To sense CO2 we used an MG811 Figaro Sensor and an amplifying circuit to read serial values into the Arduino. The sensor has 6 pins, 2 As, 2Bs, and 2Hs. The H pins are in the middle of the sensor and there is a heating coil that is inside of the sensor and runs between the two pins. One side of the H is connected to 6v the other side to Ground. The 2 A pins are internally connected as are the two B pins. This is an image of the circuit done in the style of "Open Solar Circuits".