Hi there,
I want to integrate a CO2 sensor in my clock project and I got it working, no problem for that.
However, I need to execute, at least 2 times a year, a calibration process that is initiated by grounding pin 8 (HD) for about 10 seconds (at least 7 or 8 I think. No way to do this manually since the pins will be unreachable once the clock is completely build.
Programming is no issue, but how do make the electronic circuit? I need a kind of electronic switch to ground that pin temporarily. A full blown relais seems to me a overkill. But then I stumbled on something like this: PVI5080NPBF RELAY, PHOTOVOLTAIC INFINEON ( https://sinuss.nl/componenten/relais--solenoids/solid-state-relays-contactors/mosfet-relays/9103880-pvi5080npbf-relay-photovoltaic-infineon ). Can this do the trick?
But I have no idea if this is the way to go. Can someone advise me and set me on the wright track?
Do you have a spare logic pin you could use?
Lots of detail in there, "pin 8" and so on. No mention of what sensor.
Posting the schematics would help a lot.
What voltage is normally present on this pin? If you manually connect it to ground, how much current flows from the pin to ground?
Please, read the titel of this post. It says MH-Z14A CO2 sensor ...
Please read the forum instructions. You are supposed to post clickable links to things like that (unless you post the technical information directly, inline).
Tension is 3.3V, current ... no idea. It must be very low, I guess. The pin serves as hardware reset to calibrate the sensor.
OK. Sorry, I forgot about that.
You can click here: MH-Z14A
Yes, I have spare logic pins and analog pins.
Do you mean the schematics of my project?
Why not just send the calibration command via serial?
Yes, that is an option ... and now you mentioned it, it would probably the easiest and best solution.
Thank you for this useful tip!
I saw a method via a transistor that would also work I think, but probably more complicated.
Next question, how are you going to satisfy this requirement for calibration?:
"Before calibrating the zero point, please ensure that the sensor is stable for more than 20 minutes at 400ppm ambient environment"
?
My project contains also a remote via smartphone.
The clock can send messages to the smartphone and calibration only starts at confirmation via the remote. Though this has to be put in place yet.
I also have the intention to check for that stable 20 minutes, knowing that 400 ppm has to be interpreted as, more or less and not exactly 400 ppm. Any way, checking 400 ppm manually is a difficult task and probably impossible. I still have some work to do ...
Thank you for your remarks. I appreciate your input.
Suppose your unit is measuring "low". You observe one hour of readings around 400 then make a calibration. The new reading will be around 400. But you actually calibrated it at 250. Now whenever it is 250, it will report 400. Your calibration was useless. Worse, now it is misleading.
You can't calibrate something against itself.
So, you suggest to not calibrate at all. That makes sense, I agree.
I have no idea how that "build-in" calibration process works and I was wondering how they do that. Calibration can only be done against a reference source that is 100% accurate. Nobody has such a source, except for specialized organisations.
Conclusion: no calibration, let it be?
You need a cylinder of 400ppm test gas.
You can put it outside or in a well ventilated room with no one around. Your 'calibration gas' will then be normal air. Currently 410 ppm...
It is not a coincidence that you should calibrate with 400 ppm 
!
Thank you all for all the advise. I will certainly consider them.
I try to read as much I can about CO2 meters and calibration and how to do it as accurate as possible.
My goal is not to build a precision instrument but to have an as accurate possible device that can alert me when CO2 values are getting really too high.
So, understanding fully the calibration process is important and applying those procedures as accurate as possible too.
Did I ask you to guess? Measure it!
Oh well, sounds like someone suggested a better solution in the end.
I do wonder if it comes from the factory with calibration already performed... it would be a bummer if not. Another possible issue is aging, if sensor drift is big, after a while you would have no choice if you wanted to maintain accuracy.