The “sensor box” project

Posted by on Jan 31, 2021 in Design, Environmental | No Comments

Digging through sensors I already have, I discovered that my Sparkfun Qwiic Ideation Kit came with an Environmental Combo sensor that can detect temperature, humidity, CO (carbon monoxide), and VOC (volatile organic compounds).   The list of VOCs is long but NO2 is in the list.

What’s Qwiic?  It is a standard from Sparkfun for I2C (a serial communication protocol) which makes working with complex sensors much easier, almost as easy as working with simple analog sensors.  On the Arduino you can do a digitalRead() or analogRead() on a pin and get back a value:  high/low for digital and a number between 0 an 1023 for analog.   With the Qwiic/I2C solution you can read complex sets of data with an almost-as-simple interface.

Using a Qwiic cable I tied my sensor to a RPi3, put it in the kitchen, and ran some Python code that I’d thrown together.  (I say “thrown together” because I’ve been writing in C++ since Bush41 and will write a independent post on what-language-to-use-for-physical-computing-and-why.)  I’ve let the code run for a few days and noted anything I was doing in the kitchen.

Keep in mind this sensor only reports VOC ppb (parts-per-billion) , it can’t tell you which one.  My first surprise was how the VOC jumped when I made brewed tea (sencha), from around 8 or 9 to the mid-70s then back down to ~10ppb.  Sure, the humidity went up briefly, I mean, I AM BREWING TEA, but VOC?  Then I got to thinking — I can smell the tea, is the scent a VOC?  I haven’t found a solid answer yet, but it seems possible.  Or maybe the sensor is misreading steam as a VOC?

Then another test with making toast and not using the ceiling ventilation fan.  The sensor is about a foot from our toaster oven, and within a minute the VOC rate had jumped from ~10 to 1961.  The next minute 2023, the next minute 6382, then the next minute (after the oven was turned off) it dropped to 3700.  CO also went up and back down in similar levels.  It took at least an hour for the VOC to drop back to ~10.

So clearly something is being detected but what?  Let’s look at PM2.5 (dust) levels while we’re cooking.

My next sensor is an Adafruit Air Quality Sensor, a Qwiic compatible sensor that tells you how many tiny particles are in the air.  It will be interesting to see how PM2.5 numbers compare to the VOC numbers.

I’m posting all this to a github repo if you want to try this in your home/studio.  Future work includes more sensors, a case for mounting it to a wall, a touch screen interface and whatever else I think of along the way.

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