CO2 Parallax sensor

Hi Everyone,
Has anyone worked with Prallax CO2 sensor kit (includes the amplifier)
I’m trying to wire it up but not sure if my approach is right. see attached a picture of my wiring,

So since the sensor needs more power i am using an external of 9V and sharing the sensor’s GND with Arduino GND
then I’m reading both pins CNTL and ALR as analog-ins
I have a feeling I am missing something here.
any comments or advice would be appreciated!


Looks OK to me. Does it work? If you set the trip level just below where the LED turns on can you turn the LED on by breathing on the sensor (with the CNTL line turned on, of course).

Thanks for the reply, I can't get a stable Voltage from TP1 and TP2, it starts at 3.3V and dropping very slowly and jumps back to 3.3V after a while. When the LED is on I can read the pins, but when the LED is off I only get zeros.

Not sure you should be reading both pins - as Johnwasser alluded to, it appears from the datasheet that cntl needs to be held high to get a reading from the sensor & if there's enough CO2 detected, depending on R3 setting, you'll get high or low on ALR.

As has already been said, you need to drive the CTRL pin from an Arduino output. A high on this pin turns the heater on, which is necessary for the CO2 sensor to work.

I'd ignore the ALM output and connect TP1 to an Arduino analog input instead. That will give you a readout of the CO2 level, rather than just telling you whether it is above or below the alarm level.

Thanks for the tip! It worked really well and I am reading analog from TP1 which gives me a good range to work with. The heating element is working for sure, I'm guessing it is supposed to be as hot as it is and you can leave it on for a while with out damaging the sensor.


Thanks for the feedback, I'm glad it worked for you.

What voltage did you end up using for the heater? (CNTL pin)

Also, did you ever read 0V at T1?

I just finished playing around with this sensor and got this:

1) it actually works!!! :) 2) to get graduated readings use test point 1 (TP1) 3) Alarm out or (E3 ALM) gives you the set point value for a 3.3v HIGH trigger (I cant think of an mcu type application where you would use a trigger considering graduated readings are available on TP1)

this is my configuration...

E1 > set at 5v HIGH from any available pin E2 > NA E3 > 6-12v (don't use a 9v battery as it will drain very quickly - the mg811 and sensors of this type are gas-guzzler's) E4 > ground TP1 > to analog in (graduated readings - using arduino's analog map can come in handy)

Now my big question is: how to calculate PPM?

Thanks Parallax!

u could put it in a little box and use calibration gases in a otherwise well(!) ventilated area... and then try to find a most simple formula that (almost) goes through all those known points... :) fresh air has 400ppm... if u know a paramedic: he/she has possibly 4% CO2 in his/her bag (but the rest is O2 not normal air, which could disturb the sensor slightly)...

but i dont know if the readings r reproducible... maybe the NDIR principle is more accurate?

Hi, I'm trying to set the parallax CO2 sensor, it is possible to share your code

Juan Carlos