Custom Shield - Indoor Air Quality Monitor


I backed at the custom shield level and my request was to have a Figaro TGS2602 hydrogen sulfide sensor on a custom shield.

Figaro Sensor PDF

There have been discussions with Spark about expanding the shield.

Other possibilities discussed :
Adding a combined Temperature and Humidity sensor
Adding an RGB LED or LED bar graph
Expanding into other toxic gases

Breadboard - Indoor Air Quality Monitor
Custom Shield - Indoor Air Quality Monitor (Open Source Software)
Temperature / Humidity Dashboard (dht22)

I thought I would outline my reasons why the Figaro sensor is a good choice in an IAQ solution.
It can detect some VOCs and Hydrogen Sulfide (H2S) which is a component of sewer gas.

Detecting sewer gas is useful because :

  1. In my experience the smell can overpower other smells causing confusion of the source and types of other more hazardous smells
  2. After smelling H2s the person can become unable to smell it again for some time.

I was at a friends house and they had an intermittent sewer smell in the bathroom which neither of them could smell at all. I don’t know why they couldn’t smell it, perhaps due to age or poor sense of smell. They said they smelt something a few years before and a plumber couldn’t find any problems.
3. Sewer gas smells can be tricky to locate which is why a wireless recording device would be suitable.

e.g. dry floor drain trap under the basement floor
4. Sewer gas smells can be intermittent e.g. an appliance runs and causes a trap elsewhere to be emptied, or a failing air admittance valve, or a bathroom fan pulling the smell from a failed toilet wax ring.

Which is why a device which can record intermittently is useful.
5. Plumbers don’t generally carry any equipment to identify these smells, so a visit from a plumber can be frustrating if the smell is intermittent or lead to you disregard a real smell.


An IAQ monitor is something I’d love to use as well. Not so much for the H2S, but the VOCs. My wife is an artist and works with lots of harsh chemicals so being able to monitor what’s floating around would be great. A temperature and humidity sensor would be useful for the same reason.


Hi Darcy,
I hope this shield will be useful to you.

My suggestion for an additional “toxic gas” sensor in this shield is a MQ138 sensor.

MQ Series PDF

This sensor can detect Formaldehyde and Benzene which are among the top pollutants. Formaldehyde is common since it is used in building materials, furniture, paints, glues and carpets. Benzene is bad because it causes harm in low concentrations - it is found in gasoline, oils, glues and pesticides.

This is quite an expensive sensor so my suggestion was to mount a socket on the shield so that this sensor could be optional and it also looks like it is pin compatible with some additional sensors. (Im not a hardware expert so this may not be correct).

All the components of this shield have not been finalized as I am still discussing with Zach.


I came across this Kickstarter project the other day:

This project provides, I think, a very interesting set of sensors to consider in our Indoor Air Quality Monitor shield. And it’s open source, which of course is great :smile:

@rockvole Based on your input, my thoughts for what to include on the Indoor Air Quality Monitor are:

  • The Figaro TGS2602 sensor
  • The DHT22 temperature/humidity sensor
  • The MICS-5525 carbon monoxide sensor
  • Some kind of LED display (maybe an LED bar graph or two) for displaying values
  • One or two slots for additional MQ series sensors; that way, people can add some of the more expensive and varied sensors in the MQ series for their particular use cases. @rockvole I think you’re correct that these sensors are all pin-compatible.
  • An attachment for a 3.7V battery

I think that strikes a good balance of breadth of capabilities while still being affordable. @rockvole and others, what do you think?


That list of sensors is great. The addition of a Carbon Monoxide sensor is useful if it means a household can save money buying a separate CO alarm. The regulations I have read about this state that an alarm must sound after a certain time depending on the concentration detected. I wonder if just by adding an alarm buzzer it would be possible to meet those regulations ?


Another idea I had was some kind of calibration / reset button. Apparently the MQ sensors can drift over time and I thought one way might be to take the device to a park and long hold a button and it would calibrate to 0.


For the display I thought a 16x1 LCD display would be the most descriptive. We could cycle through any values found, such as :

  • Temperature 18C
  • Humidity 44%
  • VOCs 72/1000
  • Sewer Gas Detected
  • Carbon Monoxide 1ppm
  • Formaldehyde 0.4ppm

If that is tricky / expensive then a “check your smartphone” LED would be fine with me.
I find the LED Bar graphs to be garish =O


This is pretty neat indoor air quality monitor by 3M - it looks like something from Star-Trek.
Its also a mere $7,500.

3M™EVM 7 Advanced Particulate and Air Quality Monitor Kit


Another option which we could consider if we add 2 slots for the MQ series sensors is to put the MQ-7 carbon monoxide sensor in one of those slots. It also has the nice advantage of detecting natural gas which is sometimes provided in the more expensive carbon monoxide alarms.


Re: adding a buzzer to comply with regulations — could be, although I expect that there are a lot more specific regulations that you have to meet. A cheap buzzer/speaker is an interesting idea though.

Re: calibration button, I think that makes sense, and a button is an easy add.

Re: display, I think the LCD display would make it a lot more expensive and cumbersome. My suggestion would be to have a set of RGB LEDs, one for each sensor value, and have some smart defaults for color range (green = low, red = high), but which can be modified by the user.

Re: MQ-7 — looks like a reasonable sensor to include, it’s not crazy expensive like some of the other MQ sensors. I’ll do some research into this sensor and the one I had spec’ed above for carbon monoxide to see which one looks better on paper.


Those MQ sensors look great. Thanks for the link.


An interesting video clip about indoor air quality in China. Formaldehyde is mentioned as a common carcinogen.


A set of RGB LEDs sounds fine. It might be good if its in a quantity for reasonable percentages.
4 LEDs = 25% each
5 LEDs = 20% each
10 LEDs = 10% each
I dont have a strong preference though.


Hi Zach,
What do you think of the idea of adding a connector on the shield to a Sinyei PPD42NS dust sensor ?
This would allow us to detect mold and other airborne pollutants in the home. Looks like they are around $15


Wow, this looks like a very versatile shield, but is not going to be too big? Although definitely not bigger than the average smoke detector in the bedroom. Do you think it could be used instead of that one or would that require some approvals, probably according to the laws of each country?


I did imagine that this device would be mounted high on the wall and plugged into an electrical socket like a carbon monoxide detector, also it would likely take a reading once per hour.

I dont think it would be best suited to detect smoke since those detectors are ceiling mounted and are continuously monitoring.

I guess if we add the dust sensor it would be possible to detect smoke since that sensor can detect many particles, potentially those in this list above 1 microns:


The dust sensor looks pretty enormous, plus it’s very expensive. Although considering that the interface for it is very simple and is exposed via 0.1" pins, we could probably just plan to expose all of the unused pins so that external sensor modules like this could be easily added to the design.


Excellent - the shield is looking great.