I just finished playing around with this sensor and got this:
1) it actually works!!!
2) to get graduated readings use test point 1 (TP1)
3) Alarm out or (E3 ALM) gives you the set point value for a 3.3v HIGH trigger (I cant think of an mcu type application where you would use a trigger considering graduated readings are available on TP1)
this is my configuration...
E1 > set at 5v HIGH from any available pin
E2 > NA
E3 > 6-12v (don't use a 9v battery as it will drain very quickly - the mg811 and sensors of this type are gas-guzzler's)
E4 > ground
TP1 > to analog in (graduated readings - using arduino's analog map can come in handy)
Now my big question is: how to calculate PPM?