The MQ7 and MQ135 sensors operate with voltages of 5 Volts. and they must be hot to get best measurements.
With the MQ135 sensor it was really easy to work. It is highly sensitive to different types of gases that are harmful to our health. I got pretty consistent results.
Contrary to the MQ7 sensor, which requires two operating voltages, the first of 5 volts for 60 seconds and the other of 1.8 V for 90 seconds. I did different tests applying voltage to it by PWM but the results did not really convince me. And to obtain a reading on the screen it is an aternity (Possibly I place it externally and that when it exceeds its threshold I place it on a digital pin to generate an interruption in the microcontroller and thus integrate it into the application)
I included in this screen the Infineon DPS368 atmospheric pressure sensor. I read the documentation of the library and I was able to integrate it into the application without much inconvenience. I liked how accurate it is and it doesn't fluctuate with electrical interference. As a plus it has a temperature sensor (I liked this)