MQ-3 Breathalyser
Wave a glass of beer past an MQ-3 ethanol sensor and watch the analog reading swing as the heater catches it.
Difficulty: Breadboard. Estimated build time: about 25 minutes. Estimated parts cost: about US$4.00. 5-line bill of materials. Compare supplier offers when available. A demonstration alcohol detector built around the WINSEN MQ-3 tin-oxide gas sensor — a hot ceramic substrate inside a metal-mesh cage that responds to ethanol vapour by changing its resistance.
Wire heater and signal
MQ-3 has six pins, but only four go anywhere: V_H (heater +) to 5 V, GND_H to GND, plus one of the sensor pins to 5 V and the other to a load resistor of 200 Ω–1 kΩ down to GND. The midpoint of the sensor and the load resistor is your analog signal — wire it to A0. The remaining two pins are duplicates.
Wait for the heater to warm up
When freshly powered, an MQ-3 needs about 5 minutes (24 hours for the very first burn-in!) for the heating element to stabilise. Until then the analog reading drifts upward and downward. The datasheet recommends a 48-hour preheat for accurate measurements, but a 5-minute warmup is fine for a demo.
Read the baseline and compare
Write a sketch that prints analogRead(A0) every 500 ms. Note the baseline (usually 100–300 in clean air). Now hold an open beer or a damp alcohol wipe under the sensor cage. The reading jumps within seconds — often to 500–800. Remove the source and watch it relax back over 30–60 seconds.
Set a threshold alarm
Use the comparator-like idiom: if analogRead(A0) > 600 light an LED and chirp a piezo. You’ve built an alcohol presence detector. Don’t use it for legal or medical decisions — the MQ-3’s cross-sensitivity to humidity and cooking gas is enormous — but as a teaching toy it’s a great way to feel how gas sensors actually work.