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flashing LED badges

Discussion in 'Crafts and Cooking' started by SamP20, 4 June 2015.

  1. electrokitty

    electrokitty Coffee Enthusiast

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    Wow, etching the boards yourself? I considered it for some time but when I looked at the costs involved for a UV exposure unit, the etchant and then the boards with UV resist coating it didn't really make sense and I instead use OSHpark to get all my boards fabricated. They're probably the cheapest and best quality I've seen, $5 (~£3) per square inch of board (2 layer and ENIG finish as standard) for 3 boards with no minimum order quantity and free international shipping. I never really considered making the exposure unit myself which I suppose drives to cost down a bit. I have etched some boards before at school for my electronics GCSE with the UV exposure method and ferric chloride as the etchant and they seemed to come out ok most of the time, although I'm not sure I'd trust a homebrew board like that for really fine SMD stuff, and I imagine 2 layer boards would be difficult due to lack of vias (I guess you could solder a short length of wire through a hole drilled in the board around a pad) so best of luck with that.

    The CR2032s sound like a good option, I've used them before for really small circuits that need a moderate amount of power but need to remain small. You can get holders which hold two at once stacked on top of each other, although these are quire hard to find (I think I had to get mine from the US). I used them before when I was into model rocketry with a LDO regulator to provide 5v for the electronics, I didn't measure the current draw at the time but it was enough to power a microcontroller, 3 status LEDs, an altitude/temperature sensor and datalogger (datasheet for that says it draws a max of 6mA) and the apogee detection switch/staging mechanism. Batteries had a maximum capacity of 249mAh and I replaced them before each flight to be safe but they never had a problem running those electronics.

    Buck converter sounds like a good idea too to increase current and to drop the voltage, although if you're just going to buck down to 3v again it'd probably be more efficient to just wire them in parallel if you can spare the board space :p
     
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  2. SamP20

    SamP20 A partridge in a pear tree

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    I never considered OSHpark since I thought they were a US only company. Some UK companies I tried a while ago were really expensive for small volumes, which is what made me choose the DIY approach (too late to turn back now :p). I too remember etching boards for my GCSE using ferric chloride which is really messy stuff. The great thing about the etchant I'm planning to use is you can keep regenerating it by bubbling oxygen through it and adding a little acid every now and again. And it's nowhere near as messy.

    For holders I've managed to find this one on Farnell which can hold two cells: http://uk.farnell.com/keystone/1026/battery-holder-20mm-coin-cell/dp/2293264. It would have been ideal if the LEDs have a voltage drop of less than 3V, then I could have managed with a single cell, or two in parallel. It's interesting that you managed to draw that much current from the cells. The datasheets I've read stop showing information at around 3-4mA (e.g. maxell). That being said things like pocket torches and LED throwies are probably outside the specs, yet they still work well enough.

    The model rocketry sounds like a lot of fun. Our society at university was planning to build a rocket at one point, but silly health and safety and/or politics got in the way. I think we were planning to go as far as having electronic stabilisation, so I can understand why the university were reluctant :p
     
  3. Dax

    Dax The Great and Powerful Daxie

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    WOW, definitely keep us updated with this, they look really good and it's interesting finding out how you are making them.
     
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  4. electrokitty

    electrokitty Coffee Enthusiast

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    Yea, the ferric chloride was pretty messy :p I remember sometimes it'd only take 5-10 minutes but if the etchant was really saturated it could take 45 mins-1 hour. I'd never really considered other etchants but the one you're using sounds better than that. I know what you mean about UK based fabricators being expensive, I've been quoted between about £30-100 each for small volumes of small prototype boards which is why I was so surprised when I found OSHpark. If you ever need professional quality boards, I'd highly recommend them. Only thing is it takes a while for them to arrive (about 3 weeks from ordering usually).

    That battery holder looks good, I think that might be the one I used before actually. I forgot to search for the diameter and not a specific battery type. I also forgot to consider batteries dropping more than 3V, I guess that's for the blue ones? They always throw me off, I'm used to LEDs being about 1.9-2.9v. I was working on a project quite recently actually and I needed the PCB quite quickly so I ordered it before I had finished the part selection and calculations entirely just leaving pads for reasonably sized components. When it came to calculating the current limiting resistors for the LEDs, I was surprised to find that for the blue one the LED I had picked had a forward voltage of 3.8v (With the supply being 3.3). I just soldered a bridge over the resistor for that one though and it seems to work fine, so if you only need slightly over 3v you might be able to get away with it. There's also several options for voltage conversion, you mentioned series+buck converter but there's also the option for parallel+boost. Cells in parallel will obviously be 100% efficient for doubling capacity whereas you'll loose a bit with buck or boost. I don't know if you've looked at the datasheets already but it might be the case that for this specific circuit it'd be more efficient to use parallel cells and a boost converter than series cells and a buck converter. Or, if you only need a little under 6v (5v or so) it might be more efficient to just drop the excess voltage with a LDO regulator or even a potential divider.

    Model rocketry is indeed fun, unfortunately I don't have much time for it any more and still have one half-built out in the garage from a few years ago. Getting into it through uni sounds like a good idea as I'd have thought they'd be the sort of place likely to be able to be granted the necessary certification for the larger class hybrid rocket motors. All I can play with as a hobbyist is the little black powder Estes pre-manufactured motors and for my models I was already having to use clusters of the second to largest ones you could buy to get it to lift. There does become a point though where you stop building model rockets and they become more like missiles :p. I got a chance to attend a UKRA (That's UK Rocketry Association) event a couple of years back and one guy had this model a couple of metres long with active steering (inertial guidance and servo-controlled flaps) powered by a hybrid rubber/NOX engine. Reached about 5km altitude and then he spent the rest of the afternoon trying to find where the thing had landed!
     
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  5. SamP20

    SamP20 A partridge in a pear tree

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    I spent some more time on the PCB design today. I decided to use Inkscape to lay out the circuit instead of the usual circuit editing software such as KiCAD. The reason is that the conventional software is useless for organic shapes. I am still not 100% happy with the position of everything, but it's a good starting point. The green circle shows the outline of the battery holder, and the red dots are where the LEDs are. I still need to figure out where to put the on/off switch and the safety pin. The total size is about 5cm x 9cm.
    dashie_pcb.png
    On a side note I am really grateful for all of the feedback so far. Most projects I start rarely get finished due to lack of motivation and distractions :D

    I did consider parallel+boost but it would mean I'd need a different battery holder. I could probably design and print one on my 3D printer. I also keep coming back to the idea of just powering the LEDs directly from 3V, but I'm not sure how tight the tolerances are with the LED voltage drop. The last thing I want is for some LEDs to appear brighter than others, or for all of them to become dim after a short time.
     
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  6. electrokitty

    electrokitty Coffee Enthusiast

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    Looks good! Only thing is that it appears to be missing a programming header for the micro (unless you're intending to program the chips before soldering with solderless sockets).

    Your choice of using the series+buck converter seems justified in that case if it's for board layout reasons, I wasn't sure on the size of these things so wasn't sure if there was room for two holders or not. Of course there is another option which is to use a rechargeable small flat LiPo as I did in a recent project. The charging can be done with something like a MAX1555 which is a nice small single chip solution for charging LiPos and then you'd only need a mini/micro USB socket or something for charging. I'm not sure if you're planning to sell these or not but if you are it might add too much extra cost, but for a personal thing it would certainly be worth considering if you're planning on using it a lot.

    With the LiPo option you should be able to power even the blue LEDs from 3.7v although it's a bit awkward if you don't want them all dimming as the battery drains. The solution I had to use for that was a combination of a boost converter and LDO regulator which add quite a bit of extra components and cost to something like this.
     
  7. SamP20

    SamP20 A partridge in a pear tree

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    Another quick update. I've tweaked the design slightly such that one LED is in the middle of the cloud, and the rest are spaced along the lightning bolt. Do you this this would look better than having them all evenly spaced? Image below:
    test print.jpg
    It's always a good idea to print out the design onto paper first as I discovered the pads for the microcontroller aren't wide enough apart. Also I still haven't routed the power trace through the switch (I was too keen to post this update :p). The battery holder to the right will fit onto the opposite side of the board.

    For programming I'm thinking about 3D printing a device that pushes wires directly onto the legs on the microcontroller. That or I could save myself the hassle of designing one and just buy one of these: New SOIC8 SOP8 Flash Chip IC Test Clip socket adpter BIOS/24/25/93 Programmer UK. I probably will program and test before soldering, but it's useful to have the option to program on the board if I change my mind on how I want the LEDs to flash.
     
  8. electrokitty

    electrokitty Coffee Enthusiast

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    Looking at the designs, that's probably a slightly better idea. The IC outlines are rather curious, the MCU looks like a SOIC-8 but slightly wider and the buck converter looks like a sot-23 but with an extra pair of legs. Don't think I've come across either before.

    For the programming it'd probably be easier to just get an adapter, if you really wanted to you could make an entirely separate board with a ZIF socket that does the programming and testing but I suspect that'd add a lot of extra expense to the project. I do tend to try and fit various test pads into my designs though to measure key voltages and currents so it's easy to test that everything is working as it should. I suppose if you really wanted to you could just use SMD pads for the programming but then you'd need something that could contact those pads securely for programming it.
     
  9. SamP20

    SamP20 A partridge in a pear tree

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    I may have accidentally ordered a WSOIC package instead which is wider than usual. The buck converter is a SOT-23 package according to the datasheet. It's going to be fun trying to solder it.

    For the SOIC adapter I've found this image which shows how it connects to the IC on the board. It looks like it could be used off board too:
    soic8 clip.jpg
    I bought a whole load of micros, so it'll come in handy for future projects too :)
     
  10. electrokitty

    electrokitty Coffee Enthusiast

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    Ah, that would explain it. Also today I learned SOT-23s can have more then 6 pins. I'd just never seen one like that before, looks a bit weird.

    That sort of programmer would probably work fine for this project, doesn't cost anything to add some test pads though and that's a little easier than trying to poke specific pins with a multimeter without shorting anything.

    Also while you're doing the development it's worth checking the chip manufacturer's websites to see if they do samples. They'll usually give a couple of units away free as samples for the cheaper ICs (most come from the US but they're usually shipped by courier so they get here pretty quick). Micros are pretty cheap now anyway but if you can get them for free during the development I find that quite useful (and if you're ordering on farnell you don't have to worry about a minimum order either).
     
  11. SamP20

    SamP20 A partridge in a pear tree

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    It's been a very long time since I last worked on this project, but decided it was worth another shot. The thing that halted my progress was the chemicals for home etching not being strong enough. For that reason I've decided to revisit that route later, and instead go through OSHPark as @electrokitty suggested a few posts up. It is going to cost about £30, but you get 3 boards for that.
    pcb.png
    Because I'm now using a PCB manufacturer I need to be able to give them all the right files, which meant redrawing the PCB. I chose to use KiCAD because it was free. It took a little messing around with the files to get the outline imported since KiCAD isn't really designed for natural shapes. I even had to break out my text editor and regex find/replace :p

    Before I submit I'll print it out again on paper to make sure everything fits as expected. Last thing I want is to waste £30 on something that isn't right.
     
  12. electrokitty

    electrokitty Coffee Enthusiast

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    Looks good! Probably a bit late now but I'll mention it anyway, but there's a free version of Cadsoft Eagle which I personally use, it's commonly used in industry so there are plenty of libraries available all over the internet for almost every component on the market. It's quite nice to use once you get used to it however it still isn't great at complex shapes. You can import bitmap images to use as outlines and things and I had a play around with that once to get a custom logo on the silkscreen but it's still quite fiddly. There's also a board size limit of about 10cm or so on the free version which may or may not be a problem.

    The good thing about OSHpark is that you only pay for the area your board takes up. Even with a complex shape, the price is calculated based on the actual board area rather than the rectangle around it, and they are by far the cheapest PCB fabricator for small boards I've found (I suspect for ones larger than a certain size or larger quantities it might work out cheaper to go elsewhere).
     
  13. SamP20

    SamP20 A partridge in a pear tree

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    Yea it worked out a bit cheaper, about £25 total, but they do bill for rectangular area unfortunately.
    Anyway, I stuck the files into their website and got these lovely renders from it :)
    top.png bottom.png
     
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  14. electrokitty

    electrokitty Coffee Enthusiast

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    Huh, really? Are you sure you have the board outline layer correct and it only contains the inner shape? They may have changed the way billing works but I've ordered circular and boards with cutouts from them in the past and I definitely remember them costing less than they should have done if they were a pure rectangle.
     

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