C.A.P. PPM-3 CO2 Monitor/Controller 

The PPM-3 from C.A.P. is the most affordable and simple CO2 controller available. 

Its' proven technology is very accurate at measuring levels of Carbon Dioxide in your growing area from 0 to 5000 ppm (Parts-Per-Million).

Using the PPM-3 is quite simple. Just plug your CO2 device (CO2 valve or generator) into the "piggyback" cord and it will automatically maintain the CO2 level that you specify.

The C.A.P. PPM-3 works with any make of CO2 valve or generator that is 120 volt.

More info from the Manufacturer (PDF format - Lots of detail.)