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

source
http://playground.arduino.cc/Main/MQGasSensors