Without seeing one in front of me I can't check.
1. Determine which are the 2 wires for the heater coil.
This is cunning marked as "4" and "4" and then "H" and "H" - typical unambiguous drawing from a certain neck of the woods.
You can cofirm with a resistance check using a multimeter -
what should the approx reistance read?
One of the heater pins goes to your 5V and the other to Ground (0V).
This device WILL be greedy on power, I would advice making this 'switchable' via relay or semiconductor switching or even a mechancical switch.
2. You should have 2 pins left over,
According to MY brief reading, connect either one of them to 5V and the other one is the OUTPUT to RL and then OUT to your PICAXE.
Look in the PICAXE MANUAL 1 to see the input pins of PICAXE which can be used as an ADC.
You DO NOT say which PICAXE you are using.
Let's guess and say it's an 18X.
LOOK in Manual 1 at the PINOUTS for the 18X.
See Pin 17 of the 18X.
"Input0 / ADC0 / Infrain "
This summaraises what this pin can do.
So then as Westy says above you can use READADC10 to give a good resolution value for the voltage at that pin.
Read MANUAL 2 (The BASIC language and other Info).
Once you have a value you can then compare it, within code.
Symbol MyGas = w1
Symbol Smelly = 200 ' Just a random value
MyStart:
ReadADC10 0, MyGas
If MGas > Smelly then
GoSub SwitchOnFan
Endif
Goto MyStart
Once you get used to programming you'll be able to see how to slicken it up, and how to do option, even how to calibrate it and even how to get rid of 'GOTO' completely.
Have fun. Be patient. Get some breadboard and a decent quality 5V supply.
Then we'll get onto the power supply AFTER you have got this going.
NOTE: To get good reliable ADC results, place a ceramic 100nF accross the V+/0V pins of your chosen PICAXE as close as physically possible to the actual chip pins (or 'legs' as they are quaintly called). Maybe also a few microfarads of electrolytic too in aparallel with the ceramic, say 10uF/10V for example.
This
'decoupling' at the PICAXE power supply pins can not be stressed enough if you want good ADC results.
There may be some more what-ifs and options to improve slickenitupisationability, maybe with pictures of breadboards (for which this is an ideal candidate btw) so I suspect this will run and run and confuse - well, that's my free 20 minutes worth, so good luck