Coin cell based small sensor node / handsoldering smd parts

  • Hardware Contributor

    Hey guys,

    I am planning to build small energy efficient sensor nodes for my mysensors setup. I don't really like the 2AA batteries setup because then the 2 batteries alone are mostly bigger than the rest of the sensor. So I thought about using a coin cell (like cr2032). Did someone already build something like that and wants to share his/her experience?
    The board should also use mostly smd components, because I hope to get the size down to about 3-4cm squared. A pro mini would be too big for that and also has too many components that are not really needed. So I thought to use a barebone atmega 328p. I have soldered smaller smd parts in the past but never any with more than 3-4 legs. Is it doable for a hobbyist?
    I wanted to mostly use these sensors which should work down to lower voltages to really use the coin cell. Beside temp/hum I also want to build PIR sensor nodes. Are there any pir sensors that work below 3.3V or do I need to use a boost converter?

    Thanks for the feedback and happy easter.

  • Hero Member

    @LastSamurai Sensebender?? and what about using the
    small AA designs with a coin cell. I don't think you can get much smaller. These are well suited for the sensor you mention (or si7021)

    I carried out some experiments with these designs and a coin cell.

  • Mod

    @LastSamurai said:

    Is it doable for a hobbyist?

    Yes! But first practice, practice, practice...
    TQFP (like ATMega 328) is certainly doable. Use a very fine soldering tip and preferably a (USB)Microscope to verify you solderwork.
    Drag soldering is another popular alternative (search youtube).
    The web is full of tutorials!

  • Hardware Contributor

    @LastSamurai I agree with @Yveaux too. that needs practice and the good tools. but it's doable, it depends of your goals and motivation 🙂
    Sure aa/aaa are bigger than sensor node, but if you want something as small as a coin cell, you will need to solder very tiny sensor. Or if you want to use sensors eval board and stack on it, it adds overall thickness (which is one pros of coincell, because except this advantage coincell is less available than aa and less power). Why not group all your needs on one board (so you factorise radios cost...) and benefit of aa/aaa cells power. just my point of view, we all have different goals 😉
    If you really want something small, coincell:

    can't beat these 3 small designs!

    For the PIR sensors, I don't know about reliability in low voltage..because I won't use common chinese pir module (I will use a dedicated ic for this, so it's a smaller design). Plus these are very big I think, that completely break the concept of very tiny nodes...

  • Mod

    Drag soldering:
    This is how it should be done 😉

  • Hardware Contributor

    that's how I do it too. always have flux and desoldering wick near you 🙂 (for the wick the best is not too much wide, I have multiple size). and now I have my reflow it's more magic lol
    good other youtube channels to learn soldering smt parts (and lot of other things too) : curiousinventor, eevblog...

  • Hardware Contributor

    I use the cheap Chinese mini PIRs at low voltage like this:

  • Hardware Contributor

    @m26872 : yes these are the mini version, you're right 😉 same thing as the big version as it uses biss0001. in theory biss0001 is rated for 3v min voltage. what is your min reliable voltage? I know it can work under 3v but when I did some tests I often had some trouble or false trigger (maybe bad modules) so I gave up on these.. this is why I choosed another controller on my side+a booster (less power, handle blindtime, internal ldo etc.. but not as cheap). and I know my one cell case/goal is a little bit exotic. just curious to know if these works well when your batt level decrease near 2v? The controller I plan to use is 2.7v min in my case..

  • Hardware Contributor

    @scalz It worked flawlessly down to 1.6V. I didn't test below that as it's more than enough for me.

    Edit: The false trigger is very predictable correlated to power line disturbing activities like nRF wakeup and thus easily blocked in the sketch. I didn't see increased sensitivity due to low voltage as one could expect. I have been on holidays etc but never seen a single false trip or missing trip since middle January.

    Edit2: It's very sensitive btw. I don't think it's easy to power it by anything less stable than battery.

  • Hardware Contributor

  • Hardware Contributor

    @LastSamurai, I'm close to completing this coin cell switch node. You can use the files as reference if you so wish. I'm waiting on a couple of part to arrive to just attempt to iron out the seeial uploading issue and battery monitoring, software style. But other than that i have it pretty low on power consumption (need to carry out some accurate tests on the actual consumption), very small footprint with all SMT components.

  • Hardware Contributor

    @m26872 : cool you can go as low as 1.6v with it 🙂 i was using 2xaa for the pir. no matter now i have enough boards to play with the nice e931.96 (smart with internal ldo regulator 😉 new at mouser but not available yet. I had luck to find cheap lot at aliexpress because at mouser 4e/unit!
    @GertSanders nice 🙂

  • Hardware Contributor

    I am currently working on my own version using some of the ideas from all the other concepts.
    I am including a stepup converter (can be bridged if I don't need it) for some sensors that require 3.3V or even 5V. I'm unsure how to control the power usage though. Wouldn't the sensor and the boost converter draw power even when the atmega and nrf are sleeping?
    I thought about controlling it with a transistor but how much power do they draw?
    Has anyone used a setup like this and has any tips?

  • Hardware Contributor

    I have created my own sensor baseboard which can be used with pretty much every sensor and which is quite similar to the sensebender. I have 3 of them running right now (one plant soil sensor, one humidity/temp sensor and one motion sensor). Battery life seems to be fine with a coincell although perhaps not perfect yet. During the next days I will add it to and write about it in more detail here.
    For now the code and schematics can be found here:

  • Hardware Contributor

    As promised here is my own try at creating a sensor platform based on mysensors. I have several nodes up and running for some weeks/months without problems at the moment 🙂
    Battery life with CR2032 depends on the kind of sensor used but should be 1,5 + years on everyone.
    Love to hear some feedback and/or suggestions on what kind of sensor to add next.

    I think I will add some door/window sensors next

    link text

  • Hardware Contributor

    looks nice, congrats @LastSamurai i'm sure you enjoy your work 👍

  • very cool 🙂

    Will you provie PCB at ?
    As soon as you do it I order PCB 🙂

  • Hardware Contributor

    @scalz Thanks a lot! I sure do. It's great to see that others (which gave me a lot of help here) can use my work too 🙂

    @carmelo42 Thank you too! Here is the link. They are running just fine like this but remember that the CR2032 battery holder mounting holes on the pcb aren't perfect yet (see project page).

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