This book is about designing a microcontroller based alcohol detector device, also known as a breathalyzer device. The designed device basically consists of an alcohol sensor chip, a microcontroller development system, a text based LCD display, 3 coloured LEDs, and a speech processor module. The user blows onto the alcohol sensor through a small plastic tube. The alcohol sensor generates a voltage proportional to the measured alcohol level. This analog voltage is fed to the A/D converter input of a PIC16F887 microcontroller. The microcontroller processes the signal and calculates the actual alcohol level of the subject. The result is displayed in mg/L on the text LCD.