If anyone wants to build a Gadgeteer Gas Sensor module, here is the killer feature:
So you basically build a board like a Grove (eg http://www.seeedstudio.com/depot/grove-gas-sensormq2-p-937.html?cPath=144_151) except you don’t mount a sensor on it, but mount the socket, which means you can then sell the sensor separately and people can plug in their sensor of interest. Anyone better with a soldering iron then me interested?
List of sensors
MQ-2
Sensitive for Methane, Butane, LPG, smoke.
This sensor is sensitive for flamable and combustible gasses.
The heater uses 5V.
MQ-3
Sensitive for Alcohol, Ethanol, smoke
The heater uses 5V
The Arduino blog about the “breathalyzer” using a MQ-3 : Arduino Breathalyzer: Calibrating the MQ-3 Alcohol Sensor | Arduino Blog
The MQ303A (also on this page) is like this sensor, but uses a lower heater voltage.
MQ-4
Sensitive for Methane, CNG Gas
The heater uses 5V.
MQ-5
Sensitive for Natural gas, LPG
The heater uses 5V.
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(MQ6)
The MQ306A (also on this page) is like this sensor, but uses a lower heater voltage.
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.
MQ-8
Sensitive for Hydrogen Gas
The heater uses 5V.
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.
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).
MQ135
For Air Quality
Sensitive for Benzene, Alcohol, smoke.
The heater uses 5V.
An example how to use it: AirQualityMQ135 \ Learning \ Wiring
MQ136
Sensitive for Hydrogen Sulfide gas.
The heater uses 5V.
MQ137
Sensitive for Ammonia.
The heater uses 5V.
MQ138
Sensitive for Benzene, Toluene, Alcohol, Acetone, Propane, Formaldehyde gas, Hydrogen gas.
The heater uses 5V.
MQ214
Sensitive for Methane, Natural gas.
The heater uses 6V.
MQ216
Sensitive for Natural gas, Coal gas.
MQ303A
Sensitive for Alcohol, Ethanol, smoke (just like the MQ-3)
The heater uses 0.9V
It detects the same gasses as the MQ-3, but uses a lower heater voltage.
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.
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.
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.
MG811
Sensitive for Carbon Dioxide (CO2).
The heater uses 6V.
AQ-104
For air quality
AQ-2
Sensitive for Flamable gasses, smoke
AQ-3
Sensitive for Alcohol, Benzine
AQ-7
Sensitive for Carbon Monoxide
how many ate you ordering? 
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