92 lines
4.7 KiB
Markdown
92 lines
4.7 KiB
Markdown
---
|
|
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.
|
|
|
|
[][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
|
|
|
|
[][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
|
|
|
|
[][11] [][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
|