Added gas sensor post

content/posts/2018-05-19-Improving-a-Gas-Sensor.md

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+---
+title: "Measuring Air Quality - Adding a Gas Sensor"
+date: 2018-05-19T23:25:00+02:00
+author: eNBeWe
+type: post
+categories:
+  - Allgemein
+  - Bastelkram
+tags:
+  - Wohnung
+  - Luftdaten
+  - DIY
+  - Arduino
+
+---
+At the time, there is quite some discussion regarding 
+air quality and pollution going on in germany.
+Some cities, like Stuttgart, have to deal with high levels
+of fine particle emissions.
+That is why some people came up with a simple solution to
+build a sensor network for air quality. 
+They provide detailed instruction on how to build your
+own air quality sensor for about 25 Euros.
+So, if you want to participate, just head over to [luftdaten.info][1]
+
+<!--more-->
+
+I have been running [my sensor][2] for quite a while now.
+In addition to fine particles I am also measuring temperature,
+humidity and air pressure. The data is also automatically sent to 
+[opensensemap.org][3].
+But now I wanted to add a "real" gas sensor to my collection 
+to also measure gases like ammonium (NH3) or carbon monoxide (CO).
+
+The sensor firmware from luftdaten.info allows you to connect
+more or less arbitrary sensors through the I2C protocol.
+This is also the way I am currently getting my humidity/pressure/temperature readings from a BME280 sensor.
+So far, there are no sensors for volatile gas compounds supported
+so I had to do some digging to find a suitable sensor.
+
+After some searching I found the [*Seeed Studio Grove Multichannel Gas Sensor*][4], which I ordered. Turns out, I probably should have read a bit more.
+The board primarily consists of a [SGX Sensortech MiCS-6814 gas sensor][5] for the detection and an [ATmega168PA][6] for the I2C interface.
+
+The board has a few flaws both in software (fixable) and in hardware (not so easily fixable). [Over at Github][7], paulvha gathered some of the problems he found out. In short: The board hasn't enough power to properly heat the sensor and the precision of the sensor is comromised due to bad choice of load resistors.
+
+For that reason I started to build my own controller for the MiCS-6814 sensor.
+I started out by buying a small board with just the MiCS-6814 sensor from AliExpress.
+
+[![Sensor board](/posts/images/2018-05-19-MiCS6814-250x204.jpg)][8]
+
+The board consists of the sensor with a few current limiting resistors,
+according to the reference design from the datasheet.
+To attach the sensor through I2C, an [ATmega328PA][9] that I had lying around,
+is used. For this I needed a small board to manage the heating control, 
+the programming of the microcontroller as well as potential level shifting.
+
+## Schematic
+
+[![I2C Adapter Schematic](/posts/images/2018-05-19-MiCS-6814-I2C.sch.svg)][10]
+
+The schematic above shows the wiring of the adapter board.
+The board is connected to the main air quality sensor through the 5pin connector
+at the top left. The MiCS-6814 sensor should always run at 5V so I need a pin for that. To allow for a lower main logic voltage, the VCC pin takes the voltage for the level shifters.
+The other pins are just a ground pin and the two pins for the I2C bus.
+
+In contrast to the datasheet, the resistance values from the sensor are not
+read through one single ADC pin but through three separate analog inputs with their individual serial load resistors. The resistor values are taken from the document of [paulvha][7].
+
+## PCB Design
+
+[![PCB front side](/posts/images/2018-05-20-MiCS6814-pcb-front-250x84.png)][11] [![PCB back side](/posts/images/2018-05-20-MiCS6814-pcb-back-250x85.png)][12]
+
+I designed my pcb in KiCad as can be seen above. The ICSP connector on the back is done as 6 pads, allowing for programming through pogo pins. For initial development, I just soldered a normal pin header to the pads.
+
+My initial firmware on the controller is just a port of the original firmware from the [Grove Sensor][13], allowing for reusage of the existing library. However, as I am currently getting more and more freaked out by the
+really poor code quality of the original library, I am currently doing a rewrite.
+
+More on that later.
+
+ [1]: https://luftdaten.info/
+ [2]: http://kiel.maps.luftdaten.info/#13/54.3226/10.1271
+ [3]: https://opensensemap.org/explore/594820b4a4ad59001132e6f9
+ [4]: https://www.seeedstudio.com/Grove-Multichannel-Gas-Sensor-p-2502.html
+ [5]: https://www.sgxsensortech.com/content/uploads/2015/02/1143_Datasheet-MiCS-6814-rev-8.pdf
+ [6]: http://www.microchip.com/wwwproducts/en/ATmega168PA
+ [7]: https://github.com/paulvha/multichannel-gas
+ [8]: /posts/images/2018-05-19-MiCS6814.jpg
+ [9]: http://www.microchip.com/wwwproducts/en/ATmega328PA
+ [10]: /posts/images/2018-05-19-MiCS-6814-I2C.sch.svg
+ [11]: /posts/images/2018-05-20-MiCS6814-pcb-front.png
+ [12]: /posts/images/2018-05-20-MiCS6814-pcb-back.png
+ [13]: https://github.com/Seeed-Studio/Mutichannel_Gas_Sensor