MQ-7 carbon monoxide gas sensor: specs and pinout
MQ-7 semiconductor (SnO2) sensor element for carbon monoxide on a module. The Hanwei sheet specifies a two-level heater cycle for its standard condition.
Specs
| Spec | Value | Source |
|---|---|---|
| Circuit voltage Vc | 5 V +/-0.1, AC or DC | source A. Standard work condition, p.1 |
| Heating voltage, high VH(H) | 5 V +/-0.1, for 60 +/-1 seconds | source A. Standard work condition, p.1 |
| Heating voltage, low VH(L) | 1.4 V +/-0.1, for 90 +/-1 seconds | source A. Standard work condition, p.1 |
| Heating resistance RH | 33 ohm +/-5% (room temperature) | source A. Standard work condition, p.1 |
| Heating consumption PH | about 350 mW (average over the sheet heater cycle: 5 V for 60 s, 1.4 V for 90 s). Derived, not in the sheet: held at a constant 5 V the heater (33 ohm) draws about 150 mA, about 0.76 W | source A. Standard work condition, p.1 |
| Load resistance RL | adjustable (10 kohm +/-5% in standard test) | source A. Standard work condition and C. Sensitivity characteristic, p.1 |
| Sensing resistance Rs | 2 kohm to 20 kohm (in 100 ppm CO) | source C. Sensitivity characteristic, p.1 |
| Detecting range | 20 ppm to 2000 ppm carbon monoxide | source C. Sensitivity characteristic, p.1 |
| Preheat time | No less than 48 hours | source C. Sensitivity characteristic, p.1 |
| Using temperature | -20 to 50 C | source b. Environment conditions, p.1 |
Power
| Value | Source | |
|---|---|---|
| Supply | VH 5 V for 60 s then 1.4 V for 90 s (heater cycle); Vc 5 V. Whether the module cycles the heater is not documented | source A. Standard work condition, p.1 |
| Current | about 150 mA heater current if held at a constant 5 V (derived: 5 V / 33 ohm RH, about 0.76 W); the sheet gives about 350 mW as the average of its 5 V / 1.4 V heater cycle. Derived value, not printed in the sheet; module wiring is not documented. | source A. Standard work condition, p.1 |
Chip pins
These are the pins of the chip on the module, from the chip datasheet. The module's own header may label or expose them differently.
The maker sheet describes the bare MQ-7 sensor element (6 pins: 4 signal, 2 heater), not the module board. The module's header labels, regulator, comparator and load resistor are not covered by an official document, so module labels are left empty.
| Pin | Function | Voltage | Notes | Source |
|---|---|---|---|---|
| A / B (4 of the 6 sensor pins) | Sensing electrodes (signal) | Sensor element, not the module header | source Section D, Fig. 1 and text, p.1-2 | |
| H (2 of the 6 sensor pins) | Heater | Sensor element, not the module header | source Section D, Fig. 1 and text, p.1-2 |
Build with it
Project ideas for the MQ-7 carbon monoxide gas sensor. Each link opens Soldr with the prompt filled in; Soldr assigns the pins, checks power and shows how to wire every part.
- Carbon monoxide sensor reading display: Learning project, not a safety device and not a carbon monoxide detector. The Uno has a 5 V supply and analog inputs for the module output. The heater needs about 150 mA (derived from the heater resistance in the sheet); power the Uno from USB or a 5 V adapter rated 500 mA or more.
- Carbon monoxide sensor trend display: Learning project, not a safety device and not a carbon monoxide detector or a gas detector. Shows the raw reading as a trend line, so you see how it changes with the air around it; no alarm or switching. The heater needs about 150 mA (derived from the heater resistance in the sheet); power the Uno from USB or a 5 V adapter rated 500 mA or more.
- Carbon monoxide sensor warm-up logger: Learning project. Logs how the reading settles during the long preheat. The sheet asks for a preheat of 48 hours, so leave it running that long before trusting the numbers. The heater needs about 150 mA (derived from the heater resistance in the sheet); power the Uno from USB or a 5 V adapter rated 500 mA or more.
Common mistakes
- Holding the heater at a fixed 5 V: the standard condition cycles the heater between 5 V (60 s) and 1.4 V (90 s); whether the module does this is not documented. (source)
- Skipping the warm-up: the sheet gives a preheat of no less than 48 hours. (source)
- Treating it as a safety device: this page is for learning projects; the sheet's detecting range is 20-2000 ppm CO, and it does not describe a certified detector. (source)
Not covered by the official source
- Module-level supply, outputs, pin order and heater current are not documented; sheet values are for the bare sensor element.
- Whether the module runs the sheet's 5 V / 1.4 V heater cycle is not documented.
Sources
- link (Hanwei MQ-7 Technical Data; a hosted copy)