SparkFun has a fairly good assortment of gas sensors. Only about $6 with the breakout board! They do need a 48 hour "break-in" time before you use them but once you do that they do seem easy to use. You'd only need a couple resistors and the breakout board (although even that is optional) and they seem to have good response time. A couple downsides though. You can't detect gas if it's on the other side of the room. You'd need a way to make sure you could reliably detect the gas, every time. Also, one person complained that once the sensor does detect gas, it takes a while for the sensor values to drop back down. Could just be one incident though.
Also notice a couple other things. Current draw is very high, upwards of 150 mA as far as I could see. Also, they run hot as they sense gas with a heated element. Just some things to think of. Also, as I've said before, I don't work for SparkFun, just a happy customer. [
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