Using multiple MQ135 sensors - inconsistent results and hot voltage regulator

Other Hardware General Electronics
1

Hello,

As the title says I am trying to use 2 MQ135 gas sensors. When I plug one sensor I get good results and the sensor is reacting as expected to gas exposure. I had one sensor plugged in for more than 24 hours straight, to do the burn in time and everything was fine. The voltage regulator got quite warm but there were no problems for more than 24 hours.

When I plug 2 sensors I get inconsistent results that are both wrong and change from low values to high ones and the other way around. The voltage regulator gets really hot to the point that it's painful to touch it.

The wiring is 5V to the sensor VCC, GND to the sensor GND, Analog pin to the sensor analog pin and a 1K resistor from the analog pin to GND. The digital pin is not connected. I tried with it connected but unused in the sketch and it did the same thing.

The MQ135 is the type that is soldered to a PCB and has a tiny potentiometer, an IC some LEDs and resistors soldered to it.
The MQ135 pins are VCC, GND, Analog, Digital.

I measured with a multimeter the resistance between VCC and GND without anything connected to the sensor and it was about 40 ohms. I have 3 sensors bought from 2 different palces. 2 are the same and one has a slightly different layout with the pins arranged differently.

Does anybody have an idea as to what the problem might be?
What does the small potentiometer on the PCB do?

2

!! do NOT feed all the heaters from arduinos 5V-line. U'll have to feed them 5V from a separate source.
(eg cellphone charger?)

3

Yeah - you're likely having power supply problems that are throwing off the measurements.

Those gas sensors are very power hungry, because they have a heater inside (the sensor needs to be hot for their sensing method to to work)

4

What does the small potentiometer on the PCB do?

You said that the sensor module has a digital output, which the basic sensor does not have.

I would assume that the IC will be a comparator, and that the potentiometer will set the threshold at which the digital output switches.

(but this is only a guess).

5

Thank you for your quick replies! I will try your suggestions. I used a lab power supply and each sensor drew about 120mA. The voltage regulator is rated at 1 or 1.5A.

I have not used the digital pin. Nothing changes when I adjust it and only the analog pin is connected. I will do some more research regarding the use of the digital pin.