What it does: The device measures the alcohol concentration in a person's exhaled breath using an MQ-3 gas sensor and displays the result on an LCD/OLED screen, signaling the status via LEDs and an optional buzzer.

Purpose: To provide an accessible, low-cost sobriety test tool built entirely from scratch using ATmega328P peripherals.

Starting idea: The project originated from the practical need for a simple, register-level embedded systems project that integrates multiple lab concepts into a real-world product.

Why it is useful: It demonstrates a complete embedded pipeline — analog sensing, interrupt-driven input, I²C communication, and GPIO output — while producing a functional, demonstrable device.

Module descriptions:

• MQ-3 sensor — outputs an analog voltage proportional to alcohol concentration → read via ADC (PA0)
• Temperature sensor (optional) — provides ambient temperature for MQ-3 calibration → read via ADC (PA1)
• Buttons x2 — trigger test start and calibration → connected to INT1 (PD3) and INT2 (PB2) with software debouncing via systicks
• LEDs (green / yellow / red) — indicate safe / warning / danger status → controlled via GPIO (PC0–PC2)
• LCD/OLED display — shows measured BAC value and status text → driven via I²C (TWI)
• Buzzer (optional) — auditory alert on high readings → controlled via PWM or GPIO
• ATmega324P — central MCU coordinating all peripherals

Bill of materials:

Pin mapping:

• MQ-3 analog output → PA0 (ADC channel 0)
• Temperature sensor → PA1 (ADC channel 1)
• Button 1 → PD3 (INT1)
• Button 2 → PB2 (INT2)
• LED green → PC0, LED yellow → PC1, LED red → PC2
• SDA → PC1 (TWI), SCL → PC0 (TWI)
• Buzzer → OC0B (PD5) or PD7

Diagram

Development environment: AVR-GCC + avrdude / Microchip Studio

Third-party libraries and sources:

• No external libraries — all peripheral drivers written from scratch using registers
• Optional: lightweight I²C LCD driver adapted from course lab materials

Planned algorithms and data structures:

• ADC sampling — single-shot mode on PA0/PA1; average of 16 samples per reading for noise reduction
• MQ-3 calibration — baseline resistance R0 measured in clean air on startup; BAC estimation uses the Rs/R0 ratio from the MQ-3 datasheet characteristic curve
• Temperature compensation — Rs/R0 ratio adjusted using ambient temperature reading from PA1
• Interrupt handling (INT1/INT2) — button press sets a volatile flag inside the ISR; debounce enforced via systicks counter (Timer1 CTC, 1 ms tick); flag processed in main loop
• Display logic — BAC classified into ranges (< 0.2‰ → green, 0.2–0.5‰ → yellow, > 0.5‰ → red); value and status string sent over I²C to display
• LED/Buzzer output — GPIO write for LEDs; optional PWM tone on OC0B for buzzer alert

Hypothesis:

• We believe that averaging 16 ADC samples and applying temperature compensation will improve BAC measurement accuracy by at least 20% compared to a single raw reading, because the MQ-3 is sensitive to temperature variation and ADC noise.

Performance metrics and targets:

Code profiling plan:

• Measure ISR latency with oscilloscope (button press → ISR entry)
• Validate I²C timing with logic analyzer
• Test BAC readings against known ethanol concentrations (diluted solutions near sensor)
• Verify debounce behavior under rapid repeated button presses

To be completed after implementation and testing.