Quelltext der Seite Co2 Monitor

{{Infobox Projekt
|name            = CO² Monitor
|status          = stable
|autor           = [[Benutzer:schinken|schinken]]
|beschreibung    = IoT CO² Sensor
|image           = cat_co2_sensor.jpg
|imagesize       = 250
}}

[[Category:Software]]
[[Category:Hardware]]

== Description ==

Someday a [https://www.amazon.de/TFA-Dostmann-CO2-Messger%C3%A4t-AirCO2ntrol-mini/dp/B00TH3OW4Q/?tag=bckspc06-21 TFA Dostmann CO2-Sensor AirCO2ntrol] appeared in our hackerspace - but the CO² reading couldn't be easily acquired through USB or any other connector. There are some implementations using [https://github.com/fablabnbg/airco2ntrold python] or nodejs, but they require a raspberry pi or at least a PC nearby to read the values to publish and store them somewhere.

Nowadays ESP8266, NodeMCU or in our case a Wemos D1 mini (which also has a ESP8266 on it) exist and is totally suitable for the job! Sadly, on the TFA Dostmann there's only a USB interface on the outside, and our Microcontroller does not support USB natively. So we opened the sensor and found a "CALIB" interface

== Hardware ==

Inside the TDA Dostmann CO² sensor, there's a CALIB interface, which consist of four PINs. GND and 5V can easily be determined using a multimeter with continuity test. These PINs are directly connected to the USB port and can be used to power our microcontroller. The other 2 PINs in the middle seem to send some data. After connecting them to a oscilloscope one PIN was obviously just clock, and the other data.

<gallery>
Datei:Co2monitor pinheader.jpg|LTR: GND, Clock, Data and 5V
Datei:Wemos-wiring.jpg|Add wires
Datei:Wiring.jpg|Connected!
</gallery>

Test readings using [https://sigrok.org/ SIGROK] and a logic analyser can be found on [https://github.com/b4ckspace/esp8266-co2monitor/tree/master/doc/sigrok-captures b4ckspace/esp8266-co2monitor/sigrok-captures].

== Software ==

The signals look like they're just I2C, but actually they're not. Using the scope to decode the data failed - the same is true using the Wemos. After wiring up the sensor to the Wemos we used an external interrupt on the clock pin and a digitalRead on the data pin which is basically "bit banging".

The results were promising: Using a simple python script and [https://github.com/b4ckspace/esp8266-co2monitor/blob/master/doc/pdf/AN135-CO2mini-usb-protocol.pdf the documentation from another manufacturer], we were able to verify the checksum. Also implementing the decode of the data was quite simple using the PDF:
    
 case 0x50:
  co2Measurement = (data[IDX_MSB] << 8) | data[IDX_LSB];
  break;
 case 0x42:
  temperature = ((data[IDX_MSB] << 8) | data[IDX_LSB]) / 16.0 - 273.15;
  break;

The last thing to do was connecting to wifi and publishing the data on MQTT. The [https://github.com/b4ckspace/esp8266-co2monitor source code can be found on GitHub] and is licensed under the MIT-License.