Yi-Meng,  you asked about a cheap CO2 controller, and I
wondered if you wanted it for your DIY yeast CO2. I don't
think that is a good idea. You would be stopping the flow
out of the bottle and thereby building up the pressure. I
don't think it would cause too much bodily hard, but it woul
d make one heck of a mess. When you try the correct recipe
for yeast co2 you should find the swing in PH settles down.
Then you can take steps to reduce the amount of CO2, ie: add
an airstone, remove the airstone from the co2(if you have
one). I make beer, I've seen five gallon bucket with the
tops warped 4 inches due to a clogged air lock, I would not
have liked to see that pressure in a 2 liter bottle.  Mike