Analog CO2 Sensor for LoPy, can 5v pin power it?

[ledbelly2142]

@jmarcelino
Thanks! yes, I only had 'import macine'
Now I get a reading, but it does not look like the sensor is working correctly. I am using ATTN_11DB to make the math conversion easier value * (3300/1024.0)
The raw ADC count is still high, at 883 and its not changing much.

Perhaps I am missing something in the documentation, is the ADC count a division of something? e.g. 883 count = 0.83v?

Thanks

[robert-hh]

@ledbelly2142 The range of the ADC is 0..4096. With an attenuation of 11DB, the reading of 883 tells 883*3.3/4096V = 0.71V

[ledbelly2142]

@robert-hh
Thanks, I could not find where in the documentation the ADC range?

The vendor of the CO2 sensor used the formula to get CO2 ppm:
value*(3300/1024)

I think there are 1024 is the steps on an arduino for the analog in.

So I think the step function to interpret the ppm should be:
value*(3300/4096) which would put us in the range of 711 ppm of CO2. Which sounds about right given that is on my desk right in front of me.

Thanks!

[ledbelly2142]

Here is what I used for this sensor:

import machine
from machine import ADC

adc = machine.ADC()               # create an ADC object
apin = adc.channel(pin='P16', attn=ADC.ATTN_11DB)   # create an analog pin on P16 & connect CO2 sensor

print("")
print("CO2 sensor Warming Up")
#time.sleep(180) # CO2 sensor needs 3 minutes to warm up
print("Reading CO2 Sensor...")
value = apin()
print("ADC count = %d" %(value))

# LoPy  has 1.1 V input range for ADC
co2 = value * (3300/4096.0)
print("CO2 Level = %5.1f ppm" % (co2))

[jmarcelino]

@ledbelly2142
The ADC is 12 bit so the range is 0..4095 as @robert-hh explained. The reference is 1.1 Volts.

At ATTN_11DB the full scale goes to 3.9V: 1.1 * math.pow(10.0, 11.0/20.0) so a more correct formula is value * (3900/4096)

Finally the sample code you linked doesn't just use the voltage reading (what you have now) as the concentration, it takes a few extra calculations.

    int voltage_diference=voltage-400;
    float concentration=voltage_diference*50.0/16.0;

I haven't read the datasheet but I guess you'll need to do the same in your code.

[ledbelly2142]

@jmarcelino
Thanks for your reply and help, I appreciate it. I understand the ADC bit range to 3900 instead of 3300--makes sense.

I am missing the voltage reading completely :( I was going on the ADC example code and explanations. It's gotta be in the documentation somewhere that I can't find. Even when I search the docs for 'voltage', only DAC shows up, not in ADC.

Why do we need the voltage difference -400 value?

How do I apply the concentration to get the value?
is that something like:

int voltage_diference=voltage-400
float concentration=voltage_diference*50.0/16.0
value = apin()
co2 = value * concentration(3900/4096.0)

Or would it just be

co2 = value * concentration

Thanks

[jmarcelino]

@ledbelly2142
I can't find any proper datasheet for your sensor but this plot shows everything you need to know:

At 0 ppm the sensor outputs 0.4v so you you need to subtract 400 (in millivolts) from your voltage reading (the voltage_diference in that formula)

Then you know that 0.4 volts to 2.0 volts (a 1.6 volt range or 0 to 1600 in your voltage_diference from above) equates to a range of 0 to 5000 ppm.

[ledbelly2142]

@jmarcelino
Thank you, makes sense now. Really appreciate it.
Cheers

[bmarkus]

Analog CO2 sensors does not really fit to todays IoT environment specially low power battery operated devices due to high power consumption and slow operation, long settle time. Take a look on Gas Sensing Solutions sensors. They are fast low power devices with digital interface. It is the data sheet of model I'm using with a LoRaWAN sensor (not PyCom), connected via serial line:

https://www.gassensing.co.uk/media/1050/cozir_ambient_datasheet_gss.pdf

[ledbelly2142]

@bmarkus
The COZIR sensors look nice. Are they in the sub $100 range? I also have CO2 sensors from co2meter.com, the K30. But they are a pain to work with and give very little support. At some point I wanted to create an I2C driver for micropython, but put it off because of the support. Another IoT option for low power gas sensors is Spec-Sensors We have used some of their stuff in the past, but they don't have CO2.

Thanks!

[ledbelly2142]

Here's the updated code for this sensor with everyone's help. @jmarcelino @robert-hh
In testing it, the results seem reasonable. Not super accurate but enough within 100ppm. Digy-Key has them for $58

import machine
from machine import ADC

adc = machine.ADC()               # create an ADC object
apin = adc.channel(pin='P16', attn=ADC.ATTN_11DB)   # create an analog pin on P16 & connect CO2 sensor

print("")
print("CO2 sensor Warming Up")
#need initial delay (180) # CO2 sensor needs 3 minutes to warm up
print("Reading CO2 Sensor...")
value = apin()
print("ADC count = %d" %(value))

# LoPy  has 1.1 V input range for ADC
co2 = (value-400) * (3900/4096.0)
print("CO2 Level = %5.1f ppm" % (co2))

[bmarkus]

@ledbelly2142 said in Analog CO2 Sensor for LoPy, can 5v pin power it?:

@bmarkus
The COZIR sensors look nice. Are they in the sub $100 range?

First hit of Google search gives $109 USD

https://www.co2meter.com/products/cozir-0-2-co2-sensor

As an indication you can visit the webshop of ChipCAD, a Hungarian distributor

https://shop.chipcad.hu/Welcome/Default.aspx?scenarioID=360&pid=1878

Prices are in HUF (Hungarian Forint), VAT not included. At the moment 1 USD = 262.6 HUF

But it may vary by distributor, quantity, shipping conditions, custom, etc. and model. You can ask GSS to advice where to buy it, they are good.