Hey,
I have been asked to make a simple carbon dioxide monitoring system. My only issue is that I am really only a programmer and have limited knowledge about electronics. In the data sheet, they have a diagram for interfacing with a external micro controller (http://cdn.shopify.com/s/files/1/0019/5952/files/Spec070426-K22-PWM_ed2-2000.pdf?1254409988). Although I have an idea of what to do, I do not want to damage the expensive sensor in anyway and would feel more comfortable if someone reaffirmed me.
I have already poked around online and seem to have stumbled on some working code already:
int pin=14;
unsigned long duration;
void setup()
{
pinMode(pin, INPUT);
Serial.begin(115200);
}
void loop()
{
unsigned long ppm; // adding for clarity
duration = pulseIn(pin, HIGH); // get the raw reading
ppm = duration - 177000UL; // subtract the 'zero point'
ppm /= 500UL; // divide by the gain
ppm += 350UL; // add the zero point back in
the co2 sensor and the arduino need to share a common ground voltage level...
how does ur power supply look like?
the PWM output varies between 0V and V+ of the CO2 sensor...
the arduino needs below 30% of its supply voltage for a digital LOW...
the arduino needs above 70% of its supply voltage for a digital HIGH...
but the arduino doesnt like voltages below 0V or above its supply voltage...
maybe u need a voltage divider if the PWM output voltage can be higher than the arduino supply voltage...?
i would use the following method to convert the PWM into the ppmCO2 value, because it doesnt depend on an accurate time (on my arduino i have an error of +0.3% IIRC... 0.3% of 1004ms (~=~3ms) corresponds to 6ppmCO2):
uint16_t pwm2ppm(uint8_t pin) {
while (digitalRead(pin) == HIGH);
while (digitalRead(pin) == LOW);
uint32_t h,l;
for (h=0; digitalRead(pin) == HIGH; h++);
for (l=0; digitalRead(pin) == LOW; l++);
// h+l <=> 1004msec
// 1004*h/(h+l) <=> duty cycle D in msec
// (D-177)/0.5 + 350 = ppmCO2
return ((2*1004*h+1004)/(h+l)-2*177) + 350; // +1004 for proper rounding...(?)
}
btw:
1.
do u know where pin 14 is?
i think it is analog pin 0...
after a reset all analog pins r configured as inputs...
so that u can be quite sure, that the sensor isnt damaged by them...
but maybe u want to put a resistor (2kOhm or so) between the arduino pin and the sensor PWM output... just to be sure...
In the post, he seems to have gotten correct input using just two resistors and the code provided but I wanted to be sure of where exactly he put them.
according to the K22 datasheet (page 6 & 5) u dont need ur own pull-up resistor (4.7k to 5V), because the K22 already contains its own on-board...
the 10k is just for the case, that u re-conf pin #14 as output pin and give 5V on it...
everything clearly above 125 Ohm should be safe here, because arduino pins can take 40mA...
according to the K22 datasheet there is already a 120 Ohm resistor on the K22 board...
the BC847 on the K22 can take even more than 40mA...
i like my approach (without pulseIn()) still better...
also i think ill try the method described in the other post first because i know it has been tested and works. after i get it working ill be sure to try the method you described.
I am a representative from CO2Meter and have noticed this forum post. I'd just like to add that we have a guide written specifically for Arduino boards that will guide you through interfacing with the sensor over I2C:
Hey Andrew,
Thanks for the post! I did get the CO2 sensor working via PWM. The guide you linked seems very helpful. I thought about using I2C in the beginning but I wasn't too comfortable with it.
One question I have is what do you think a average house CO2 level should be? I read in around 600-750 ppm on average. I want to make sure I did my math right and it is calibrated.
Hi Andrew,
I just finally logged back on today and got your message.. yes 600-800 ppm is where most residences sit.
If you have your space filled with green plants like a greenhouse then the co2 can go lower as it is consumed by the respirating plants.
