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 NXP Semiconductors


  • Breath Alert Alcohol Tester

  • Created: Feb 18, 2014

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Summary

The breath alcohol tester is an electronic device used to test the blood alcohol content (BAC) in a person’s blood stream. 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.


When power is applied to the circuit, the heater coil in the sensor is energized by the 5-V output of U5, a 7805 voltage regulator. Breathing into the sensor with alcohol on your breath will lower the sensor’s resistance; consequently, the input voltage to the detector circuit will change. The detector circuit consists of quad op amp, U2 and its associated circuitry. All sections of the detector circuit circuit are calibrated via R3 and R4, and the inputs to each section are controlled by the voltage-divider network R21 through R23. As each section is triggered, the outputs decrease, and sample-and-hold circuits, U3 and U4, will latch onto the highest input value and drive the appropriate LED. The different colored LEDs represent alcohol levels from 0 to 0.16%.


If the level of alcohol is above the legal limit, or 0.16%, part of another quad op amp, U1D, will turn on both the optional buzzer and LED5. That is an indication of a high level of alcohol present in your blood, and you definitely should not drive.


After a test is taken, the sensor takes a few seconds to ready itself for another test. When the sensor is ready, its input to U1B, adjusted via R2 to a threshold of 0.5 V, causes LED4 (ready) to light. That, in turn, causes U1C to reset the rest of the circuitry. The last section of U1 is biased via R15 and R16, and used to indicate a low-battery condition - when the battery voltage drops below 6.8 V - which could result in an inaccurate breath test.

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