I built this carbon dioxide monitor at my first job (Picture taken with permission). This was my very first PCB design. It makes use of a PIC16F877A microcontroller. The sensor is interfaced to the microcontroller via the I²C interface. It also controls a 16×2 LCD display. 

The microcontroller triggers the buzzer when it crosses the threshold of 800 ppm. The unit had push buttons to adjust the trigger threshold. I also wrote the firmware for this unit.