Breathalyzer is a project mainly written in C, it's free.
This breathalyzer uses a PIC16F876A attached to an analog MQ-3 Gas Sensor and a 4-Digit 7-Segment Display driven by SPI.
It is compiled in MPLab IDE v8.60 using the HI-TECH C Compiler V9.80.
When the unit is turned on, the display continually updates the raw A/D value of the sensor (every 0.1s), more work needs to be done to convert that value into a reliable BAC.