Alcohol Detection with Engine Lock using Arduino

Drunk driving remains one of the deadliest preventable causes of road accidents, and the most reliable intervention point is before the vehicle ever moves. Interlock devices exist commercially, but they are black boxes — a student-built prototype makes the sensing and decision chain transparent and examinable. This project builds that chain: an MQ-3 semiconductor gas sensor, sensitive to alcohol vapour, feeds an analog signal to an Arduino Uno; the firmware compares it against a calibrated threshold with hysteresis, and on detection it de-energizes a relay in series with the ignition — demonstrated with a DC motor standing in for the engine — while a buzzer and LCD announce the lock. The demo triggers the sensor with an alcohol sample, the motor stops, and the system stays locked until a clean reading and manual reset. Students learn how real safety interlocks are reasoned about: sensor warm-up, threshold calibration, fail-safe relay wiring and the honest limits of a semiconductor gas sensor.

1. On power-up the firmware runs the MQ-3 heater warm-up timer; the LCD shows WARMING and the system stays disarmed.
2. Once warm, the Arduino samples the MQ-3 analog output continuously and smooths it with a moving average.
3. Each averaged reading is compared against the calibrated threshold; crossing it (with hysteresis) is a detection event.
4. On detection the firmware de-energizes the ignition relay — the demo motor stops — sounds the buzzer and updates the LCD to LOCKED.
5. The system remains locked while readings stay above the release level; only a clean reading plus manual reset re-arms the ignition.
6. Below threshold the relay stays energized, the motor runs, and the LCD shows READY with the live level bar.

Tech stack:

• Arduino Uno (ATmega328P)
• MQ-3 alcohol gas sensor module (analog + digital outputs)
• Relay module (ignition-cut)
• DC motor as engine stand-in
• 16x2 LCD (status display)
• Buzzer (audible alert)
• Arduino IDE (C/C++ firmware)
• 5 V / 12 V supplies per rail

• [MQ-3 alcohol sensing] The semiconductor gas sensor's analog output rises with alcohol vapour concentration, read by the Arduino's ADC for the lock decision.
• [Relay engine lock] A relay in the ignition path cuts power to the demo motor on detection — the direct physical analogue of an ignition interlock.
• [Calibrated threshold with hysteresis] The trip point is set through a documented calibration procedure, with separate engage/release levels so the lock does not chatter at the boundary.
• [Buzzer and LCD alerts] Detection triggers an audible buzzer and an LCD status screen showing LOCKED/READY state — the driver gets unambiguous feedback.
• [Sensor warm-up handling] The firmware enforces the MQ-3's heater warm-up period before arming, since readings are meaningless on a cold sensor.
• [Manual reset procedure] After a lock event, the system requires a clean reading plus a deliberate reset action — no automatic restart while alcohol persists.
• [Live level display] The LCD shows a continuous relative alcohol-level bar, so the examiner can watch the reading climb toward the threshold during the demo.

• Working interlock prototype (Arduino, MQ-3 sensor, relay, demo motor, LCD, buzzer)
• Complete firmware source code (warm-up timer, averaging, threshold logic, lock state machine)
• Sensor calibration procedure (threshold setting with alcohol samples, documented step-by-step)
• Circuit and wiring documentation (including fail-safe relay wiring notes)
• Project report PDF (gas-sensor background, interlock logic, calibration method, safety analysis)
• PPT presentation for final review
• Viva Q&A preparation document (semiconductor sensors, ADC, hysteresis, relay driving)
• Setup and demonstration guide