Alcohol-Triggered Fuel Cut
Introduction
A safety system that prevents the vehicle’s fuel pump from powering on if alcohol is detected in the driver’s breath. The project uses an MQ-3 gas sensor and an Arduino Uno to monitor alcohol concentration in the surrounding air. If the measured level exceeds a set threshold, a relay module mounted in series with the fuse cuts off the current to the fuel pump.
Overview
MQ-3 alcohol sensor measures the level of alcohol in the air.
Arduino Uno reads the sensor value and compares it to a threshold.
If the level is too high, Arduino deactivates the relay.
The relay is mounted in series with the fuel pump fuse and cuts power to the pump.
Hardware Design
List of Components and Technical Justification
| No. | Component | Rated Voltage | Typical Current | Connection to Arduino | Interface | Technical Notes and Justification |
|---|---|---|---|---|---|---|
| 1 | MQ-3 alcohol sensor | 5V | ~150 mA | AO → A1 | Analog Input | Powered from 5V. Output compatible with Arduino ADC (0–5V). |
| 2 | Arduino Uno | 5V | ~50–100 mA | — | — | Powered via USB or 5V from LM2596. |
| 3 | Active Buzzer | 5V | ~30 mA | D4 | GPIO | Controlled via `digitalWrite()`. Low power usage. |
| 4 | 5V Relay Module (with internal transistor) | 5V | ~70–100 mA | D7 | GPIO | Triggered with LOW. Controls fuel pump power. |
| 5 | LM2596 Step-Down Converter | IN: 12V, OUT: 5V | up to 2A | Not logically connected | Power Supply | Provides stable 5V for Arduino and peripherals. |
| 6 | Red LED | 5V | ~10–15 mA | D3 | GPIO | Turns ON when alcohol is detected. Needs resistor (~220Ω). |
| 7 | Green LED | 5V | ~10–15 mA | D6 | GPIO | Turns ON when NO alcohol is detected. Needs resistor (~220Ω). |
| 8 | Pushbutton | 5V (with pull-up) | ~0 mA (standby) | D5 | GPIO + Interrupt | Used to mute the buzzer. |
Compatibility and Additional Notes
- All components operate at 5V → no voltage level shifting is required.
- The relay draws too much current to be controlled directly by Arduino → a TIP41C NPN transistor is used as a switch (turns
- out it is already incorporated into the relay).
- Arduino, relay, and sensor share a common GND.
- The entire system is powered from the 12V car outlet, stepped down to 5V using an LM2596 voltage regulator module.
Software Design
Software documentation + source code: https://github.com/IleaDorin/pm_alcohol-fuel-cut
Results
Conclusions
One thing I noticed during the build is that placing the Arduino closer to the sensor and LEDs would’ve helped avoid long wires running across the car. It would’ve made the setup cleaner and reduced the risk of loose connections.
Resources
