List of All Gas sensors

MQ-2

Sensitive for Methane, Butane, LPG, smoke.
This sensor is sensitive for flamable and combustible gasses.
The heater uses 5V.
The MQ-2 at seeed: http://www.seeedstudio.com/wiki/Grove_-_Gas_Sensor%28MQ2%29

Search for datasheet: http://duckduckgo.com/?q=%22mq-2%22+gas+sensor+filetype%3Apdf

MQ-3

Sensitive for Alcohol, Ethanol, smoke
The heater uses 5V
The Arduino blog about the "breathalyzer" using a MQ-3 : http://arduino.cc/blog/2010/09/23/arduino-breathalyzer-calibrating-the-mq-3-alcohol-sensor/
The MQ303A (also on this page) is like this sensor, but uses a lower heater voltage.

Search for datasheet: http://duckduckgo.com/?q=%22mq-3%22+gas+sensor+filetype%3Apdf

MQ-4

Sensitive for Methane, CNG Gas
The heater uses 5V.
Search for datasheet: http://duckduckgo.com/?q=%22mq-4%22+gas+sensor+filetype%3Apdf

MQ-5

Sensitive for Natural gas, LPG
The heater uses 5V.
The MQ-5 at seeed: http://www.seeedstudio.com/wiki/Electronic_brick_-_Gas_sensor%28MQ5%29 and http://www.seeedstudio.com/wiki/Gas_Sensor_-_MQ5

Search for datasheet: http://duckduckgo.com/?q=%22mq-5%22+gas+sensor+filetype%3Apdf

MQ-6

Sensitive for LPG, butane gas
The heater uses 5V.
The MQ-6 at seeed: http://www.seeedstudio.com/wiki/Electronic_brick_-_Gas_sensor%28MQ6%29
The MQ306A (also on this page) is like this sensor, but uses a lower heater voltage.

Search for datasheet: http://duckduckgo.com/?q=%22mq-6%22+gas+sensor+filetype%3Apdf

MQ-7

Sensitive for Carbon Monoxide
The heater uses an alternating voltage of 5V and 1.4V.
A library for the MQ-7 : http://thesis.jmsaavedra.com/prototypes/software/mq-7-breakout-arduino-library/
The MQ307A (also on this page) is like this sensor, but uses a lower heater voltage.

Search for datasheet: http://duckduckgo.com/?q=%22mq-7%22+gas+sensor+filetype%3Apdf

MQ-8

Sensitive for Hydrogen Gas
The heater uses 5V.

Search for datasheet: http://duckduckgo.com/?q=%22mq-8%22+gas+sensor+filetype%3Apdf

MQ-9

Sensitive for Carbon Monoxide, flammable gasses.
The heater uses an alternating voltage of 5V and 1.5V. It depends on the gases how to use that alternating voltage. If only Carbon Monoxide is tested, the heater can be set at 1.5V.
The MQ309A (also on this page) is like this sensor, but uses a lower heater voltage.

Search for datasheet: http://duckduckgo.com/?q=%22mq-9%22+gas+sensor+filetype%3Apdf

MQ131

Sensitive for Ozone
The heater uses 6V.

The load-resistor is 100k...200k, which is a lot higher than for other sensors. This sensor is also very sensitive. It measures in ppb (parts per billion) where other sensors measure in ppm (parts per million).

Search for datasheet: http://duckduckgo.com/?q=%22mq131%22+gas+sensor+filetype%3Apdf

MQ135

For Air Quality
Sensitive for Benzene, Alcohol, smoke.
The heater uses 5V.
An example how to use it: http://wiring.org.co/learning/basics/airqualitymq135.html

An example with calculation of the CO2 value: http://davidegironi.blogspot.it/2014/01/cheap-co2-meter-using-mq135-sensor-with.html

Search for datasheet: http://duckduckgo.com/?q=%22mq135%22+gas+sensor+filetype%3Apdf

MQ136

Sensitive for Hydrogen Sulfide gas.
The heater uses 5V.
Search for datasheet: http://duckduckgo.com/?q=%22mq136%22+gas+sensor+filetype%3Apdf

MQ137

Sensitive for Ammonia.
The heater uses 5V.
Search for datasheet: http://duckduckgo.com/?q=%22mq137%22+gas+sensor+filetype%3Apdf

MQ138

Sensitive for Benzene, Toluene, Alcohol, Acetone, Propane, Formaldehyde gas, Hydrogen gas.
The heater uses 5V.
Search for datasheet: http://duckduckgo.com/?q=%22mq138%22+gas+sensor+filetype%3Apdf

MQ214

Sensitive for Methane, Natural gas.
The heater uses 6V.
Search for datasheet: http://duckduckgo.com/?q=%22mq214%22+gas+sensor+filetype%3Apdf

MQ216

Sensitive for Natural gas, Coal gas.

Search for datasheet: http://duckduckgo.com/?q=%22mq216%22+gas+sensor+filetype%3Apdf

MQ303A

Sensitive for Alcohol, Ethanol, smoke (just like the MQ-3)
The heater uses 0.9V
An example for the MQ302A http://insapio.com/2011/12/27/mq3-or-mq303a-alcohol-sensor/
It detects the same gasses as the MQ-3, but uses a lower heater voltage.

Search for datasheet: http://duckduckgo.com/?q=%22mq303a%22+gas+sensor+filetype%3Apdf

MQ306A

Sensitive for LPG, butane gas
The heater uses 0.9V.
It detects the same gasses as the MQ-6, but uses a lower heater voltage.

Search for datasheet: http://duckduckgo.com/?q=%22mq306a%22+gas+sensor+filetype%3Apdf

MQ307A

Sensitive for Carbon Monoxide
The heater uses an alternating voltage of 0.2V and 0.9.
It detects the same gasses as the MQ-7, but uses a lower heater voltage.

Search for datasheet: http://duckduckgo.com/?q=%22mq307a%22+gas+sensor+filetype%3Apdf

MQ309A

Sensitive for Carbon Monoxide, flammable gasses.
The heater uses an alternating voltage of 0.2V and 0.9V. It depends on the gases how to use that alternating voltage.
It detects the same gasses as the MQ-9, but uses a lower heater voltage.

Search for datasheet: http://duckduckgo.com/?q=%22mq309a%22+gas+sensor+filetype%3Apdf

MG811

Sensitive for Carbon Dioxide (CO2).
The heater uses 6V.
The signal from this gas sensor can be connected to the Arduino, but it's better to amplifly the signal with a OpAmp.
An example of this sensor (without an OpAmp) : http://itp.nyu.edu/physcomp/sensors/Reports/MG811

Search for datasheet: http://duckduckgo.com/?q=%22mg811%22+gas+sensor+filetype%3Apdf

AQ-104

For air quality

Search for datasheet: http://duckduckgo.com/?q=%22aq-104%22+gas+sensor+filetype%3Apdf

AQ-2

Sensitive for Flamable gasses, smoke

Search for datasheet: http://duckduckgo.com/?q=%22aq-2%22+gas+sensor+filetype%3Apdf

AQ-3

Sensitive for Alcohol, Benzine

Search for datasheet: http://duckduckgo.com/?q=%22aq-3%22+gas+sensor+filetype%3Apdf

AQ-7

Sensitive for Carbon Monoxide

Search for datasheet: http://duckduckgo.com/?q=%22aq-7%22+gas+sensor+filetype%3Apdf