🔷 Overview of MQ Series Gas Sensors

The MQ series gas sensors are metal-oxide semiconductor (MOS) sensors that detect gases by measuring changes in resistance of a tin dioxide (SnO₂) sensing element when exposed to gases.

They are widely used in:

Gas leakage detection
Air quality monitoring
Safety systems
IoT and Arduino projects
⚙️ General Electrical Specifications
Operating Voltage (Module): 5V DC (typical)
Heater Voltage (VH): 5V ±0.1V
Power Consumption:
Heater: ≤ 800–900 mW
Current: ~150 mA typical
Load Resistance (RL): Adjustable (commonly 2kΩ–47kΩ)
🌡️ Operating Conditions
Operating Temperature: −10°C to +50°C
Humidity Range: ~5% to 95% RH (non-condensing, typical)
Standard Test Conditions:
20°C ±2°C
65% ±5% RH
🔥 Sensor Characteristics
Sensing Material: Tin dioxide (SnO₂)
Heater Type: Internal heating coil (nichrome wire)
Detection Principle: Resistance changes with gas concentration
Resistance Range (Rs): ~2kΩ – 20kΩ (varies by gas and model)
⏱️ Timing Characteristics
Initial Preheat Time:
24–48 hours (recommended for calibration)
Warm-up Time (each use):
≥ 20 seconds to a few minutes
Response Time: < 10 seconds
Recovery Time: Fast (seconds to minutes)
📊 Detection Characteristics
Detection Range: Typically 10 ppm – 1000 ppm (depends on sensor type)
Sensitivity: High (varies per gas type)
Selectivity: Low (responds to multiple gases, not highly specific)
🔌 Module Interface Specifications

Most MQ modules (e.g., MQ-2, MQ-135) include:

Output Types:
Analog Output (A0): proportional voltage
Digital Output (D0): threshold-based (TTL)
Comparator IC: LM393 (commonly used)
Sensitivity Adjustment: Onboard potentiometer
Indicator LEDs: Power + digital trigger
Pin Configuration (Module):
VCC
GND
A0 (Analog)
D0 (Digital)
📏 Physical Specifications
Sensor (metal can):
Diameter: ~16 mm
Module Size:
~32 mm × 20 mm (typical PCB size)
Mounting: 4 holes on PCB
Weight: ~5–10 g
🧪 Gas Detection Types (by Model)

Each MQ sensor targets different gases:

Sensor Detects
MQ-2 LPG, methane, smoke, hydrogen
MQ-3 Alcohol, ethanol
MQ-4 Methane (CH₄)
MQ-5 LPG, natural gas
MQ-6 LPG, butane
MQ-7 Carbon monoxide (CO)
MQ-8 Hydrogen (H₂)
MQ-9 CO + flammable gases
MQ-135 Air quality (CO₂, NH₃, benzene, smoke)

⚡ Key Features
High sensitivity to target gases
Fast response and recovery
Long operational life
Low cost and easy to use
Compatible with Arduino, ESP32, Raspberry Pi
⚠️ Limitations
Not highly selective (cannot distinguish exact gas)
Requires calibration for accurate readings
High power consumption (heater)
Sensitive to temperature and humidity changes