Customer:
I need to know the internal resistance of the MQ135 breakout board to calibrate the sensor for a project.
Originally submitted via email support on 12 July 2025. Migrated to the forum for community visibility.
Soldered Support:
The MQ135 sensor has a 10k ohm resistor connected to it on the breakout board. You can analyze the whole breakout board schematic on the GitHub page here: GitHubSolderedElectronics/Air-quality-sensor-MQ135-breakout-hardware-design/blob/main/OUTPUTS/V1.1.1/Air%20quality%20sensor%20MQ135%20breakout%20Schematics.pdf
If you are calibrating the sensor, it requires a preheat time of 48h (connected to power doing nothing) in an environment without the gas you want to measure. You can also use the Arduino library which has examples on calibration as well as taking measurements: GitHubSolderedElectronics/Soldered-MQ-Gas-Sensor-Arduino-Library