Category: PCB

Pixhawk-CHDK trigger progress

10. June 2015 11:07 by Jens Willy Johannsen
Categories: Drones | PCB | Projects

The Pixhawk-CHDK trigger is coming along nicely. The prototype is done and the firmware is almost done.

The firmware is actually very simple. I'm using 32 bit timers for both measuring incoming RC PWM signal for for outputting 5 V pulses of specific lengths and all the USB CDC functionality is copied-and-pasted from a previous project. The most complex part is actually for command-line interface (or "CLI") parser.

To configure the device you plug into your computer using a mini-USB cable and then connect using Telnet, screen or other terminal application.

The command set is pretty simple and supports the following commands:

list                    -- shows all defined intervals
add 1000,1200,85        -- adds a new interval
delete 1                -- removes entry 1
change 1=1000,1300,85   -- changes entry 1
pulse 50                -- sends an output pulse of 50 ms
show                    -- shows all settings parameters
show <setting>          -- shows the value of a single parameter
set <settings>=<value>  -- changes the specified settings parameter
help                    -- shows available commands
?                       -- same as "help"

No need to "save" or similar. Everything is stored in EEPROM as soon as it is entered.

PCB

The PCB design is also almost done. I just need to check it a couple of times (time well spent, trust me) and verify footprints (likewise time well spent). The size is 5 x 2 cm and even though I probably could squeeze it down some more, I think I'll keep the current size seeing as how I'll be hand-soldering some of the parts and applying solder paste and components all by hand.

Here's a rendering (from the excellent 3D Gerber Viewer from Mayhew Labs:

Pixhawk-CHDK PCB. 5 x 2 cm.

Pixhawk-CHDK PCB. 5 x 2 cm.

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Schematic component naming

9. August 2013 10:45 by Jens Willy Johannsen
Categories: General | PCB

Looking at some of my old schematics, I can see that I haven't been entirely consistent when naming components. Nowhere near consistent, actually.

Sometimes I have used "T" as prefix for transistors and sometimes "Q". And especially IC's that are prefixed either with "U" or "IC" – often I have used both in a single schematic. And also connectors that are named "CON" or "J" or "JP". Sloppy…

Well, no more! This page on Wikipedia has a nice table with designators for various component types (even though it does not comply with with IEEE- or ANSI-whatever standard).

So this is what I will use henceforth:

Designator Component type
R Resistors
C Capacitors
L Inductors, coils
D Diodes – all types including Zener, Schottky and LEDs
U IC's of all kinds. Technically it is used for "Inseparable Assemblies".
J Connectors. The more fixed part of a connector pair (often the part soldered to the PCB).
P Plugs. The less fixed part of a connector pair (i.e. the one on the wire).
K Relays
M Motors
Q Transistors – all types including MOSFETs, IGBTs, BJTs etc.
S Switches
Y Crystals and oscillators
X Transducers/sensors (no, X is not for crystals anymore).
FB Ferrite beads
BT Batteries
JP Jumpers. Not connectors but only actual jumpers that are linked or unlinked.
TP Test point (pad or pin for measuring)
MP Mechanical parts (standoffs, screws, mounting hardware etc.)
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3D Gerber viewer

24. April 2012 21:45 by Jens Willy Johannsen
Categories: PCB

This online 3D Gerber viewer is awesome.
Simply drag-and-drop your Gerber files and it'll generate a 3D model of the PCB which you can pan and zoom and save to an image.

Like this one:

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Creating custom enclosures

29. December 2011 21:04 by Jens Willy Johannsen
Categories: PCB

For another project (that I have shamefully neglected to write about) I decided I needed to put it into some kind of enclosure. And I since I couldn't find any off-the-shelf enclosures that fit I decided to make my own.
A little bit of googling let me to Shapeways that lets you create 3D printed models from your own 3D files in several different materials.

The models themselves can be created in pretty much any 3D modeling application (including the free SketchUp or Blender). I used Luxology's modo.
Obviously, exact dimensions are important. So I got PCB dimensions and coordinates from Eagle and used a caliper to measure the size of the battery and plugs and so on. And I created dummy objects for the battery and the PCB to make sure that everything fit. This is what I came up with:

Enclosure (click for larger version)

The stand-offs have 4.1 mm holes for threaded inserts to use with M3 machine screws (like these ones from RS).

And here with the dummy objects visible:

With dummy objects (click for larger version)

I exported the object to Wavefront OBJ format and uploaded it to Shapeways (I needed to rotate 90 degrees about the X-axis first in order for the preview image to render correctly on Shapeways) and specified the units as meters (since that's what modo uses by default).

The price came to €20.14 plus shipping (€8.38 for UPS shipping) for "white, strong, flexible" material. So now it's just wait and see how it turns out.

Bear in mind, that this is my first attempt at making a 3D printed object. I haven't even smoothed the edges or added support ribs or anything. Not to mention that I have only made the bottom part of the enclosure (it does however have a "lip" for mating with the top half).

For a much nicer custom enclosure, take a look at this one (also on Shapeways).

 

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PCB ordered

24. November 2011 9:22 by Jens Willy Johannsen
Categories: PCB | Projects

The PBCs have only undergone minor revisions and Gerber files have now been sent off to ITead Studio for fabrication.

Here's a 3D rendering of both sides of the PCB (yes, several of the components were unknown to Eagle3D and I didn't bother creating custom parts). Also, the largs pads for the RGB LED are actually mostly covered by soldermask:

Eagle3D rendered PCBs (click to enlarge)

 

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PCB layout done

19. November 2011 22:29 by Jens Willy Johannsen
Categories: PCB | Projects

First version of the PCB for the RGB Tumbler is ready. Although usually I do through a couple of revisions before I send it off and get it made.

The PCB ended up somewhat larger that I had originally planned. It is currently 3.6 x 3.6 mm.

Here are pictures of the PCB printed in 1:1 scale and populated with components to verify that everything fits (click for larger images):

Top side (click for large version)

Bottom side (click for large version)

I need to move the slide switch a bit up to make room for the 10 µF capacitor. And I will also move the battery holder up a little bit. The NCP1402 voltage booster almost touches it. Apart from that, everything looks pretty good.

Another problem is that I ended up placing several vias directly under thethe battery. This may not be the best thing. Especially since the vias on ITead's PCBs (ITead Studio is where I get my PCBs made) are not always completely covered (even though the Gerber files say they should be covered). I guess I'll just have to cover the vias myself with some tape or something).

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High-power LED driver

10. June 2011 10:09 by Jens Willy Johannsen
Categories: PCB

For the airsoft flag device project I have been working with normal or semi-high-powered LEDs. And they just weren't bright enough so I had to step it up to proper 1W or 3W LEDs.

However, that required me to change the circuits since it's not a good idea to drive a 1W LED with a series resistor since way too much power will get wasted in the resistor. The correct way of driving high-power LEDs is with a dedicated constant current circuit.

So I decided to make a small board with just the driver circuit and external leads for power, GND and LED control as well as leads for the LED itself.

Of all the many ways of making a constant current regulator, I decided to go with the ZXLD1350 IC for reasons I can't remember.

Here are links to the datasheets:
ZXLD1350
ELL5PS SMD inductor
ZLLS1000 SMD Schottky diode

Schematic:
Board:
EAGLE files: Eagle files

Terms of use: Yeah sure, go ahead. No guarantees, obviously.

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Board assembled

6. January 2011 13:38 by Jens Willy Johannsen
Categories: PCB | Projects

I have received the PCBs and assembled the first one. Everything expect connections to external components, that is.

Here's a picture. It looks pretty much like the rendering. And I know: some of the soldering joints could have been made better but bear in mind that this is my first SMD soldering attempt so I'm actually pretty pleased with the result.

PCB with all SMD components (click for larger version)

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Eagle3D layout

20. December 2010 21:38 by Jens Willy Johannsen
Categories: PCB | Projects

Here is a 3D model of the board created with Eagle3D (here's a tutorial and here's some information about getting it to work on Mac OS X):

Eagle3D model of the Airsoft Rounds Counter PCB (click for larger version)

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Final layout?

12. December 2010 12:19 by Jens Willy Johannsen
Categories: ATtiny | PCB | Projects

Schematic and layout revised again. I added the 10 µF capacitor (that really ought to be there) and a LED with accompanying resistor for debugging purposes.

Here a print-it-out-and-see-if-it-fits test. And it does fit. Note that all the resistors are the same value (doesn't matter of course since all 1206 resistors are the same size) and that I've placed a 1206 resistor instead of a 1206 LED. Also, the B sized 10 µF capacitor isn't placed on the printout but it shouldn't pose any problems.

Layout, revision 3 printed 1-to-1 with components (click for larger version)

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Soldering SMT components

11. December 2010 15:59 by Jens Willy Johannsen
Categories: PCB

Until now I have only made through-hole circuits.But the Airsoft Rounds Counter project's demand for a small PCB made me decide to use SMT. The components are a lot smaller so you can fit a lot more onto a small board but they are more difficult to solder by hand. (But easier and faster to assemble automatically. And cheaper.)

So I have to practise my SMT soldering. Fortunately there are a couple of great tutorials on the net. I used these:

And it's actually not that hard. I have – so far – had much success with flux-pen-and-solder-on-the-tip-of-the-iron method. A flux pen if your friend – trust me.

Here's an example of my handiwork (the ting at the top is a millimeter scale ruler):

One of the first attempts: a 1206 resistor

The resistor in the image above is a fairly large SMD component: a 1206 sized resistor. The smallest I will be using to begin with are 0805 sized capacitors and SO-20 and -24 ICs. You can see the various SMD sizes here:

Image from fpga4fun.com: SMD sizes (click for larger version)

(Note: SMD means "surface-mount devices" and SMT means "surface-mount technology". I use the terms pretty much interchangeably. Which is wrong. I know.)

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PCB Layout Revised

10. December 2010 21:55 by Jens Willy Johannsen
Categories: PCB | Projects

Oops. The photo interrupter's collector wire wasn't connected to INT0 on the ATTiny. That has been fixed now. Also, the board is now 1 mm wider so there is enough clearing from the segment A wirepad to the edge of the board.

And the VCC and GND traces are now 24 mils and all other traces are 12 mils wide. Refer to the first layout if you can't remember what the various wirepads are for.

SMD layout, revision 2 (click for larger version)

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SMD layout

10. December 2010 0:53 by Jens Willy Johannsen
Categories: PCB | Projects

Here's the first draft of an SMD PCB layout for the Airsoft Rounds Counter project.

The wire pads are as follows:

  • VIN
    9V power input
  • AN, CA
    Anode and cathode the the photo interrupter LED
  • CO, EM
    Collector and emitter for the photo interrupter transistor
  • A-G, DP
    Segment anodes on the display
  • 1-4
    Digit cathodes on the display
  • RST/PRG SW
    Reset and program tact switch

Actual size is 49.53 x 20.32 mm.

SMD layout

SMD layout (click for larger version)

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Learning from your mistakes

15. September 2010 11:43 by Jens Willy Johannsen
Categories: PCB

In electronics as in everyting else, the more you mess up the more you learn (hopefully). With that in mind I figured, "what the hell, let's give it a go" and I sent an order to Micron 20 .

It is a pretty simple PCB for an ATmega168 (or -328) with voltage regulator, pull-up resistor for the reset pin, a crystal with associated load caps and a 3mm led with resistor.

The order specifications are as follows:

  • Single-sided
  • 6.9 x 2.8mm (2700 x 1100 mils)
  • Solder mask (blue) on both sides
  • Silkscreen (yellow) on top

Minimum PCB size is 0.7 dm2 (square decimeters?! Why would anyone ever use square decimeters as a unit?) so I will received 4 boards.
I chose to have the boards done in 7 work days (you can choose between 24 hours to 15 days) and airmail delivery and the grand total was €66.

Now, we'll see what kind of mistakes I have made :)

It looks like this:

(click to see larger image)

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Learning PCB layout

15. September 2010 0:17 by Jens Willy Johannsen
Categories: PCB

So far I've only been concerned with designing circuit schematics. That is, an illustration of how the various components are connected to each other but which not necessarily have any correspondance with how the actual, physical components are placed on a board. I have been using the free version of CadSoft's EAGLE which is very capable and has all the features I will need for the forseeable future. And it runs on Mac OS X (as well as Windows and Linux). It does have kind of a weird user interface but you get used to that.

EAGLE has both a schematic editor and a PCB layout editor and until recently I had never spent more than five minutes looking at the PCB layout editor so I had some learning to do. This tutorial from SparkFun and this PDF helped a lot. As did this list of layers from PCB-Pool.

Three more things I learned:

  1. Choose the right components when making the schematic - or later.
    It quickly turned out that I had to go back and spend some more time in the schematic editor. Because up until now I just chose components without much regard for the "footprint" – i.e. size and shape. I didn't care exactly what resistor I was using as long as it had the right symbol and I could specify name and value.
    But for PCB layout you really need to know whether there are 0.1 or 0.2 inches between the capacitor leads, for example.
    So choose the right components when you make the schematic OR just drop in some components and then go back and change them when you starting working on the PCB layout.
    Yes, I know that most engineer types out there will yell stuff about always choosing the correct component to begin with. But honestly, when I make the schematic I have no idea whether a given resistor will be 10 mm or 8 mm long. And I don't want to spend time thinking about at that time. So there!
  2. Make your own or adjust the packages
    For some components I needed to change some details on the footprint. For the polarized capacitor I would like a square pad for the positive lead and for the 7805 volt reg I wanted to remove a lot of unnecessary silk screen print. This tutorial from SparkFun is helpful (http://www.sparkfun.com/commerce/tutorial_info.php?tutorials_id=110).
    As is knowing how to copy everything from an existing package. Here's how:
    1) to identify the package and the library it's part of, choose the info tool and click on the part
    2) open the library and edit the existing package
    3) click the layers tool and show all layers
    4) choose the group tool and select everything
    5) click the cut tool (the scissor) – yes: cut. It will actually copy and not cut.
    6) open or create the new package
    7) click the paste tool (next to the cut tool)
    And then you can add or remove stuff and save the package. Then edit the part, add a new package and connect pins and pads and then replace the old part with the new variant in the schematic editor (And choose Library -> Update all).
  3. Print and check
    Print out a copy of the board in actual size by choosing File -> Print… and setting the scale factor to 1.
    Then place all the components on the paper (push the pins through if possible) and make sure everything fits and the footprints are correct. I caught several mistakes by doing this...
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The next step

10. September 2010 15:46 by Jens Willy Johannsen
Categories: PCB

Having made a couple of prototype-like projects I thought it would be nice to put the projects on proper made-for-the-purpose circuit boards. So far I've been using these boards and they have worked very well: the size is perfect for an ATmega168 with support components (like a 7805 volt reg and a couple of capacitors) and they are laid out like a breadboard.

However, I'm running out of space for components on those boards and using single-pad proto boards seems kind of kludgy.

So I began looking into making my own PCBs. But all that stuff with printing and ironing and splashing around with acids and whatnot just looks like too much hassle.

So if I don't want to make my own, I guess I'll have to buy them. It might be a bit more expensive but they will look soo much cooler with solder mask and silkscreen and stuff.

After a couple of hours of Googling I ended up confused on higher level. It turns out that there are lots and lots of companies making PCBs to order but specifications, requirements, prices, minimum quantities etc. vary wildly. My requirements are:

  • Simple boards: typically only one copper layer and (for the time being) no small SMD components so layout isn't too complex.
  • Small quantities: like one or two or six boards at a time.
  • No hurry: I can wait a couple of weeks.
  • Price: I don't want to spend too much money on something that is just for fun (actually, that's probably too late but still...)
  • No hassle: by which I mainly mean import tax and VAT.
  • Solder mask: I'd like a solder mask (http://en.wikipedia.org/wiki/Solder_mask) to help protect from shorts and to avoid having to tin all copper traces to prevent corrosion of the copper (not strictly necessary). Also, it look cool.
  • Silkscreen: Not strictly a requirement but it would be nice to have outlines and labels silkscreened unto the board. And again: it looks cool.

Two providers I'm currently looking at are:

Micron20 / PCB Boards in Bulgaria which is very cheap and
PCB Pool in Germany which is a bit more expensive but looks more professional.

Both are in the EU so there's no import tax and VAT, both accept designs in EAGLE .BRD files (EAGLE is the schematic and layout program I use), and both are able to deliver single and double-sided boards with solder mask and silkscreen in small quantities for a reasonable price.

(original image by Henry Herndon)

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