Use this simple circuit to detect the presence of Alcohol. This kind circuit is used by Police officials to prevent drunk driving. It gives a beep when it detects the Alcohol vapour. MQ3 Alcohol sensor is now readily available and you can make the device very easily. When the Sensor detects alcohol vapour, it gives output voltage depending on the concentration of alcohol molecules in the air.
MQ3 sensor has six pins and pins 1, 2, and 3 can be shorted and connected to positive rail. Pin 5 goes to ground. Pins 4 and 6 are shorted and used as output. Resistor R1 and Pot VR act as load for the sensor and adjusts the sensitivity. Output voltage triggers T1 which drives the Buzzer and LED. MQ3 sensor works off 5 volts DC so regulator IC 7805 is used to give 5 volts.
Assemble the circuit on a Perf board and use 9 volt PP3 battery to power the circuit. It can be tested using some Spirit. Take spirit / alcohol on some cotton and place the cotton close to sensor. Adjust VR till buzzer just to turn on. The Sensor takes few minutes to stabilize so calibrate patiently.