I've searched a lot on this, you want to get one of these, don't waste
your time with $5 or $10 sensors, they're too sensitive to humidity
and other gas changes:
http://www.co2meter.com/products/k-30-co2-sensor-module
THen you just need a 2-way valve and a gas regulator and CO2 cylinder,
all of that will be available from any soda/beer distributor. The
valves they have may not be microcontroller power level compatible
(i.e. the solenoid is 12V or 24V or something) so you'll likely need a
MOSFET or relay so the micro can actuate based on sensor readings.
Luckily for non-reactive gasses/liquids these valves are pretty cheap.
http://www.ebay.com/itm/New-Electric-Solenoid-1-4-Brass-Valve-Air-Water-Gas-Etc-12v-DC-Volt-NPT-/111135813629?pt=LH_DefaultDomain_0&hash=item19e035effd
http://www.ebay.com/itm/New-Electric-Solenoid-1-4-Plastic-Valve-Air-Water-Gas-12V-DC-Volt-NPT-NC-/121154250036?pt=LH_DefaultDomain_0&hash=item1c355b2534
http://www.ebay.com/itm/CO2-magnetic-Valve-Solenoid-Valve-Night-Time-Cut-Off-For-Aquarium-220V-/310828115616?pt=LH_DefaultDomain_0&hash=item485ecca6a0
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