The breath alcohol tester is an electronic device used to test the blood alcohol content (BAC) in a person`s blood stream. Several other methods of alcohol detection exist, such as chemical reactions, optical- interference, infrared absorption, and heat generation from a catalytic reaction on a hot platinum wire. These methods, however, have the disadvantages of being difficult to handle and maintain. They are also very expensive and have a very limited lifetime.
This device utilizes a semiconductor sensor to analyze a person`s exhaled breath. This method is both simple and affordable. The semiconductor device offers the an added advantage in that it is robust; it is highly immune to shock and vibration, and therefore more reliable.
The Sensor
The sensor consists of a semiconductor material mounted on a ceramic tube; it is protected by a double layer of 100-mesh-per-inch stainless steel screening. When the alcohol tester is turned on, the sensor is heated to normal operating temperature. During this process, oxygen is absorbed into the semiconductor surface. This causes a depletion region to form betw1een the surface and the body of the sensor. In turn, this increases the resistance of the sensor.
When an intoxicated person blows on the sensor, the alcohol is absorbed into the semiconductor surface and reacts with the oxygen that is already there. This decreases the sensor`s overall resistance in proportion to the alcohol concentration.
The Circuit
When the alcohol tester is turned on, the heater coil in the sensor is energized with 5 volts from a 7805 voltage regulator (IC5). The circuit cycles through a self-testing and warm-up period. The circuit resets itself to 0-alcohol reading and sets off the ready light.
When an intoxicated person blows into the device and the sensor detects alcohol, its resistance decreases; this changes the input voltage to the detector circuit. The detector circuit is made up of a quad op-amp. The detector circuit is calibrated by 2 resistors (R3 and R4), and the inputs to each section are controlled by a voltage-dividers (R21-R23). As each section is activated, the outputs go low and the sample-and-hold circuitry will latch onto the highest input value and trigger the appropriate LED. Different colored LEDs represent different alcohol levels.
If the detected alcohol level is above the legal limit (0.1%), another op-amp will trigger a buzzer and the LED corresponding to an "over the limit" level of alcohol. This indicates that the person is too drunk to drive.