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In-the-Air-Challenge: Exploring internally NDIR CO2 monitor

janisalnis

Previous post: In-the-Air-Challenge: Measuring CO2 levels during lectures with EXTECH CO2 monitor

Next post: In-the-Air-Challenge: NDIR CO2 meter connected to IoT via TI CC3200 Wi-Fi connectivity board

 

Opening the case of made in China EXTECH CO100 air quality monitor reveals a very compact NDIR CO2 sensor made by SenseAir.com (Sweden) type model S8 004-0-0062. Connected to other electronics with just 3 wires: 5V, GND and output signal. On the sensor output signal is present a TTL pulse once a second (exactly every 1009ms) with PWM lineary proportional to CO2 concentration.

 

At 500 ppm CO2 the pulse width is 50ms. That is what I got today by opening the room window.

At 1200 ppm warning starts beeping and oscilloscope shows 120 ms pulse length.

At 10000 ppm the pulse width is 1s. Basically only a very short peak going down that is useable for triggering.

 

imageimage

Visually one can see through the air diffusion membrane that a small lamp is turning on once a second for a short while, and it is not very bright, so emmision peak is in the Mid-IR and lamp can last for 15 years according to specs.

 

imageimage

 

Schematics is nicely designed and assembled. On one side of the PCB following components can be identified. Large chip - LCD display driver. Small chip - touch sensor buttons driver. Moisture/temperature sensor model SHT. 3V Li cell for clock backup. 5V regulator working from 6.2 V at the input jack.

On the other PCB  side is a MSP430F microcontroller. USB and FTDI chip place is not populated. No RX or TX signals coming from the microcontroller.

So the only way how to connect external electronics is to measure the output of the CO2 sensor directly.

 

Measuring precisely the pulse length on one second time scale should be an easy task to do with a CC3200 board (or TI MSP430 + ESP8266). It was nice to learn this methoid how to enclode analog signals using PWM.

 

Conclusion and recommendation to electronics guys is: Buy just the NDIR sensor!

 

Continue reading: David Giorni NDIR CO2 detector

L. Crawley 2008 Dr. Thesis about NDIR trace gas detection

  • in reply to DAB

    You are right that silicon sensor  like used in simple black and white cameras can sense infrared up to 1.1 microns.

    Such cameras can be used for survilance during night with IR LED illumination. During sunny days they show picture too much red as sun has quite a lot of IR.

     

    I did not know about possibility to use a floppy to let pass through just the IR. Thanks for this hint.

    Some things around us like money, vegetation and veins under skin look quite different in IR.

     

    CO2 molecules have absorption bands around 2, 3 and 4 micron wavelengths. Here is a link to a picture of spectrum:

    http://davidpratt.info/climate/clim7-1.jpg

    One has to avoid water absorption bands. So, I guess, commercial NDIR CO2 detectors use 4 um. That is outside the range of Si sensor.

    As detectors are used pyroelectric crystals, same as in IR motion-detector alarms or light switches.

     

    With a silicon webcam one could may be try visualize absorption from water vapour at 1 um, or oxygen at 760 nm coming out from plant leaves during photosynthesis.

    Thanks for good idea may be wil try some time in future. Some consumer cameras, like Canon EOS and Nicon have 14 bits of intensity resolution and could be used to photograph smog over city. In internet people report removing IR filter from their sensor. Absorptions from gasses are weak so 8 bits of intensity is usually not eneough.

  • If you already have a silicon sensor, you can set it up to read only NIR by covering it with a piece of floppy disk material.

     

    The plastic and magnetic coating do a good job at blocking the visible spectrum, but do a good job at passing IR above about 750 nm.

     

    DAB

  • in reply to michaelkellett

    In SensAir web page there are 2 datasheets for slightly different modules. Datasheet states they put out 1 kHz PWM. The module used by EXTECH puts out slower 1s PWM.

    Probably if you write email to SenseAir they will send you a quote.

    I just looked in Ebay. Could not find SenseAir module.

    But found exactly looking CO2 monitor as by EXTECH

    Rilevatore/Misuratore di Anidride carbonica CO2 Trotec (BZ25) 170 EUR.

  • in reply to michaelkellett

    Hi Michael,

     

    I think you're right. I can see the S8 sensor here, but the last two digits are different from both of them compared to the one in Janis' instrument (perhaps firmware mods).

  • It's not clear to me that this is a standard sensor that is generally available. I can't locate it on the SenseAir web site.

     

    MK