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Hi,
I purchased the above O2 sensor for an Air Quality project, but I am unable to get reading from it. I have it connected to an ESP 32 board using the Arduino IDE.
I stripped back the code to just display the ADC read level and it is always 4095. I connected my multi-meter to it and read 3.295V, no matter what supply voltage 3V or 5V.
My understanding is that it should output a voltage somewhere between 0V and 3.3V that would indicate the standard 21% oxygen content of the air around us.
Is there something wrong with the sensor? How can I test it?
If a top seal is present, removing it would likely have an effect; the device is electrochemical in nature and has a limited functional lifetime, so shipment in a sealed state to limit consumption of working life during shipment and storage would be a reasonable measure to take.
That said, being a current-output device, I’d not expect the amplified output to be stuck in a high state when subjected to an oxygen-deprived environment. If the sensor’s impedance in this state is high enough that the amplifier’s input bias current have no place to flow, this could be normal behavior. Otherwise, some defect in manufacture may be possible; check for missing/broken parts if the behavior persists after any seals present have been removed for a half hour or so.
Thanks, have been all over that site and all I get is the one reading.
That’s why I stripped it back to just reading the ADC value and it is always the max value as it only send 3.3V
I’ve even put it in a vacuum bag with a handheld O2 meter, sucked out the air and got a reading of 8.8% on the handheld meter and still had the highest ADC level on the sensor. Which lead me to conclude it is not working, or the white circle needs removing, but every picture I see of it has the white circle on it.
If it appears fibrous/porous, the white circle is likely a gas-permeable membrane intended to keep the innards in and dust/particulates out.
The schematic for the whole matter isn’t particularly complex; just a 3-terminal linear regulator and a non-inverting op amp circuit with some gain. The sensor’s being in parallel with a 100ohm resistance should eliminate the possibility of the bias currents being a problem, so if the circuit is indeed intact, I see no reason why you should be seeing the output stuck high. Might be worth taking your meter around the op amp to see what’s there, and if that seem to be working right or something’s off. A bad solder joint somewhere might be a possibility, or the board might’ve been zapped such that the op amp is not working. If the output of the board is indeed stuck at 3.3v, something’s not working as it ought.
Honestly I am not sure what I am looking for on the op amp with the system powered. Without power and the sensor removed I confirmed there is 100Ohms between pins 1 and 2 on the sensor. That pin 1 is connected to ground and that pin2 is connected to pin 2 on the op amp. On pin 4 there is 100Ohms between that and ground. Pin 2 is also connected to ground.
Powered on I can see 3.3V on pins 1 and 5. I can also see 0.03v across pins 1 and 2 on the sensor.
30mV across the sensor is significantly out of spec; one should only see about a third of that at normal O2 concentrations, per the device documentation. Might be the sensor itself, might be the op amp, but something’s not working as it’s supposed to.
Separating the amplifier and sensor would be necessary to distinguish between an amplifier problem and sensor problem, though that may be tricky do do without causing further damage. Carefully lifting pin 3 (+ input) would be an option, allowing separate measurements of sensor output, amplifier offset & bias current.
It may be more to your advantage to simply request a return/exchange on the basis that of the device not functioning to spec.
If you are also using WIFI in the ESP32, you need to use ADC1 channels for analog readings, all ADC2 channels are being use by the WIFI so it doesn´t let you use its analog pins, giving you always the 4095 reading problem.
Hope it helps.
