DIY co2 reator and yeast bottle

This is one I used early on when I first started with DIY. I found it easy to understand and it worked well for me for quite a while til I switched to pressurized.

http://www.animalnetwork.com/fish2/aqfm/1997/oct/aquatic/default.asp

One thing to keep in mind. You'll probably start with one 2-liter bottle but will eventually realize that you'll need 2 bottles to get the kind of ppm you'll need for your plants in your 40 gal. I was using two bottles and was able to push about 24ppm into the tank(also a 40 gal.).
Len

This is a good site.

http://www.qsl.net/w2wdx/aquaria/diyco2.html

I just made the reactor from it yesterday and so far, everything is fine.

That's the best one I've found.