Customer:
I’m having trouble reading anything from the Analog output port of the MQ3 gas sensor using a multimeter. I have 5V supplied to the 5V input and ground to the ground input. When I place a cotton swab with alcohol near the sensor, the voltage reading is not changing. I have the multimeter red side connected to the AO port and the COM side connected to the same ground which is fed to the sensor. Am I doing something wrong? Also is the AO supposed to be 0-10V voltage signal or 4-20mA current signal?
Originally submitted via email support on 22 August 2024. Migrated to the forum for community visibility.
Soldered Support:
The AO pin outputs 0-5V level voltage which is measurable with analogRead on 5V logic boards like Dasduino CORE.
The MQ sensor series has a warm-up period. The sensors need to be left operating for a while in order to be accurate and to give some sort of meaningful measurement. Leave the sensor powered on for a couple of hours and then check if there will be a change in voltage when alcohol is present.
Customer:
I waited a couple of hours before testing. Without any alcohol near the sensor, the AO voltage reading was 2.462VDC and AC voltage was reading 0VAC when I switched the multimeter to AC voltage setting. When I put a cotton swab with alcohol near the sensor, the AO voltage output was reading 4.57VDC and the AC voltage reading was still zero. Shouldn’t the AO voltage output pin be reading in the AC voltage setting on the multimeter?
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Soldered Support:
This behavior is expected. The Analog output pin (AO) outputs only DC voltage. There is no AC voltage in small electronics, it’s all DC. You can read AO with a microcontroller’s Analog Input pin.
Customer:
Is there a reason why there is a voltage reading even without the alcohol swab near the sensor? Shouldn’t the voltage reading stay at zero? Why is the analog output called analog if it’s only DC voltage?
Soldered Support:
A DC signal is still analog, because the voltage from 0 to 5V can be infinitely precise (1.42453V is not the same as 1.42454V). Voltage reference sources, potentiometers, and DC motors all use DC analog signals.
These sensors work on electrochemical principles and it’s just a matter of how they’re physically constructed that the output is some baseline level voltage. You can read this value with a microcontroller and interpret it in software (when you read 1.4V, map that to zero in your code).
You also have a digital output which is either high or low, and its threshold is modified by the onboard potentiometer - you can use that to fine tune.