Thursday, November 27, 2014

A Sophisticated Heat Beam Which We Called a "Laser."

Having recently acquired a stylish collection of potentiometers and knobs from Tayda2009, I wanted to mount them in a mechanically solid way. An excellent opportunity to learn how to use the Epilog laser cutter at FabLab Zürich

I started with a box design built at MakerCase, rearranged the pieces to fit an A4 sheet of poplar plywood, and added the other features (cutouts for the potentiometer shafts and wires, outline for the breadboard, etc) to the top surface.

After struggling unsuccessfully with Inkscape (which was not overly fond of the 0.01mm thickness of lines signaling to the laser cutter to cut instead of rasterizing, and created PDFs that omitted all circles with that thickness), I spent a well invested dollar on EazyDraw 3 (there appears to be a separate app for every version from 3 to 7, with prices neatly staggered, which I didn't know when I bought the oldest one, but it was perfectly suitable for my purposes) and redid the SVG, creating a PDF that opened just fine.

15 minutes' or so worth of cutting and rasterizing got me this: 

The design has a number of flaws (a.k.a teaching moments for next time):

  • I left off the bottom surface, because it would have been useless, but I forgot to adjust side surfaces to remove the fingers for the bottom. Oh well, I'll pretend these were meant for ventilation.
  • The box is considerably deeper than it needs to be. 2cm would have been ample, I made it twice as deep.
  • The potentiometers I bought have a little pin at the edge, which I had not noticed. In order to be able to mount them flush, I had to cut small notches for those pins, which luckily was no problem with the plywood.
  • The hole for the wires are a bit close to the potentiometer shaft hole, as the pins of the potentiometer are fairly long. On the other hand, there's no reason why the pins of the potentiometer have to point in any particular direction (the knob will mount in any orientation),   so I might just cut the pin notches so the potentiometers are facing at a 45 degree angle to the wire hole next time.
Overall, though, I'm pleased with the result, shown here demonstrating three simple audio circuits (On the left, a VCO and a Metronome, using the two timers of a 556. On the right, a non-functional attempt to build an OpAmp based circuit driving a piezo element).

Thursday, July 31, 2014

Lötet, freie Schweizer, lötet!

The recent introduction of chasing electroluminescent (EL) wire inspired me to give this a try in a seasonal installation:

admittedly, the sound on the video is not recorded off the installation—I overdubbed the same MP3 file the installation plays on top of the video, so the fairly soft rendition of the audio through the simple speaker I used did not detract from the majesty of the visuals and the occasion.

I noticed that so far I had never written up an EL wire project, mainly because the software generally has all the sophistication of an LED blink sketch, the hardware is 95% off the shelf (I'd rather not tinker too much with the high voltage AC current involved in an EL wire project), and my craftsmanship with the wire shaping is undistinguished. Nevertheless, I thought it was time to show how a project like this can be done.

Bill of Materials

  • EL Wire — I mostly use Sparkfun's brand of EL Wire, bought mostly through Boxtec. Make sure whatever wire you buy has the right plugs. I bought some of my EL wire from Seeedstudio, and spent some rather instructive time and effort refitting it with the plugs I needed (some adhesive copper tape and a bit of heat shrink tube ended up working quite nicely). The wire I use is 2.3mm thick (the thinner, the more tightly the wire can be bent).
  • Adapter cable for the chasing EL wire
  • An EL Sequencer — my Sparkfun EL Sequencer still works, despite the nasty accident it suffered when I first powered it up.
  • An EL Inverter to generate the high voltage high frequency power to drive the EL wire.
  • Electrical tape to mask off the sections of EL wire that I don't want to be seen.
  • Garden mesh to mount the wire. I bought a roll of 3x3.5mm garden mesh and cut it to size as needed.
  • An Arduino Uno with a MP3 Player Shield to play the music.
  • speaker for the audio output. With some additional circuitry, the MP3 shield is also capable of sending a fairly reasonable quality output signal to an external amplifier (e.g. your stereo system).

Arranging the EL Wire

There are numerous ways of getting EL wire to keep whatever shape you want to form it. The technique I'm using (which I may have invented; I don't recall having seen it anywhere else) is to use a tight garden mesh with holes a bit bigger than the diameter of the EL wire, and weave the wire through it:

it turns out that it does not matter whether the EL wire runs on the front or the back side of the mesh: it is bright enough that it will be equally visible on both sides. EL wire can bend quite a bit, but if abused too much, it will eventually break, so my wiring tends not to be all that tight.

The EL wire I buy generally comes with a plastic cap at the end, and the mesh is tight enough that I need to remove the cap or cut off the end of the wire. I've generally not had problems with this practice, but in the new chasing EL wire, doing this seems to have created some sort of shortcut between the individual strands of wire, so I had to separate them a bit and put some electric tape around the end to fix the problem.

Code for EL Wire

Basically a glorified blink sketch, as I said. Rather than synchronize line by line, or note by note, I just synchronize with the start of the song, which happens to be recorded at a fairly steady 60bpm (except for a ritardando at the end which I'm ignoring). For the chasing wire, I'm using an adapter cable using 3 channels; I do the chasing with 2 channels on, 1 off at a time, although the other way around also works, of course.

Playing the MP3 File

For playing the MP3 file, I used Bill Porter's very convenient MP3 shield library, so the actual code to play the file was trivial: 

The synchronization is simply done by connecting the TX pin of the Arduino to the RX pin of the EL sequencer, and then sending some data when ready. It certainly could have been made more precise, but for my purposes, I thought it close enough.

Monday, March 31, 2014

Introducing Muggeseggluino

One of the ideas that fascinate me in my electronics projects is minimalism in form factors. Having started out on an Arduino Uno, I started building ATtiny84 (14 pin DIP) based projects after a while, then ATtiny85 (8 pin DIP) based projects.

That's where things remained for a while, but eventually, I could not resist the temptation of tinkering with the most minimalist of the ATtiny series: The ATtiny4/5/9/10, coming in a 6 pin SOT-23 package. Despite the modest specs of the series (The ATtiny10, as the luxury high end model, features 32 bytes of RAM and 1K of flash), there is decent compiler support (don't expect to run an Arduino core, of course), but programming the MCUs is a bit tricky. Pulling out the MCU from the circuit and reinserting it for every programming cycle would require rather massive amounts of soldering, and putting it onto a breakout board sort of defeats the purpose of having such a small form factor in the first place.

I decided to try a different approach: 

Every pin of the MCU (Except for GND) is broken out to a pin in the right row of headers. The left row is connected to the rightmost row in the prototype area. That way, the two rows of headers can be connected to operate the circuit, and separated when reprogramming the MCU. After the software is final, the headers can be removed and the two rows of pins bridged (The astute reader will notice that the space savings over a DIP breakout board, if any, are minimal, but I still like the design).

The name of the board, "Muggleseggluino", derives from the Alemannic word for a small but distinctive amount. It seemed an appropriate name for situations where just a tiny bit of microcontroller was needed as an ingredient.

Right now, I'm still playing with software options. The easiest solution out of the box is Wayne Holder's ATtiny10 IDE. The high voltage circuit is only needed when using pin 3 as an I/O pin, otherwise, SHDN can be used on the RESET pin directly. If you already have a 12V power supply, the single transistor circuit used by ScratchMonkey should also work.

Tuesday, January 28, 2014

Digispark & Co

Boxtec discussion recently brought up the fact that the last year has seen a good number of ATtiny85 based boards with USB connectors:
While of these, I own only Digisparks (having been one of their Kickstarter backers), the others seem close enough in design for me to offer a few thoughts about the idea.


Electrically, all four boards seem to be based on the Digispark design: An ATtiny85 MCU, an USB connector directly connected to the MCU. Within that basic design, the boards have quite a bit of variety in the mechanical aspects:
  • The Digispark 
    • Features a male USB connector that is simply designed into the PCB. While this is an eye catching and original design, in practice it caused me no end of mechanical problems. Even with perfect mechanical tolerances (which at least the early Digispark boards did not have), having the board sit right next to the USB plug was rarely convenient, so I ended up using a f-m USB extension cable, pretty much negating the point of having a male connector in the first place. 
    • Has a voltage regulator on board and can be externally powered.
    • Has an unique and elegant, squarish form factor that Digistump is trying to build an ecosystem around.
  • The Adafruit trinket
    • Features a standard female USB mini connector, which in practice probably will work out much better than the male connector.
    • Voltage regulator, can be externally powered.
    • Available in 3.3V and 5V variants.
    • Reset button on board.
    • Board looks like a tiny Arduino Micro, pins brought out DIP style. Mounting holes might be quite useful.
  • The Olmexino
    • Female USB mini connector. [Update: Now (wisely) changed to a full size USB connector]
    • No voltage regulator, though it looks like it might be possible to power externally with appropriate voltage. Hardware reset button.
    • Board comes as a DIY kit, which for the most part seems like a fun idea. The only board of the four featuring the through-hole variant of the ATtiny85, and even a socket for it. My one concern is the SMD USB connector, which is not my idea of fun to solder, especially since mechanical reliability is a major concern for this part. [Update: It seems that Olmexino came to the same conclusion. Rev B of this board features a through hole, full size USB connector — unfortunately, nobody seems to make through hole USB mini connectors].
    • Pins brought out SIP style.
  • The Iteaduino Tiny
    • Appears to have a female USB micro connector, which I'm not terribly fond of. In my experience, mini connectors are much more convenient for frequent plugging/unplugging.
    • No voltage regulator, no reset button.
    • Instead of bringing out all pins, this design seems to have a male ISP header. I'm not at all convinced this is a good idea. The whole idea of this family of boards is to be able to do programming through USB. Furthermore, it's impossible to fall back to High Voltage Serial Programming on this board because some of the pins are only brought out through USB.
    • To add to the hardware design issues, Iteaduino apparently did not disable the RESET functionality of the MCU, so of the 4 I/O pins brought out by the header, only 3 can be used safely.
All four of the boards share the concerns of the original Digispark design: Of the 6 I/O pins available on an ATtiny85, only 3 (PB0, PB2, PB5) are truly unencumbered: PB3/4 are connected to USB (which in practice rarely seems to be a problem, but would make me a bit nervous when the board is plugged in), and PB1 has an LED attached, which is a problem in some applications (early Digispark designs had the LED on PB0, which made I2C unusable, but this has long been fixed).


All of the boards use about 2K (the Adafruit Trinket a bit more) of the flash memory for a boot loader allowing the boards to be programmed through USB. If you're used to modern Arduinos, getting used to these boards might have a bit of a learning curve, as reprogramming them is somewhat timing sensitive. None of the boards works with the Arduino IDE out of the box—the Adafruit Trinket requires some manual configuration or a custom Arduino IDE downloadable from their site, while the others use the Digispark customized Arduino IDE.

In addition to regular ATtiny85 activities, all of the boards have some capability to work as an USB device (e.g. a keyboard or a mouse). Generally, the software USB libraries work as advertised, but it tends to be somewhat more fragile than using a true USB capable MCU.


One of the attractive features of the Digispark was its low price, and the clone boards are priced even more aggressively. 

Final Thoughts

The Digispark was a rather original design when it first came out. From the neat form factor, to the minimalist USB connector, to the software USB and boot loader support, it featured several novel ideas, and demonstrated they could be done. As for whether they should be done, the situation might be less clear:
  • In hindsight, I don't think the male USB connector was a good idea.
  • The small form factor is really convenient for space constrained projects.
  • The boot loader certainly adds convenience for beginners. If you're comfortable working with external programmers, I'm not convinced the extra convenience of a boot loader is all that useful.
  • Though the software USB support might be a bit finicky to use, it's undoubtedly a benefit for some applications.
In summary, I think these boards are at their most useful when you want USB capabilities in a project, and space and/or money are an important concern. In some projects, having a voltage regulator on board might be convenient as well. For many other projects, it seems to me that a plain ATtiny85 (possibly of the through hole variety) and an external programmer are far more flexible and not all that much harder to use.