How can I measure a sensor's load resistance which is adjusted by a potentiometer?

Question

I have this MQ136 sensor that comes with a shield. Like most of the MQ series of gas sensors, you can get them with or without a shield. Unfortunately, I've been unable to find a datasheet that actually displays the shield, much less even mentions it. However, this datasheet does say that the load resistance (R_L) is adjustable, and since the potentiometer is the only adjustable component on the sensor I'm guessing that must be what it's for. Since the datasheet is also pretty short on what R_L actually does, I did some poking around and found this blog on gas sensors which states:

[gas sensors have] a built in variable resistor that changes its value according to the concentration of gas. If the concentration is high, the resistance decreases. If the concentration is low, the resistance increases. Besides the built in resistor, it is necessary to include a load resistor. This resistor serves to adjust the sensor’s sensitivity and accuracy. The value can range anywhere from 2k Ohms to 47k Ohms. The higher the value, the more sensitive the sensor becomes. The value depends on whether you want to measure high concentrations of gas or not. If you choose to measure high concentrations of gas, but choose a high value for the load resistor, then the readings won't be very accurate.

First of all, I correct in my understanding this potentiometer is responsible for adjusting R_L which controls the sensitivity of measuring gas concentrations?

Second, how can I measure the R_L with a meter? I played around with the potentiometer, and when spinning clockwise I saw a (previously unnoticed) LED light up and it stayed on for a small arc and then it turned off and stayed off when I turned even further until I hit the stop point. Going counterclockwise the LED came back on during the same arc. Any idea on what this means?

Answer 1

The MQ series sensors have two circuits within the sensor.

The first is the heater circuit which is connected to a voltage source and this heats up the active element.

The second is a chemical sensor which has a variable resistance according to the concentration of the target gas. The resistance of the sensor varies wildly between different sensors.

The sensor is connected in series with the load resistance to produce a voltage, due to the variation in the sensor resistance this is the pot you have on the board. Adjusting the pot adjusts the output voltage. Generally the rest of the circuitry is a voltage comparator set to raise or lower an output when the voltage from the sensor-Rl section reaches a set level.

To measure Rl measure the resistance over the pot. It is impossible to tell for sure from your description but I suspect the LED comes on when the output voltage from the sensor-Rl combination lies within a certain range. Measuring the voltage at the junction between RL and the sensor while adjusting the pot will give you some insight into what is happening with this particular circuit.