Custom Shield - Indoor Air Quality Monitor


A brief update about the MICS sensors:
I got my quote from, the sensor itself (MICS-4514, qty < 10) costs 9,72 USD, but shipping to Italy starts from 36 USD (no tracking), upto 88-94 USD (UPS). Definetely too much for getting 2 sensors, the quantity I had in mind. I will see if a friend of mine in the US agrees to have the sensors shipped to him, costs would be 8 USD. And I guess a normal letter from the US to Italy will not cost that much :smiley:

About the screw terminals, I agree with @Rockvole , I will use that kind of terminal (not populating the footprint) together with the USB connector in my board

@mohit : why the need to place a transistor in series with Rs for powering / depowering the MQ/Grove sensors? Would not be possible to place it “cutting” the GND connection of the sensor? (just asking!)


I think the idea is to conserve power and limit heat.


Yes I never would have thought getting accurate temperature and humidity readings would be the trickiest part but its where I have spent the most time on this project.


@rockvole, I have not used the MQ sensors, so I can’t speak to the heat issue, however I can say that I have had a DHT22 alongside a BMP180 and they read within +/- 1 degree. I also have a DS18B20 that when placed near the other two I get a similar reading. So I’m confident the DHT22 is doing a good job. As I suggested earlier, the SmartCitizen example had separated the PCB with an airgap around the DHT to minimize the thermal conductance.

With all that said, have you measured the settling time for the heaters on the MQ sensor? Do you know how long it takes to stabilize the temp for an accurate reading? I’m sure there is some amount of time for the heater to stabilize the sensor, you may find that switching it on & off to limit the thermal issues will affect the sensitivity & accuracy.


From what I can read, MICS sensors are better operated in constant power mode so perhaps the power for the off-board grove sensors is not controlled. For the on-board sensors, I believe the warm-up time is in the order of seconds. If you do your DHT22 samples while your sensors/board have “cooled”, you should get good readings. In this product, “real time” is really measured in minutes I suspect. Good ventilation in the case is needed for the sensors anyway so “some” cooling is intrinsic in the design.


@mtnscott - yes I think we should have power to the DHT22 all the time @mohit

I was expecting to take a humidity / temperature reading right before switching on the heaters to leave the most time for everything to cool. The highest frequency reads I have taken are 10 minutes and I don’t expect to take readings more frequently than that. I guess we could have a diagnosing mode with 10 min intervals and a monitoring interval of an hour.

I did look previously about the warm up time required for the MQ series and some said 3 mins was all that was needed. The TGS2602 is supposed to be kept on for a week initially to burn off residue on the sensor.

Yes I think the DHT22 does a stellar job for $3, but I do have a couple which have never given accurate readings (including a grove one) which is the idea behind the socket.


@peekay123 - yes I’m not concerned if the readings are a few minutes late, but I am concerned the time is synchronized - which is what my code does. I have a current reading ability in my code when you press a button it will take a reading after the warm up time (warm up time is fictional in V1 of the shield but I put the code in for it).

I guess the transistor may not be toggled by everyone as they will want always on - but I think its a useful feature.

I planned to put a plastic grill on the whole front of the enclosure for good ventilation.


@cerocca I am in France and would be interrested in MiCS too. You can see some work I’ve done on arduino here:


@mtnscott I understood the idea (conserve power and limit heat), my question was “why place a transistor there” = in series with RS.
If you put a logic level transistor (NPN) with collector to GND of the sensor, and emitter to GND, wouldn’t be helpful to reach the same goal (conserve power and limit heat), without adding noise to the output pin (where Rs) is?


Guess I missed the detail of using it in series with the Rs. Yes, the better place to locate it would be between the GND and the sensor.


@mohit - do you have everything you need from me for the shield ? Do you think you will get time to manufacture the PCB before you come back from China ?


Hi @mohit ,
may I ask you which transistor did you choose for powering / depowering the TGS2602 and the MQ sensor?
Did you put them in series with Rs or between the sensor and GND?


@Rockvole Sorry about the radio silence. I’m down with cold right now (blame it on the air). I’ll be here for another 4 weeks, so enough time to spin the boards and send them over. I think I have everything from you, I now need to finalize the components and lay the board.

@cerocca I’m planning to go for a logic level MOSFET instead of a transistor. I’vnt decided on the exact part yet.


@mohit - here is a photo of the Figaro SR7 socket which I just got in the post.
I have never seen a photo of them on the web. These are 62c in 100+.

SR7 socket modelled by TGS2602 :smile:


They are out of stock of the Figaro SR6 until next month so I couldn’t get any. These are $3.00 in 100+


@mohit - you mentioned that we can put an air gap in front of the DHT socket, but I just realized there is nothing on the schematic. Is that missed or do you just drill it after ?


@Rockvole Since I took the power regulator circuit out, I did not add the air gap for the DHT module. The airgap is added on the milling layer of the PCB and is easy to do so. You reckon we’ll still need one?


The TGS2602 and many of the MQ series sensors have heaters in them, although they are raised off the PCB by the sockets. I cant say for sure if it will make a difference - so its your call.


Just a hint, on some gas sensor board there is a plastic socket to limit the heat transmission to the board such as this one: gas sensor board xl (beware this 2SH12 sensor has no datasheet, this is just a board example)


hi, this is a long and very interesting thread.
Without annoying anyone, is there any loose target timescales or rough guess when the board could offered for sale ?