We will make use of the pySerial library (https://pyserial.readthedocs.io/en/latest/shortintro.html#opening-serial-ports) for interfacing the carbon dioxide sensor:

1. As per the sensor's documentation, the sensor output can be read by initiating the serial port at a baud rate of 9600, no parity, 8 bits, and 1 stop bit. The GPIO serial port is ttyAMA0. The first step in interfacing with the sensor is initiating serial port communication:

       import serial 
       ser = serial.Serial("/dev/ttyAMA0")

2. As per the sensor documentation (http://co2meters.com/Documentation/Other/SenseAirCommGuide.zip), the sensor responds to the following command for the carbon dioxide concentration:

3. The command can be transmitted to the sensor as follows:

       ser.write(bytearray([0xFE, 0x44, 0x00, 0x08, 0x02, 0x9F, 0x25]))

4. The sensor responds with a 7-byte response, which can be read as follows:

       resp = ser.read(7)

5. The sensor's response is in the following format:

6. According to the datasheet, the sensor data size is 2 bytes. Each byte can be used to store a value of 0 and 255. Two bytes can be used to store values up to 65,535 (255 * 255). The carbon dioxide concentration could be calculated from the message as follows:

       high = resp[3] 
       low = resp[4] 
       co2 = (high*256) + low

7. Put it all together:

       import serial 
       import time 
       import array 
       ser = serial.Serial("/dev/ttyAMA0") 
       print("Serial Connected!") 
       ser.flushInput() 
       time.sleep(1) 

       while True: 
           ser.write(bytearray([0xFE, 0x44, 0x00, 0x08,
           0x02, 0x9F, 0x25])) 
           # wait for sensor to respond 
           time.sleep(.01) 
           resp = ser.read(7) 
           high = resp[3] 
           low = resp[4] 
           co2 = (high*256) + low 
           print() 
           print() 
           print("Co2 = " + str(co2)) 
           time.sleep(1)

8. Save the code to a file and try executing it.