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

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