1) What the MQ-7 senses (and what it does not)

The MQ-7 is designed to detect carbon monoxide (CO), a colorless, odorless, and highly toxic gas produced by incomplete combustion.

MQ-7 is sensitive to

• Carbon monoxide (CO)

MQ-7 has limited response to

• Hydrogen (cross-sensitivity)
• Some hydrocarbons at high concentration

MQ-7 is NOT suitable for

• LPG, propane, or methane detection (use MQ-2 or MQ-4)
• Certified CO safety monitoring
• Accurate ppm measurement without calibration gases

2) How the MQ-7 carbon monoxide sensor works

The MQ-7 uses a heated tin dioxide (SnO2) sensing layer. Carbon monoxide reduces the surface resistance of this layer when present.

• Clean air ? higher sensor resistance
• CO present ? resistance decreases
• Resistance change ? voltage change at output

3) Why heater cycling is required

Unlike most MQ sensors that use constant heater power, the MQ-7 requires alternating heater voltages:

• High temperature phase: burns off contaminants and resets the sensor
• Low temperature phase: optimized for CO sensitivity

Typical cycle:

• 5V heater for ~60 seconds (cleaning phase)
• 1.4V heater for ~90 seconds (measurement phase)

4) CO selectivity and cross-sensitivity

The MQ-7 is optimized for CO but is not perfectly selective.

• Strong response to carbon monoxide
• Moderate response to hydrogen
• Lower response to alcohol and methane

This makes it suitable for CO monitoring but not for complex gas discrimination.

5) Module overview and onboard electronics

The MQ-7 module typically includes:

• MQ-7 sensing element with heater
• Load resistor for voltage divider output
• LM393 comparator for digital output
• Threshold potentiometer
• Status LED

Most modules expose both analog and digital outputs, but the digital output still depends on proper heater cycling.

6) Pinout and electrical characteristics

• Heater current can exceed 150 mA
• Designed for 5V operation

7) Power supply and heater control

The heater must be driven with two different voltages. This is usually done using:

• A transistor or MOSFET controlled by Arduino
• A resistor divider or DC-DC converter for 1.4V phase

8) Analog vs digital outputs

Analog Output (AO)

• Must be read during the low-temperature phase
• Provides relative CO concentration information
• Required for meaningful use

Digital Output (DO)

• Comparator-based alarm signal
• Threshold adjusted via potentiometer
• Only valid if heater cycling is correct

9) Wiring to Arduino

MQ-7 VCC ? Arduino 5V
MQ-7 GND ? Arduino GND
MQ-7 AO  ? Arduino A0
MQ-7 DO  ? Arduino D2
Heater control ? Arduino digital pin (via transistor)
    

10) Arduino Example 1: Heater cycling control

/*
  MQ-7 Heater Cycling Example
  Simplified demonstration only
*/

const int HEATER_PIN = 8;

void setup() {
  pinMode(HEATER_PIN, OUTPUT);
}

void loop() {
  // High-temp phase (5V)
  digitalWrite(HEATER_PIN, HIGH);
  delay(60000); // 60 seconds

  // Low-temp phase (approx 1.4V via resistor network)
  digitalWrite(HEATER_PIN, LOW);
  delay(90000); // 90 seconds
}
    

11) Arduino Example 2: CO level reading

/*
  MQ-7 CO Sensor - Analog Read
  Read during low-temperature phase
*/

const int MQ7_AO = A0;

void setup() {
  Serial.begin(9600);
}

void loop() {
  int value = analogRead(MQ7_AO);
  Serial.print("CO level: ");
  Serial.println(value);
  delay(500);
}
    

12) Arduino Example 3: CO alarm logic

/*
  MQ-7 CO Sensor - Alarm Logic
*/

const int MQ7_AO = A0;
const int BUZZER = 9;
const int CO_THRESHOLD = 300; // adjust after calibration

void setup() {
  pinMode(BUZZER, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  int value = analogRead(MQ7_AO);

  if (value > CO_THRESHOLD) {
    digitalWrite(BUZZER, HIGH);
    Serial.println("WARNING: CO detected!");
  } else {
    digitalWrite(BUZZER, LOW);
  }

  delay(500);
}
    

13) Calibration and baseline setup

Calibration is relative and requires a clean-air baseline.

1. Allow full heater cycle stabilization
2. Measure baseline in clean air during low-temp phase
3. Set thresholds above baseline
4. Adjust using test gas if available

14) Environmental effects and limitations

• Humidity affects sensitivity
• Temperature drift alters readings
• Cross-sensitivity to hydrogen
• Sensor aging over time

15) Typical applications

• Carbon monoxide monitoring experiments
• Educational safety demonstrations
• Engine exhaust detection
• Indoor air quality experiments

16) Troubleshooting

Unstable readings

• Incorrect heater cycling
• Reading during wrong phase
• Power supply instability

No response to CO

• Heater not driven correctly
• Incorrect wiring
• Sensor damaged or aged

17) Quick checklist

MQ-7 Carbon Monoxide Sensor (#522) Checklist
-------------------------------------------
? Use stable 5V supply with sufficient current
? Implement heater voltage cycling
? Read sensor only during low-temp phase
? Treat readings as relative values
? Expect cross-sensitivity to hydrogen
? Do NOT use as certified safety device
? Recalibrate periodically