http://www.420magazine.com/forums/do-yourself/115134-diy-co2-generator.html this has good reviews, and yes they work, approx 200 ppm ea with this set up.
some more helpful info on determining amount of sugar and yeast.
Just some helpful info I came across on the topic.
A few things about this, I've just been doing a lot of research on this and there is some miss(ing) information. In the air outdoors, CO2 comprises about .038% of the total gas (380ppm). If you have a closet growspace with the door open and a fan running, this allows for gas to equalize so you will have ~380ppm. 380ppm is on the low end of the continuum of most plants ability to use it as fuel for photosynthesis (which stops completely at 200ppm), plants getting sufficient light grow much heavier and faster if supplied with 1500ppm (.15%, more than 4 times atmospheric levels). CO2 is only used in the presence of light and the amount of light available directly effects how much CO2 your plant can photosynthesize: at 4600fc a plant can use up to 600ppm CO2, if you increase the light to 5500fc it can use between 1200-1300ppm of CO2, if you go over 7500fc they can use up to 1500ppm.
To do calculations of how much CO2 will be generated:
1lb of CO2 (.45kg) = 8.7 cubic feet (.246 cubic m)
1cubic foot of gas increases the percentage of gas in a 1000 cubic foot room (10x10x10) by 1,000ppm
-during fermentation about half of the sugars weight is CO2, so if you want 1lb CO2, use 2lb sugar
-To determine how much weight you need, first figure out how much light you have and find the appropriate ppm CO2. Then find cubic area of growspace. Use these: 600ppm is 1:666(if you want 600ppm, multiply cubic area by .0006, so a 1000 cubic food room requires .6 cubic feet, or .0168 cubic meters or CO2) -- for 1000ppm it's 1:1000, so multiply area by .001 -- 1200 ppm is 1:1,000,000, multiply by .0012.
-When you make your hose, puncture it up and down the length of the hose (like for a drip irrigation system) and place it above your plants (CO2 sinks).
-if you want to use CO2 outdoors (which you should!!) a large compost pile in the middle of the garden works, or in a smaller garden putting a small compost pile under every plant. This is a super easy + effective way to enrich CO2 outdoors.
-CO2 generators are VERY effective under certain conditions and at least somewhat effective in nearly all conditions.
credit to spitinthacool