- 10000ppm LPG
- Adjustable sensitivity with the potentiometer
- Size: 32mm x 20mmx 18mm

MQ-7 Carbon Monoxide Sensor Module. The sensor detects the concentration of CO (carbon monoxide) and flammable gases. It has a goldpin connector raster 2.54 mm. It is powered by 5 V. It has digital and analogue outputs, which makes it compatible with most runtime modules including Arduino.

MG-5 Gas Methane Sensor Module for Arduino features:
- Main chip: MQ-5 natural gas sensor and comparator LM393
- Working voltage: 5V
- Current: 150mA
- Digital output: TTL 0/1
- Analog output: analog voltage sensing value
- Quick response features:
- Detection concentration:
- 200ppm - 10000ppm LPG, LNG, natural gas, propane,
- Adjustable sensitivity with the potentiometer
- Size: 32mm x 20mmx 18mm

MH-Z14A NDIR infrared gas module is a small size and common type sensor using the non-dispersive infrared (NDIR) principle to detect the existence of CO2 in air, with good selectivity, non-oxygen dependent and long lifetime.