CO2-400 Carbon Dioxide Controller Simply set the desired CO2 level and the CO2-400 does the rest. The Sensor Unit has a display and 3 buttons (SET, +, and -). Normally it displays actual room CO2 PPM (Parts Per Million). Using the buttons, you can display and change the set point (also in PPM). The Sensor Unit is connected to the Control Unit by an 8 foot cable. The Control Unit plugs into any 120V outlet. If you wish to defeat CO2 dispensing when your lamp is off, plug the Control Unit into your lamp timer or a timer running the same schedule. The CO2 dispensing equipment (tank solenoid valve or generator) plugs into the 120V outlet on the front of the Control Unit.