EEVblog #130 – The µCalc Credit Card Scientific Calculator / Computer

Dave describes the design of his open source hardware µCalc credit card sized calculator / computer.
(NOTE: Not to be confused with the program uCalc
Specifications: PIC 24F 16 bit Processor, 256KB Flash, 16KB SRAM, 64KB serial flash user memory, 3-Axis tilt sensor, Micro SD card, capacitive touch sensing keypad, 128×64 dot matrix display.

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  1. Do you have a trick for your SMD soldering work? I’ve seen various tutorials on sites like sparkfun, but for a one or two unit product the by hand method always sounds a little sketchy.

  2. Nice video, Dave. Thanks for waking me up! Thumbs up!

  3. Which circuit design software are you using / what would you recommend for hobbyist?

    Also interested in Luke’s SMD question, what do you think about products like the SchmartBoard for prototyping?

  4. About SMD IC soldering: you have to use a lot of flux, then there is no need of caring for precise soldering. The flux makes the solder joints quite precise and acurate, with minimum shortcuts. Other elements like resistors or caps you simply do by hand, holding them with tweezers.

  5. Dave,

    Do you do anything to help protect against ESD when using exposed PCB as the outside panel/case?



  6. Dave,
    I have a suggestion regarding joining the 3 layers and transferring power from one layer to another – you could have those pads on touching each other after all (in the video you say the pads should be connected with wire that you then squeeze between the boards) , just place a thin layer of solder on each of the pads that touch each other, then squeeze the layers in a vise or something and heat with hot air. The solder will melt and will give a more permanent bond than super-glue and also transfers power or other signals from one layer to another, this probably will allow you to also have buttons on the back or maybe other components. Also this type of product is more serviceable than super glue because you can actually disassemble it again with hot air.

    • i was thinking something similar, but perhaps he could get bits of 2-2.5 mm routed board like he said, but shaped like the outline of the calc (minus the battery and connector slots), with some big plated trough holes cut in half so you can solder everything sandwiched together from the outside with a standard iron. it would also give the hole assembly a better look imo since it would look sealed.

  7. Man, you love hearing your own voice. This whole thing could have been shrunk to 5 minutes.

    • Please point us to your non-scripted video blog…
      But I’ll go out on a limb here and bet that you don’t have one and you simply enjoy mouthing off at others who actually produce content.

      • Actually, I don’t need to be producing videos to notice that you enjoy hearing your own voice. I watched your early videos. You started as a humble professional sharing ideas to an arrogant blabbermouth in the later videos. Calling others as being dickheads, morons. Get over it, you are just not that important.

        • I’m hoping that ‘Rob’ no longer exists as a user here and that this was some low-life form of spammer.

          I’m sure many people who appreciate Dave’s fine work would like to meet ‘Rob’…

        • Looks like I was spot on.
          First you complain about my long winded style (which has been there from day one), and now you bitch about my language I chose to use on MY blog.
          If you don’t like my content why don’t you just stop watching? why waste your life with pointless unproductive comments?
          Why don’t YOU show me and everyone else how it’s done?

        • Rob, the most humble response for you was to give it no response at all, so your comment wasn’t important. Unfortunately, as you’re appointing a direct attack how Dave makes his blog, I am going to respond.

          Your comments are weak. Maybe you are right, but so what? This is Dave video blog. He makes it the way he wants. I love watching every video he makes and what he does for the community.

          It looks you’re kind of the guy who says ‘humpf, my >ideas< are better and what you do is crap. However, I am not going to make those idea's happen, because I am already better!'

          I'd like to hear that saying to your clients as well if you ever get to speak them, I wonder what their response is! If you don't like the blog, how hard is it to walk away? You won't buy groceries from a store that you know is appalling?

      • For the record, some of us actually do like your voice (certainly I do). Form of presentation is important and it’s one of key reasons your blog is so interesting.

        Is it long winded? Hell, yeah! But if that’s what it takes to get more of these videos online I’m fine with that. I guess that’s 80-20 rule at work.

    • OF course he loves hearing his own voice and we love hearing his voice too!

  8. How much does a board like your front panel cost to produce? Who do you use for production?

  9. Rob, you like the look of your own text……… If you don’t like what Dave has to say don’t watch it…… Its amazing how brave people become when hiding behind their monitors and keyboards.

    Crawl back into your hole trool.


  10. Nice work — really good idea to use PCB as a core. Is the cost worth the benefit over spacers and a bit more glue, though? Guess we’ll see! Love those block soldermasks with gold plating.

  11. Nice video!

    You might consider spring clips soldered to one board to connect to the other one. They are often used in cell phones to connect to the antenna assembly or to battery terminals. That way you could easily dis-assemble it later if need be. They are typically gold plated metal of some springy type (e.g. Be + Cu) and can easily handle the current you would need for this project.

    Thanks again!

    • I forgot to mention those! They are nice, but I could not find ones of a suitable height and a low cost off-the-shelf.
      I have a similar problem with my uWatch2 project, but that requires a lot more connections.

  12. Nice project and video. I hope this makes it to kit form sone day.
    I have been looking for a micro scientific calc for a long time

  13. How easily these CR batteries come out from that calc, is there a hole from witch they can be pushed out? Really nice project anyways!

    • I was thinking the same thing- My idea was to make “battery cartridges” out of 2-mm PCB that would hold the batteries and provide a bit of a handle. A simple C shape with a tab on the back would work like a charm. You could even rout a small piece out of the top and bottom boards to provide a place to grab the cartridges without them sticking out. The cartridges could even be part of the core PCB, held in by small un-routed tabs that would be cut/broken off during preparation/assembly.

  14. Why use PCB for back panel, and not just use molded plastic and use fitted leads to hold the batteries in place to make contact with the upper PCB? You could also make holes in the outer corners of your front PCB to secure the layers with screws/rivets. This would also allow you to add spacers so that the case is more solid, and it remains completely sealed. You could also add plastic battery holders with tension clips to hold them in place and make them easily replaceable.

    • That means custom NRE for the plastic. Sure you could maybe try and Maker-Bot it, or eMachineShop it, but the plastic will probably have to be much thicker than a 0.5mm or 0.8mm PCB.

      • You are right, haven’t seen any CNC extrudes that have good enough accuracy to do anything that thin.

      • I also meditated how to get flat battery out of the calculator.
        I resolved that I would place a slot through the back panel over the centre of each battery (perpendicular to the longer side) staring just beyond the outline of battery and about an 1 cm long so that you can stick a pin there to push the battery out.

  15. Very nice! Now we just need an RPN firmware on that and I’ll take two.

  16. Really cool project, Dave, really inspiring!

    You showed that the LCD screen is actually in a DIP style package and that you have to make some hand modifications in order to incorporate it into your design. Have you considered the implications of that regarding assembly cost and time if you decide to mass produce this?

    • Realistically, if this was to be mass-produced, you could probably just ask the company producing the parts to keep the leads unbent. At the very worst, you could just make a jig that straightens the legs for you quickly.

    • Yes. This was realistically only ever going to be a niche product with small production runs. So the thought of making a hundred thousand of them isn’t really a consideration.
      This option was the best I could find that allowed me to produce a suitable looking and low NRE cost functional prototype and small scale runs without going to custom tooling.
      Sure, medium volume would cost more to produce than a proper custom solution, but you can absorb that into the final cost for niche products like this.
      Otherwise if you try and shoot for the brass ring on the first production run then you end up like, for example, Mitch Altman who spent $30K tooling up for his TV-B-GONE before he even sold the first one. That’s a big risk that can be mitigated by using off-the-shelf and low NRE part solutions like this.

  17. Dave, the poularity of your blogg is evident. I suggest we all ignore non-constructive criticism. Altium rules.

  18. Why not use a spring instead of a nasty wire to join the two boards? – It’s an industry standard for shielding, why not?

  19. Thanks for the video Dave. Thumbs up!
    I only wished you could told us more about your design trade-offs because that is most valued.
    For instance, we mentioned MSP430 and XLP and since they both are low power what criteria triggered your final decision?

    • I’m also interested in mcu-choice, is there any reason you chose a 16 bit PIC instead of msp430 or a 32 bit ARM-microcontroller for instance?

      • He mentioned it has a capacitive touch peripheral built in and is a low power device. Not to mention, Microchip RULES!.

        But seriously, for small builds, why would you anything other than Microchip? I’d go to them for projects like this for no other reason than guaranteed availability and even support if you need it. I’d use to say something similar about TI but more and more they seem to be going down the tubes, unless your 100k+ of their parts, but at that quantity, everybody is good to you.

  20. This is great! I love every part of this design. It has inspired me to think out of the box for some of my designs. I plan on using the LCD mounting concept on my next project. Can you recommend a very small (like the size found in really cheap wrist watches) 4 – 5 character numeric LCD with a built-in driver?

  21. struth
    I stopped by here because of the voice and the delivery.

    I stay because of the content.

  22. I am not sure how thick the LCD actually is, but you could cut the pins completely and solder like QFN package.

    Also, to save on wires/work running from bottom to the top board you could use metal springs that would make contact by pushing battery slightly outward, making contact on the edge of the battery, not on bottom/top of it.

  23. Mannnn… Many thanks for the inspiration!
    Your videos are great! I hope one day I can design something like that 🙂

  24. Take a look on this project:
    (a combined calculator, osciloscope, multimeter, .. etc)

  25. Hi Dave!

    What is the purpose of the accelerometer? I can’t imagine any “useful” program what can make advantage of it. The button matrix can be used as a “big” 2D touch-pad.

    Some kind of expansion/communication port is not bad to have (I don’t know if in this certain MCU the serial port is re-mappable to the pins what you use to program it).

    How does you imagined the “turn on” of the device (because the cap sens buttons can be easily triggered unnoticeable)?

  26. Hello Dave, I love the entire project and the use of Microchip is a wonderful choice as I have been tinkering with there line of PICs far more than Atmel. My question is are you going to produce this for the store here, I would love to buy it … I think you mention you did this for a small run … why not do a pre-order and get a waiting list going with a deposit or something. Should facilitate initial production run.

    Also will the end result have the serial programing port exposed to the outside? Would be nice to program your own additional firmware to it making new functionality and extending it to be also a programmable calculator.

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  28. Hi Dave,

    How is the project going?
    I think the final versions is going to be great.


  29. come on David!

    tell us something please!


  30. One suggestion I would have would be to modify a little bit the routing so that the 5 ISP pads are a bit further away from the edge. This way you could actually solder that female ISP connector inside the calculator, aligning its edge to the calculator edge. This would allow you access to the ISP even after the boards are super glued together, leaving the calculator with 5 small little holes on the side.

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    this is great news to as calcutor a computer and prossocing works

    md nazeer
    wide Circles

  35. good to hear a calculator computer can we do small job works with this then it great to have it

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  36. Any hope of a release date for sale?

  37. Hey Dave I am a machinist and I really would like one of these scientific calculators, to put into my work binder. I was wondering if you are willing to release the cad files for this project so I can have the pcb’s made.

    Thank you,

  38. Hey Dave,
    Is it a processor or a controller ? can you specify the part number ?

  39. Dave,
    Awesome project! Any news on a release date? I’ll definitely buy a couple!

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