My Slim 2AA Battery Node

  • Hardware Contributor

    Board releases
    (other colors might be selected when ordering)

    • Version 2.0 (black) [order] Now designed in KiCad. "Final release". I'm not developing it further atm, but I know others have some projects going.
    • Version 1.4 (red) My latest version in Eagle. Known issues are wrong references due to panelization and broken circuit diagram links.
    • Version 1.2 (blue) Some less convienient placed components and the panelized verision has a faulty via.
    • Version 1.0 (green) The one described below in this first post. Working but not panelized and lacks a few features.

    Share stats and info
    The panelized versions 2.0, 1.4 and 1.2 have until today (2016-11-12) been shared 138, 13 resp 2 times at boardhouse. Together with a few shares of the non-panelized version and my own orders, and the usual 3x10-11 boards/order, it means a lot of boards! Guess very few build nodes with every board, but at least the design should be well proven by now. This also means a few $ to, since $/order will be donated. Great thanks to everyone who has orderd this board! I'll keep this share-info updated for transparency purposes. IMPORTANT: Please understand that is a non-profit community service, with a lot of manual support required. So please be patient and nice to their support in general. A new site is under development. Read more at their support site. EDIT 2017-06-22: Despite the new site it is still a hassle every time to get a reply from them and then the share credits. If anyone have some more info on this, please let me know.

    This project describes a successor Node concept to my first 2AA battery sensor. I have combined a few simple design options to a result that I find rather useful myself and I think should be shared. The application specific sensor/-s of your own choice has to be added to this Node design, nor here any example sketches provided here except from a few links further below. I use this design for all my door and window reed switches, temperature (calibrated internal or thermistor), LDR and similar simple sensor types. But, nothing prevents the use of more sophisticated sensors like Si7021 here as well. A few links to sensor examples based on this node will be presented further down in this post.


    • Simple, in the sense that it consists of a minimum number of components and common available material.
    • Cheap regarding choice of components, assembly work effort, energy storage and power consumption (battery type and life time).
    • Flexible universal design base equipped with various sensors. PCB pads used as port connections or prototyping area for extensions.
    • Small and discrete to fit in confined spaces and to reach WAF level

    And more concrete:

    The uC
    A "bare bones" ATMega328p 28pin PDIP (with or without socket). Bought from here and here. My reasons to not use Arduino Pro Mini here are

    • The APM width is too big.
    • APM has no prototyping/near connection area. There's no spare pads for separate connections unless you accept to use pads connected to softwise inactive ports.
    • Radio module connection has to be manually made to the APM.
    • Low power hacks like removing power led and voltage regulator are needed.
    • Necessary support components (resistors and capacitors) are few and can easily be added to a custom pcb.

    Since I prefer Arduino IDE for programming (flash) and debugging, I need a bootloader. Bootloader instructions are found all over the internet, but here's anyway how I do it. I use this precompiled bootloader from here. It's an Optiboot with 1MHz internal clock and 9600 baud serial communication. Fuse changed to BOD disable. According to this you should use minimal startup time to reduce power in every 8s sleep cycle, but for the moment I don't care and stick to the default 65ms. I use Avrisp mkII avr programmer for fuse and bootloading similar to this procedure. Arduino as ISP, Avr/USBtiny or whatever any other should of course be just as good. Avr Studio 4.19 is a good choice for Avrisp mkII (perhaps for others too) and 4.19 is the last version before the gigantic (and for me useless) IDEs were released.
    I add this new board to my "boards.txt". Fuse settings, don't forget to set the lock bits. If programming a large batch, the ELF production file is handy.
    Here's a great tutorial for those who use Arduino Uno as ISP.
    UPDATE 1: Today (2017) a lot has happen since I wrote about this. Some things has made it easier for us. A very good selection of precompiled bootloaders is now found here at MySensors. And you don't need to (and shouldn't) mess with the boards.txt any more. Instead I recommend the installation of MiniCore to the Arduino IDE.
    UPDATE 2: There have been reported issues with MySensors 2.x freezing on SlimNodes running at 1MHz, which I've confirmed. Recommended solution when using MyS 2.x, is to use 8MHz (internal) instead.

    The Radio
    A standard NRF24L01+ radio module is used. The width align with the AAs and no mods is needed (like with my other one). As always I try to keep the antenna part of the module free from shading metal.

    The Board
    At first I planned the build on a proto board, just to stick to the cheap-and-standard concept. But with today's low prices on custom made PCBs, it wasn't any longer an option. Space, quality and work effort are so much more attractive.
    Latest design files are open and available at the site. Please click on the image-link below to access where all design files such as latest BOM, kicad-files and circuit diagram (pdf) are found.

    Board (v2.0) Top Side:

    Board (v2.0) Bottom Side:

    The Enclosure
    UPDATE: If you dont't like my primitive casing descibed below, in this post the user @buxtronix made a nice 3D-printed case which you can find here.

    An important overall part of this design idea was to align minimum dimensions of the components and get rid of "expensive" parts like battery holder. It turn out (see below) that the enclosure's functionality as battery holder wasn't needed even though it was the initial idea. The cable duct case has been discussed earlier, but rejected by some due to lack of ways to seal the endings. I still haven't the perfect solution, but I've since many years simply used (cheap) white tape. With some care it looks ok, and still does 5-10 years later. There are often proper terminators/endings to buy, but for some reason to unrealistic high prices.
    I used this cable duct with the dimension 17x20mm. Unfortunately it turned out that this particular type I used (Thorsman TMK T20) is now "professional grade" and dimension 17x20 is no longer very commercially available for consumers (here in Sweden at least). Eg. to get it, you have to pay >5$/m from places like this or buy it in bulk (50m) from a professional store (preferably as a professional with discount). The 50m bulk batch will give you 263 sensor nodes of standard length (19cm).
    Standard consumer dimension cable duct is e.g. 15x15mm from what I've seen. It'd be nice to design a 2AAA node in that one. If only there is a thin radio module? (Future project.)
    box1.jpg box2.jpg box3.jpgbox4.jpg

    The Battery pack
    Easy home made 2AA battery pack. Maybe it looks more demanding and time consuming than it is. (Usually its the other way around in my experience.)

    1. Start by taping the two (connecting) batteries together.
    2. Prepare the wires and make a small bun at the battery connecting ends.
    3. Attach the wires with tape.
    4. Tighten the cable ties and carefully note
    • that the wires are pressed to make good contact with the battery poles
    • how the cable tie ends must be placed to not steal lateral space
    • that the wire from the bottom must be routed near the cable tie to not steal space.
    1. Make the pack more rigid by taping one or two times around at the top, bottom and middle.
    2. Trim wires and solder the female connector. If desired, leave at least a small part of one wire naked for current measurements.
      A battery change is done fast when cables a already made (use solid wires that preserves its shape). So why pay for a battery holder when you can remake a pack with fresh batteries in 1-2 min and your low power sensor will live 5-10 years before anything needs to be done?
      bat1.jpg bat2.jpg bat3.jpg

    The Interface/Connections
    Convenient there's the 6 pin standard serial interface exactly like on the Arduino Pro Mini. Perhaps it's mirrored here, but I think everybody double checks Gnd and Vcc before connecting. The Vcc and Gnd pins also serves as a connector for the battery pack. (CTS is connected to GND on the PCB.)
    "Under" the radiomodule are pads for the ICSP pins. The idea was to have a socket for the radiomodule instead of the "expensive" 328p socket and still have easy future access to the SPI/ICSP interface. Perhaps not very useful. But nice to have Gnd and Vcc in this end of the board for general purpose.

    The Sleep Mode Power Consumption
    I measured the sleep mode current draw to be 1.5uA when it's set to interrupt wake up and 5.8uA when it's set to timer wake up.
    power1.JPG power2.jpg

    Sensor Examples and more
    Reed Switch Sensor: post 116
    Humidity Sensor: Slim Node Si7021 sensor example
    Motion Sensor 1: Slim Node as a Mini 2AA Battery PIR Motion Sensor
    Motion Sensor 2: Slim CR123A (2AA) battery node..
    Scene Controller: Slim Node scene controller/ keypad
    (work in progress to collect more examples here)

    Not Sensor exemples, but some nice to see "node variations" from @AWI:
    Here (post 88) and here (post 233). And now there's also @AWI 's My Slim 2AA Battery Node Tools.

    Still not "slim" enough? Check out Very Narrow and Minimal Switch Node ! by @GertSanders

    And also, there's this 5V-slim-node a 5V-slim-node mod by @Soloam

    Feature Requests
    Here's a collection of suggestions and development ideas for future versions of the board (or other parts). If anyone else make their own board where some of this is included, I'd be happy to reference it from here.

    • Pin labels/references also on board top side.
    • Turn the nRF footprint to make the assembly shorter.
    • Make the board suitable for the nRF SMD version.

    More Pictures
    Some photos. First a comparison next to My (old) 2AA battery sensor, one painted and one not. (Note the high WAF of the colour even without the paint.) Then some placement examples. Reed switch nodes for all my doors and windows are my first priority.
    20150901_220448.jpg 20150901_220505.jpg 20150901_220659.jpg 20150901_220847.jpg 20150901_220948.jpg IMG_2065.JPG IMG_2063.JPG IMG_2064.JPG

  • Nice job and give me another tool to use in my tool box. Thanks.

  • Hardware Contributor

    Very nicely done ! Great idea, makes the sensor blend in. Fortunatly in Belgium we do have access to cheap cableducts. I did not think of using them as a case, but given that we have access to various widths, I will check.

    May I ask, which boardhouse made the PCB's for you ?


  • Hardware Contributor

    @GertSanders I ordered from DirtyPCBs. It was 51mm but it didn't complain.

    Hm. I can now see few strange spot in one of the PCB-photos above. I'll look into what that can be...

  • Hero Member

    Thanks for the detailed information about this project! I was/am looking for a similar solution to check whether doors are opened or closed.
    Have you ever considered drilling a hole in the top of the door, and placing the entire node inside this drilled hole like the video below? Similar to this video?
    That was something I planned to do, but your solution seems really nice as well in case drilling a hole in the door doesn't work out as expected.

  • I see u don't use a external 8mhz quarz. Because any technical reasons (=low current?) or for reasons of space?

  • Hero Member

    @m26872 Great design, did you share the design on dirtyPcb's?

  • Hardware Contributor

    @aproxx I've never thought about it, but I think it's an excellent idea for some situations.

  • Hardware Contributor

    @AWI No, but the Eagle files are attached above. I'm new to all this open hw/sw sharing. You're very welcome to tell me how I should do it the right way. PM if you like.

  • Hero Member

    @m26872 When you check out your order you can set the "sharing" options:
    upload-821ec565-96e5-4214-baa4-72564a63c07a and even get a small profit (or a donation to MySensors..)

  • Hardware Contributor

    @m26872: very nice idea 😃
    just a little question, is your cable duct pvc. I think so. Alloy maybe would not be great for radio...
    yoyur idea can be very helpful thx for share.

  • Hardware Contributor

    @scalz Yes, it's pvc.

  • Hardware Contributor

    @Harrdy said:

    I see u don't use a external 8mhz quarz. Because any technical reasons (=low current?) or for reasons of space?

    Low power, low cost, less space, less work. Stability might be an issue at different ambient temperatures etc.

  • Hardware Contributor

    Added a boardhouse link if anyone would like to order the board straight from there (thx @AWI). Couldn't see any cons.
    I think it's enough tested, but personally I'd wait for a tested v2.0 or make suggested improvements myself first.

  • Hardware Contributor


    great job! I think i will use your board to for small window -sensors.
    But one question: i cannot upload sketches via arduino-ide when the 1mhz bootloader is flashed. i checked the wires and everything else and i tested a 8mhz/internal bootloader - works fine. but the 1mhz-bootloader just does not accept uploads?

  • Hardware Contributor

    @ahhk There's also the precompiled bootloader for 4800 baud in the link above. Did you try that one too? I think 9600 baud was maximum serial speed at 1MHz. Lower should be safer.
    Or what do you mean with "does not accept uploads"? Is it locked?
    Double checked boards.txt settings?

  • Hardware Contributor


    i can upload via RX/TX with arduino IDE with the 8mhz/internal bootloader. but if i flash the 1mhz bootloader from the link above (atmega328_1a.hex), i cannot upload the same way...

    TX/RX is connected correctly. I can upload a sketch via "Upload with programmer" and receive and transmit characters with 9600baud. tested with the example-sketch "SerialEvent".

    IT is a brand new 328P - not locked...

    boards.txt shows

    ############################################################## Optiboot internal 1MHz noBOD 9600baud

    i didnt try the 4800baud bootloader until now....i will do now...

  • Hardware Contributor

    @ahhk Reset issue? Maybe try other startup times (low fuse 42h or 52h), other usb2serial or adjust reset R and C.

  • Hardware Contributor

    For info. I've prototyped and ordered a slightly improved version of the pcb. Note that this is UNTESTED and experimental.
    Eagle brd-file and board house link. Use it for inspiration. Don't expect it to be and tested until next month at least.

  • Mod

    Great work @m26872
    Thanks for sharing!