MQ-5 Gas Sensor (LPG/Natural Gas)
Target Gases: MQ-5 is another combustible gas sensor, advertised primarily for LPG (Liquefied Petroleum Gas) and Natural Gas detection. LPG is usually a mix of propane and butane. So you can think of MQ-5 as tuned for propane detection (and it does fine with methane/natural gas too). It has similar range to MQ-4: roughly 200–10000 ppm for propane/methane. It’s often recommended for gas leak alarms involving cooking gas (since many regions use propane/butane mixes in cylinders for cooking).
Interestingly, MQ-5’s datasheet mentions it can detect LPG, natural gas, and town gas (which could include hydrogen or carbon monoxide in some contexts). It’s somewhat a hybrid between MQ-4 and MQ-6 in coverage.
How it Works: Similar SnO₂ sensor with a heater, likely optimized for slightly heavier hydrocarbons (propane C₃H₈ has a higher boiling point than methane). The MQ-5 might run at a slightly different heater temperature or have a thicker coating to balance sensitivity between methane and propane. In practice, MQ-5 and MQ-6 are often considered interchangeable by hobbyists for LPG detection – but MQ-6 is typically said to specialize in LPG, while MQ-5 handles LPG and natural gas both. For your project, if you primarily worry about propane/butane leaks (heavier-than-air gas) near the floor, MQ-6 is a good pick; if you want a sensor a bit more responsive to methane as well, MQ-5 might be chosen.
Physical Specs: Same package as others. Four pins on the module (often labeled “MQ-5”). The module includes the usual comparator for digital output. No surprises here – size and pinout identical to MQ-2, etc.
Wiring: Exactly like previous sensors.
Sensor Placement & Ventilation: A known challenge with LPG detectors is avoiding false alarms from other fumes (like solvents at floor level). While MQ-5 is less sensitive to alcohol than MQ-2 or MQ-3, heavy vapors of gasoline or paint thinner might trigger it. If installed in a garage, for example, car exhaust (which contains unburnt hydrocarbons) might cause some response. Ideally, keep it in a location where it’s not exposed to frequent benign vapors.
Calibration: If calibrating for LPG (propane), note that 1% volume of propane is 10,000 ppm and is roughly 1/5 of the Lower Explosive Limit (LEL ~2.1% for propane). You might aim to alarm at say 2,000 ppm (0.2%) which is 10% of LEL – a very safe early warning. The analog reading corresponding to 2,000 ppm can be derived from the MQ-5’s curve. If no test gas, a hack: use a cigarette lighter (but don’t flick the flint or ignite). Just press the valve near the sensor – it will release a small amount of butane gas. See how the sensor responds. That can give you a reference to adjust the threshold (the digital out pot) so that D0 goes LOW when a small butane release is detected. Caution: do this in a ventilated area and only a small amount, as butane is flammable. Always err on the side of triggering too early (false alarm) rather than too late for safety.
Average Price: Same ballpark, $2–$5. It’s not as commonly included in basic kits as MQ-2 or MQ-3, but still easy to find. Often sold in sets. No special cost considerations beyond the generic MQ points.
MQ-6 Gas Sensor (LPG/Butane)
Target Gases: MQ-6 is very similar to MQ-5 in purpose, but specifically marketed for LPG (Butane, Propane) detection. It’s often the sensor recommended for household LPG leak alarms, where the gas in question is mostly propane/butane (like a gas cylinder or bottled gas). MQ-6 has high sensitivity to isobutane (a common component of LPG) and propane, and also can detect methane but with slightly lower sensitivity than MQ-5 or MQ-4. Think of MQ-6 as the “propane sensor”. Detection range: ~100–10,000 ppm for propane/isobutane.
How it Works: The MQ-6’s internal design is tuned so that it’s extra responsive to slightly larger gas molecules like butane. Winsen’s site notes MQ-6 is good for “LPG, isobutane, propane” and even mentions it can detect these gases anywhere from 0.1% to 1% concentration in air. Because LPG is heavier than air, like MQ-5, MQ-6 should be placed near floor level or wherever leaking gas would accumulate (low points).
Physical Specs: Same as others. MQ-6 modules are widely sold and look identical except for label. According to Winsen, MQ-6 sensor’s body is actually a tad different size: they list it as φ19×24.2 mm (with pins), whereas others are φ16. But in practice, any breakout you get will fit it; it might just have a slightly wider base. Not really noticeable unless comparing.
Wiring: Same 4-pin scheme. Provide 5V and ground. Use analog out to ADC, digital out to GPIO if needed. One note: Because MQ-6 is highly sensitive, sometimes the analog baseline might be a bit higher. Ensure your ADC reading covers the range.
False Alarms: MQ-6 is pretty specific but can still respond to other flammable vapors. For example, if you spill some high-proof alcohol or gasoline near it, it will likely respond (though not as strongly as it would to propane). In a kitchen, strong solvent cleaners could transiently trigger it. But day-to-day, it should be quiet unless real gas is present. To avoid false triggers, adjust the threshold (potentiometer for D0 or in software) so that normal background levels (which should be near zero) don’t trip the alarm, only significant increases do.
Calibration: For MQ-6, calibration is often referenced to 1000 ppm LPG in air (datasheets typically provide a point like Rs/R0 at 1000 ppm is X). If you can get a calibrated gas or even a butane lighter test: one method used by hobbyists is to release a small burst of butane ~20 cm below the sensor and observe the analog jump. If the analog goes from, say, 0.3V to 1.5V with that, you know that’s a relatively high concentration in the immediate vicinity (maybe a few thousand ppm for a moment). You might then set your binary threshold a bit below that peak to catch smaller leaks. Again, caution with open butane and no flames, etc.
Average Price: In line with MQ-5. These sensors (MQ-5 and MQ-6) sometimes cost a hair more than MQ-2 because they are slightly less mass-produced, but still in the few-dollar range. There are also pre-made LPG gas alarm modules (with relay and buzzer) that incorporate an MQ-6 – those can be bought and then hacked to integrate with an ESP (like reading the module’s alarm output). But building your own with an MQ-6 and ESPHome is straightforward and cheaper if you already have the ESP.
MQ-7 Gas Sensor (Carbon Monoxide)
Target Gases: MQ-7 is designed to detect Carbon Monoxide (CO). CO is a deadly, colorless gas produced by incomplete combustion (think vehicle exhaust, furnaces, generators). MQ-7 has a detection range roughly 20 to 2000 ppm CO. It’s quite sensitive to CO in that range – note that 100 ppm CO is dangerous over long exposure, 400 ppm can be immediately harmful, so 2000 ppm is very high (life-threatening) level. MQ-7 is used in DIY CO alarms, but caution: true life-safety CO alarms usually use an electrochemical sensor which is much more stable and accurate at low ppm. MQ-7 can detect CO but has some quirks.
Unique Operating Requirement: Unlike other MQ sensors, MQ-7 requires a cyclical heating to get accurate CO readings. Specifically, MQ-7’s datasheet suggests a heating cycle: heat at 5V for 60-90 seconds (to clear the sensor and adsorb oxygen), then heat at a lower voltage (~1.4V) for 90 seconds and read the sensor during that low-temp phase for CO concentration, The reason: MQ-7’s tin dioxide surface needs a cooler period to properly adsorb CO and get a stable reading. At continuous high temp, it’s less sensitive to CO.
In practice, many DIYers don’t implement this cycle, and they still see the sensor react to CO, but not as accurately. If using ESPHome, one could implement the heater cycling by controlling the heater power via a MOSFET or using PWM on a spare pin to simulate 1.4V (since directly providing 1.4V is tricky from a 5V source without something like a resistor network or a special driver).
For simplicity, if you don’t implement cycling, MQ-7 will still respond to CO but you might get false positives or a baseline drift. This is a big gotcha with MQ-7 that sets it apart.
Physical Specs: Looks like the other MQs. On some MQ-7 modules you might find a note about powering or an extra resistor for heater. But usually, modules treat it as a normal sensor and expect you to supply 5V (some might assume you won’t cycle and thus aren’t “officially correct” usage). Pin-out is same 4 pins.
False Positives: MQ-7 might also respond to hydrogen gas somewhat (like from charging batteries), and maybe to high concentrations of natural gas or ethanol, though much less than to CO. If you expose MQ-7 to cigarette smoke or cooking smoke, it will probably respond because smoke contains some CO and other reducing gases. So an MQ-7 alarm could sometimes trigger from heavy smoke (which is not a bad thing since where there’s smoke, CO usually follows). But if you want it only for CO, be aware of that crossover sensitivity.
Calibration and Accuracy: Honestly, MQ-7 is one of the harder ones to calibrate precisely due to the heater cycling requirement. Without cycling, you may find it saturates or doesn’t zero properly at low concentration. If you can smell car exhaust a bit and the MQ-7 reading jumps to like 20 ppm, that might be used qualitatively. But for reliably alerting at low CO levels, it’s safer to use a metal-oxide sensor specifically designed for CO (with built-in compensation) or an electrochemical one.
Average Price: Slightly more expensive than MQ-2, maybe $5 for a module, sometimes due to being less commonly bought. Still inexpensive compared to real CO sensors.
MQ-8 Gas Sensor (Hydrogen)
Target Gases: MQ-8 is a sensor primarily for Hydrogen (H₂) gas. Hydrogen is a combustible gas, and MQ-8 is tuned to detect hydrogen leaks. The detection range is typically quoted as a few hundred ppm up to 10000 ppm of H₂. It’s less commonly used by hobbyists, unless working with fuel cells, lead-acid batteries, or other hydrogen sources. But it exists for scenarios like battery charging rooms (where H₂ can be released) or hydrogen fuel systems. It also has some sensitivity to lighter hydrocarbons but is mainly for H₂.
How it Works: Similar MOS principle. Hydrogen, being the smallest molecule, diffuses very quickly and the sensor can respond rapidly. MQ-8’s heater likely is set for an optimal temperature for hydrogen’s reaction. If we reference typical info, MQ-8 can detect concentrations roughly 100–10000 ppm H₂. It may also respond to very high concentrations of CO (since H₂ and CO both reduce SnO₂), but it’s calibrated for H₂.
Hydrogen is lighter than air (it rises fast), so detectors for H₂ leaks are placed high. For instance, near the ceiling of a battery charging area or around the top of a hydrogen storage tank.
Physical Specs: Standard MQ package. On the module labeled MQ-8. Same 4-pin wiring. Power at 5V for heater.
Wiring: No differences. Use 5V and analog out to ESP.
Calibration: Hard to get known H₂ concentration without specialized equipment. One trick: charging a battery – if you charge a lead-acid battery intensely, it will produce some hydrogen and MQ-8 might pick that up if placed just above the vents. Not a quantifiable amount easily though. Alternatively, some people use a balloon filled with hydrogen (if accessible, like from a chemistry lab or by electrolyzing water) and release a small amount near sensor. Extreme caution: hydrogen is highly flammable/explosive! It’s odorless and invisible, so be very careful if deliberately testing – ensure no ignition sources.
False Positives: MQ-8 might respond a bit to Methane or lighter gases but not strongly – MQ-8’s sensitivity to hydrogen is much higher than to LPG or CO (unlike MQ-2, which is broad). It might have slight cross-sensitivity to CO at high concentration. But in general, if MQ-8 triggers, likely H₂ or possibly a very high concentration of some other flammable gas is present. It’s one of the more specific MQ sensors.
Average Price: A bit less common, might be $5 for module. If you buy the whole MQ set (MQ-2 to MQ-9, MQ-135 etc.), MQ-8 is sometimes not included because not everyone needs hydrogen detection. But it can be found separately.
One note: If you can’t find MQ-8, MQ-2 also detects hydrogen fairly well, (MQ-2’s list includes H₂). So, in pinch, MQ-2 can serve as a rough hydrogen detector too, though MQ-8 is more optimized for it (less interference from other gases).
MQ-9 Gas Sensor (Carbon Monoxide & Combustibles)
Target Gases: MQ-9 is a dual sensitivity sensor for Carbon Monoxide (CO) and combustible gases (like LPG, methane). It’s often described as detecting CO and flammable gas. In essence, MQ-9 is kind of a hybrid of MQ-7 (CO sensor) and an LPG sensor. What sets MQ-9 apart is that its tin dioxide surface might be doped or used with temperature cycling to differentiate CO from other gases.
Interestingly, the MQ-9 datasheet also suggests a heating cycle (like MQ-7, but different): It sometimes is used with two heater voltages to detect CO at a certain temperature and CH₄ at another. However, many implementations just run it continuously at one heating and treat it as a broad sensor.
Detection ranges: roughly 10–500 ppm for CO and 100–10000 ppm for combustible gas (propane, methane). It’s often used in indoor air quality monitors that want to catch both CO (a deadly poison) and gas leaks (fire hazard). For example, an MQ-9 could be used in a home unit to detect either a CO buildup (from a furnace malfunction) or a gas leak from the stove.
How it Works: MQ-9’s internal heater can be cycled between high and low to distinguish CO vs CH₄. At a lower heater temp, CO is more readily oxidized on SnO₂, whereas methane needs a higher temp. So by reading sensor resistance at two different heater levels, one can roughly tell if the gas is CO or a hydrocarbon. However, doing that distinction is complex in code. If you just use MQ-9 with constant heating (often at 5V), it will respond to both CO and flammables, but you won’t know which it is – just that “something bad (CO or gas) is present”.
For IoT alarm usage, that might be fine: either case, you’d want ventilation or alarm. But if you need to know exactly which gas, MQ-9 alone can’t tell you unless you implement the heater switching and some math.
Physical Specs: Standard form. The module pinout is the same. Possibly the module’s PCB might label some difference if expecting cycling, but likely not – they treat it like normal.
Wiring: Standard 4 pins, 5V power.
Calibration: If focusing on CO – calibrate in clean air (0 ppm) and maybe test with a source like a lit incense stick or piece of paper smoldering near sensor (that produces CO ~ tens of ppm). Or vehicle exhaust (careful!). If focusing on combustible, calibrate with a lighter gas as described earlier.
Average Price: Similar to MQ-7, maybe $5. It’s not as widely in beginner kits, but available. Sometimes MQ-9 is used in some commercial detectors labeled as “CO + Gas Alarm 2-in-1” (the ones you find on eBay often use MQ-9 or a pair of sensors).
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