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Thread: Appallingly small LiPo charger add-on for Teensy 3.1

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  1. #1
    Senior Member onehorse's Avatar
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    Appallingly small LiPo charger add-on for Teensy 3.1

    FYI

    The LiPo charger add-on for Teensy 3.1 has been added to my store at Tindie.

  2. #2
    Senior Member pictographer's Avatar
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    Sweet! This will fit perfectly in a couple of projects I'm working on.

    Would you be willing to post some documentation, like a PDF of the schematic and description of all the I/O? I see that the schematics and board file are available at OshPark, but I bet I'm not the only one who would like to read a bit more before firing up the tools.

  3. #3
    Senior Member onehorse's Avatar
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    Of course, I will post a schematic and description of the pinout of the board tonight. The board is designed straight from the "typical application" circuit from the data sheet. I had posted schematics on previous items until I realized they were accessible on OSHPark as you point out. But I agree, the more info the better!

  4. #4
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    That's nice!

    Thanks.

  5. #5
    Senior Member onehorse's Avatar
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    ...who would like to read a bit more...
    Added some details and the schematic/board layout. If more info needed, just ask!

  6. #6
    Senior Member pictographer's Avatar
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    Thanks for the details!

    I ordered a few of these from Osh Park and now I'm trying to order the components from Mouser. Was able to find everything except the switch. Closest I found was SSAJ120100. Is there a typo in the BOM?

    Also, I'm wondering about omitting the power indicator LED since the Teensy will be in an enclosure and there will be other ways to tell it is on. Looks like I can just omit it and omit the 330 Ohm resistor for it?

  7. #7
    Senior Member onehorse's Avatar
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    You may omit the green LED and the corresponding 330 Ohm resistor. Here is the switch I used:

    http://www.mouser.com/Search/Product...688-SSAJ120100

  8. #8
    Senior Member pictographer's Avatar
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    Assembled one board. Wondering how to troubleshoot.

    Tried briefly plugging in USB but with no battery connected. No signs of life. Verified that 5V was making it to the LiPo Charger board. Verified that I'd successfully cut the Teensy 3.1 connection between VIN and VUSB. Didn't feel anything getting hot or smell any smoke. Tried the switch in both positions. After a few minutes, I started wondering if I'd killed the T3.1, so I cut the four wires I used to temporarily connect the two boards. Added a blob of solder to reconnect VIN and VUSB. The Teensy returned to life.

    My assembly job leaves a bit to be desired. I soldered everything manually with a decent iron, but not the ideal tip. I'm uncertain that I succeeded in soldering the switch. I didn't have the 100K resistor so I improvised with two 47.5K resistors in series tented across the pads. My cheap DMM says 928K Ohms for the pair of resistors. Checked especially carefully for shorts between the resistors and the caps.

    Click image for larger version. 

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    Possible next experiments:

    • Test with a dummy load, not a Teensy 3!
    • Battery only
    • Battery and USB
    • Wait for the proper 100K resistor and replace the "tent"
    • Wait for the proper 100K resistor and assemble a new board
    • Assemble a new board with wires in place of the switch


    I don't have an oscilloscope or adjustable power supply, though it could be time to buy more tools.

    Suggestions?

  9. #9
    Senior Member onehorse's Avatar
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    I have had trouble with soldering the switch and I suspect that is where your problem lies. Check the switch function with a voltmeter. Make sure the tiny connections on the back of the switch are not solder bridged and that the middle pad alternatively sees high impedance and zero ohms when tested against the VIN. I am assuming you had the ground pin connected. I only ask because the first time I hooked one up to the Teensy I forgot to solder this pin!

    As far as the design is concerned, the 100 kOhms is not critical. 47.5 kOhm would work just as well. It is only to pull up the active low CHG signal from the MAX1555. The 100 kOhm is to limit the current, but 47.5 kOhm or anything conveniently high will be fine.

    This board works as designed, I have verified it many times. If you can manage to get the switch soldered properly with a soldering iron you are a better man than I!

  10. #10
    Senior Member pictographer's Avatar
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    Thanks for the quick reply!

    As you suspected I shorted the switch. Unfortunately, in my testing since my last note, I swapped + and - on the battery connection and let the smoke out of something, presumably the MAX1555.

    Next time I think replace the switch with a wire bridge and test with dummy loads instead of the Teensy.

    I have no doubts about the board design and I'm grateful you published the design and answered my questions.. I've got two boards more to go. If I still can't get it to work in two more tries, I'll just buy a finished ones until I manage to improve my tools and skills.

    I can see the value of having a power supply that allows you to dial in a current limit.

  11. #11
    Senior Member pictographer's Avatar
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    TL;DR: Got the board to work by soldering wires to alternate locations for GND, VIN, and Battery '+'. The microswitch is near impossible with an iron. Good learning experience and good product!

    Here's the long version for what it's worth.

    The signal pads for the switch are entirely underneath the switch. The switch can't be repositioned much because the pads are tiny. The tight spacing between the signal pads and the metal case add to the difficulty.

    I had no trouble soldering 0603 components or the gullwing MAX1555. The iron I used was a temperature-controlled Weller with a conical tip that might be a bit large for the job. I used 23 gauge leaded solder. I tried very lightly tinning the pads and using copper braid and Chipquik SMD291 no-clean paste flux. I didn't have a good solution for holding the switch and board in place while soldering.

    On my second attempt, I tried starting with the switch because it is by far the hardest. I even tried soldering 27 gauge magnet wire to the pads instead of the switch, but the pads were too small and too close even for this. Finding alternate soldering locations on the schematic worked much better.

    What worked: soldering wires to GND, VIN, and Battery '+'. To turn on the T3, short Battery '+' to VIN. To turn off the T3, disconnect battery '+' from VIN and short VIN to GND for equivalent behavior to the switch or simply leave VIN disconnected from Battery '+'.

    Pro Tip: Test the board using a dummy load such as an LED and resistor rather than your Teensy.

    I haven't figured out if the battery charges when switch is "on". This seems to be implied in the Tindie description if I'm interpreting it correctly. However, since the battery passes through to VIN, it seems to me the device load would confuse the battery charge monitoring.

    I used 1K Ohm current limiting resistors for the LEDs and they are still satisfyingly bright.

    Wishlist:

    • An alternate microswitch with gull wing leads or at least bigger pads for the microswitch.
    • An alternate hole to solder a lead for Battery +. Maybe one of the N/C holes. Ideally, three holes aligned for a three-pin header.
    • A method to control the charge rate.
    • More space between the LEDs so position helps indicate the mode.
    • Protection diodes, maybe? Could make assembly and testing safer.
    • More and sharper pictures in the documentation with arrows and labels.
    • More layers used in the layout.


    Overall, assembling One Horse's LiPo charger was a challenging yet satisfying learning experience and I'm happy with the result. Now that I've been through it, I'll probably buy assembled ones in the future. Many thanks, One Horse!

  12. #12
    Senior Member onehorse's Avatar
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    I appreciate the detailed description of your efforts.

    I agree, the switch is the weak link in the assembly of the design. It's a trade off between very low profile and difficulty/robustness of assembly. In general, reflow soldering is the way to solder the switch. Still, I am tempted to make the solder pads twice as big to aid it since it requires a lot of solder paste relative to the area of the pad to get a good bond, and precise alignment. Both can be achieved routinely now (I have built more than a dozen successfully), but not with hand soldering! If I could find a switch similarly thin with through-hole mounting I would use it. Suggestions anyone?

    The board is designed specifically to mount on the Teensy 3.1. If I had intended a generic battery charger I would have done things very differently. It is a minimal design in that it provides 100 mA of current to charge a battery through the USB; the DC charging port is just there since it is a capability provided by the MAX1555. I never really intended anyone to use it since it isn't convenient to put a DC jack there. But if you wanted 280 mA charging current, that is where it would come from.

    I might start using 1 kOhm resistors for the LED on the charger since I have been using them on similar boards and I agree, the leds are plenty bright.

    I will replace the pictures with better ones but I must say taking clear pictures of a quarter sq. inch board is not that easy with an iPhone!

    Right now I am limited to two layers but hope to graduate to four or six layers soon.

    Lastly, I would just say that this is the least capable battery charger that is still useful; I agree that there is plenty of room for improvement and I appreciate the suggestions. Feedback makes for a better product and I will take your recommendations into account in version 2.

    Sometimes I am just amazed it works at all!

  13. #13
    Hi, I'm a new developer and wanted to clarify some things about your LiPo charger for the Teensy before I go ahead and buy it. This is what I understood from your page; I was hoping someone could confirm it for me:

    1) Once the add-on is attached, the micro-usb port on the teensy can still be used as a serial port and can upload code, but will not supply power to the teensy. Instead it will charge the lipo battery.
    2) The only pins used are Vin, GND, VUSB, and RX1 (pin 0). If I am currently using pin 0 for my project, do I need to move what I have attached to pin 9 (a Serial2 pin)?
    3) The board attaches underneath the teensy. It looks like it fits very nicely. Once this is attached, can the teensy no longer be placed onto a breadboard?
    4) Can the header pins of AGND, 3.3V, and TX1 really just sit through the add-on board with no risk of an unwanted connection, like in this picture?

    Click image for larger version. 

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    5) What LiPo batteries work with this add-on? Would it be outlined on the MAX1555 data sheet? Also, does anyone have any recommendations for LiPo batteries (or other external power sources) for a teensy project that requires not much current (sorry I don't have an exact amount--includes bluetooth module and a small motor) but that values small size and easy charging?

    Thanks!

  14. #14
    Senior Member onehorse's Avatar
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    1) Once the add-on is attached, the micro-usb port on the teensy can still be used as a serial port and can upload code, but will not supply power to the teensy. Instead it will charge the lipo battery.
    Yes

    2) The only pins used are Vin, GND, VUSB, and RX1 (pin 0). If I am currently using pin 0 for my project, do I need to move what I have attached to pin 9 (a Serial2 pin)?
    Yes, but you can a) not solder the CHG pin and b) cut the thin trace on the LiPo charger so there will be no interference with your RX1 usage.

    3) The board attaches underneath the teensy. It looks like it fits very nicely. Once this is attached, can the teensy no longer be placed onto a breadboard?
    It dependes, if you use extra long headers it would fit in the traditional way; you could also mount the charger on the top of the Teensy but it would be harder to see the leds!

    4) Can the header pins of AGND, 3.3V, and TX1 really just sit through the add-on board with no risk of an unwanted connection, like in this picture?
    I don't see why not. There is no electrical connection whatsoever between these through holes and the charger circuitry.

    5) What LiPo batteries work with this add-on? Would it be outlined on the MAX1555 data sheet? Also, does anyone have any recommendations for LiPo batteries (or other external power sources) for a teensy project that requires not much current (sorry I don't have an exact amount--includes bluetooth module and a small motor) but that values small size and easy charging?
    Pretty much any 1 cell LiPo battery can be used. The typical LiPo is 3.7 - 4.2 V and varies from 40 mAH to 1800 mHA or more. I like the smaller ones available at Sparkfun and I really like this one since it is small enough to fit within the Teensy form factor and with very low power applications enables a nice portable (wearable) device.

    I forgot to mention that I have sold more than 150 of these so they seem to serve their purpose well enough.
    Last edited by onehorse; 03-08-2015 at 05:59 PM.

  15. #15
    Great, thanks for all the info! And thanks for such a prompt response. It seems that this will be great for my project.

    Another quick question that came up--I want to turn my teensy on and off by a switch that is not on the board (my project will be held in a case, so I can't access the on board switch). Is the only way that I can stop the teensy from running code (and therefore stop the battery from draining) by using a switch before Vin? To do that, would I have to not attach the add-on to the header pins and instead solder wires, soldering a toggle switch between the Vin connections?

    Thanks again to whomever answers!

  16. #16
    Senior Member onehorse's Avatar
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    I think you will have to solder the toggle switch.

    However, if the USB is accessible when mounter in your case, would the on-charger switch just underneath it not also be accessible?

    If the USB is not accessible, what's the point of having an on-board charger then?

  17. #17
    I'm 3D printing a case that will leave just the usb port and an on/off switch accessible. I could maybe make it so that the on-board switch is reachable too, but I was hoping to use a larger switch to make the product more user-friendly. It seems that in order to make the on-board switch accessible, I'll have to expose some of the board and the user would have to use quite a small switch. I'd certainly like to be able to mount to board onto the teensy, but perhaps an additional switch is necessary and I can still hide away your board quite nicely. Thoughts?

  18. #18
    Senior Member+ MichaelMeissner's Avatar
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    Depending on your soldering skills, I could also imagine de-soldering the switch on the lipo charger, and running the 2 wires up to the case, or just soldering wires to legs of the switch, and leave the switch in the off position. But it probably simpler to do as you say, and not attach the charger directly to the Teensy.

  19. #19
    That's a good point. Pictographer said earlier on this thread that soldering the microswitch proved quite difficult, but perhaps I can grab a friend who is more skilled, or just not attach the charger directly to the teensy. With those two options, I should be able to work something out.

  20. #20
    Senior Member onehorse's Avatar
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    You could also order the charger without the switch and solder wires on the pads instead, which is a much easier task than soldering the SMD switch. One person has ordered them this way for exactly the same reason. Just add a note with your order if you want the switch unpopulated.
    Last edited by onehorse; 03-09-2015 at 04:01 AM.

  21. #21
    Great, I'll do that! That's pretty good service there. Appreciate the help.

  22. #22
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    It looks fantastic. I really like it.

  23. #23
    Senior Member+ Frank B's Avatar
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    I just noticed you great display.

    Some years ago, i made a NIXIE clock with an avr - it still works good. This display would be a great extension (together with an updated MCU, Teensy LC..)
    I hope i can find the old schematics somewhere in my chaos.. :-)

    EDIT: Sorry, wrong Thread
    Last edited by Frank B; 03-15-2015 at 08:56 PM.

  24. #24
    Senior Member onehorse's Avatar
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    The thing I like about the bubble display frontpack is that it only takes two wires for sixteen digits. Very efficient.

  25. #25
    I ordered this without the SMD switch and am going to solder wires to the pads in order to connect my own switch. Before I do that, I just want to make sure I'm doing this right. Which of the five pads would I solder wires to in order to add my switch? Thanks!

    Click image for larger version. 

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