๐Ÿ’ฌ MySensors RFM69W multisensor node(CR123)




  • Hero Member

    Hello, that's a nice tiny project !

    I have 2 questions :

    • where do you get these battery holders ?
    • why do you use a dc-dc converter ?

  • Hardware Contributor

    I did the same on my nodes : The main idea is to get a consistent 3.3V to supply the atmega and radio module, whatever battery voltage you have. So, If you decide to power it with a 3V lithium battery, it will still be giving the radio and CPU 3.3V , and even when the lithium battery will start failing, it will continue to work down to 1V !
    I can even use a single AA battery to power my nodes. I use a MAX1724 step-up converter, which start working from 0.8V ! At this voltage, any AA battery is almost 100% empty ;)


  • Hardware Contributor

    @Nca78

    Battery holder: http://www.tme.eu/en/details/bhc-cr123a/batteries-holders/comf/

    It's out of stock now but it will be available soon. Regarding the step-up converter @napo7 already provided the answer:). Now I don't think that it will work down to 1V or below because when the battery voltage reaches that level then the internal resistance is also pretty big because of the internal chemistry stuff. And when the internal resistance has a significant value then the power delivered by the battery is weak too so it can die instantly because the step-up converter will try to get even more in order to keep the output level required by the load. In short yes, a step-up converter will try to get all the juice from the battery but that's only in theory and as we all know the reality is not so tolerant:) unfortunately. So now you wonder why I still choosed to use a step-up converter anyway: well it's still better than nothing imho and I can stay assured that I get the constant 3.3V output no matter what.


  • Hero Member

    @mtiutiu well my humble opinion is the step-up converter with a lithium cell is a waste of energy and battery life.
    Basically at 2.4v lithium cell battery is nearly dead, and at 2V you can't get anything from it. With updated bootloader atmega can easily run down to 2V. So can NRF24 (1.9V) and si7021.
    With increased voltage, these components will also naturally increase their current consumption, so in addition to a bit of waste from the converter you have more waste because of the increased voltage.

    For AA / AAA the boost converter makes sense, for lithium cells it's just a burden and a hit on the battery life I think.

    Just realized I posted on the wrong subject, my message was intended at the CR2032 sensor, which has a hefty reserve capacitor already so should run for a veeeeeery long time without the booster.


  • Hardware Contributor

    Yes, you're right for the NRF which can run from 1.9V, but I don't know for RFM, which can also have higher power consumption ?


  • Hero Member

    From datasheet it can run at 1.8V and they claim "Constant RF performance over voltage range of module" so there should be no problem.


  • Hardware Contributor

    Do you know what is the influence on the consumption to use a step-up converter ?

    Edit:
    Oh, sry, i am just seen the nca answer and i found lot of informations about the use of step-up converte.
    But if we use a step-up converter with an AAA battery, do we increase the current used in a sleep mode ? If yes what about the magnitude (mA? ยตA? nA?) ?


  • Hardware Contributor

    Yes, the step converter has an efficiency, ranging from 50 to 90 % regarding the output current and input voltage.
    There is also a quiescent voltage inside the converter, so the converter will ALWAYS eat some current, either when the devices are sleeping !

    The pros and couns should be measured : I hope we can consome "more" of the battery with the converter (till a lower battery voltage), but perhaps due to efficiency of the converter, we'll have wasted more power !


  • Hardware Contributor

    if you want to know efficiency of your converters i put the formula there if you want ;)
    https://forum.mysensors.org/post/52319


  • Hardware Contributor

    @scalz Thanks you a lot scalz, exactly what i want ;)


  • Hardware Contributor

    Good reading for dc-dc converters when designing for low power: http://www.digikey.com/en/articles/techzone/2014/mar/the-advantages-of-pulse-frequency-modulation-for-dc-dc-switching-voltage-converters

    L6920 can operate in both modes(PWM/PFM) and it will switch to PFM on light loads. The quiescent current is around 10uA in general for this device as the manufacturer promises. But and there's a big BUT: in reality a dc-dc converter will always have a drawback on battery life to some degree as stated by the other users that replied in this thread so it would be ideal to accomplish things without it. I wanted to test it in real life anyway and that's why I used in my designs. For the CR123 based board it can be bypassed using the provided smd jumpers. On the other design(CR2032) which is more critical I will provide that too in the next hw revision.


  • Hardware Contributor

    I was about to make tests with the 2 same modules, powered by a single CR2032 coin , a boost converter (MAX1724) for one node, and nothing for the other.
    Since those readings, I'm not sure that the boost converter is needed anymore.

    Just to prove myself and my friends that it is useless, I'll do the test, with each node waking up the same amount of times and two fresh cells (I'll have to reduce the sleeping time for the test to be quick enough...).

    I'll post the results on the forum, as soon as the batteries will die, in a few month ;)



  • That is one sexy board! I am humbled by the 3d kicad outputs... My question is how did you make the 3d-files? Do u have any good explanation for getting these done? I found nothing really recent..


  • Hardware Contributor

    Launch the 3D Viewer from PCBNew, then do screen-shots ;)



  • yeah :) alt+3 is one off my favorites. But for example the rfm69W module and the battery holder 3D wrl-file are hand mande. I guess. Do you have a lean toolchain for this? I found so may multi-programm toolpath..


  • Hardware Contributor

    @mr_red

    The rfm69w module was made by me in FreeCAD. Some of the 3d models are made by me when I don't find one online. You can find lots of 3d models(step files) here: www.3dcontentcentral.com and www.tracepartsonline.net. Then you need to use FreeCAD to import the kicad footprint and the downloaded 3d step model to align it with the footprint. I use the kicad stepup tools FreeCAD macro for this: https://sourceforge.net/projects/kicadstepup/. Then you can export the scaled wrl file using the macro dialog. It requires some work to get all things done but it works just fine(patience is your friend here or should be).

    The big advantage of using the kicad stepup tools FreeCAD macro is that you can have both the step and wrl files in place for exporting the board afterwards for mechanical CAD integration. Because in the end you'll want maybe to do some 3d printed enclosure for the project. For now kicad stepup tools is the best friend in doing electronics to mechanical CAD exports in order to design the plastic enclosure(a big congrats to the authors for making that possible).



  • Thanks for the nice rundown! I will try that.
    Yes the mechanical integration is my problem. I have quite a tight case to fit a sensornode with some other parts and I am actually routing parts so I have a nice flat area on the baord
    I also need to feed in my own CAD for some oddball parts.


  • Hardware Contributor

    @mr_red

    I have a zipped snapshot of all the 3d step/wrl models that I used to build my projects so far. It was quite some work to gather them all and align with the footprints using the kicad stepup tools. It's not complete - still needs some more work:simple_smile: but it should help you or anyone else to get started into MCAD integration using FreeCAD and kicad stepup tools. Maybe it's not very easy to start when using free tools but hey they are "FREE" so we cannot ask for more. But the nice thing is that once you get used it will be really easy to do your stuff and to go forward. Oh and FreeCAD can be so powerful after you learn it - I was really amazed what you can do with it(it has it's quirks and downsides but once you master it - oh, well...).

    There's also: https://www.onshape.com/ - pretty powerful this one too and it has more advanced features than FreeCAD but the downside is that it's a cloud based solution: it's free for public documents but you never know when they will go to paid accounts only so yeah...I don't really like the idea to depend on something that it's hosted and the application is not on my PC along with the documents that I create with it. Cloud solutions have their flexibility but you're locked and you depend entirely on that hosted service.

    Ok, enough talking...I uploaded the zip file here: http://www.mediafire.com/file/kzh1l9uo40cpj1u/kicad_stepup_packages.zip



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