Exchanging data with other computers can be a daunting task with Arduino. No matter if you just want to pass some information to Processing, to a Web Service or something else – You always have to encode the data and decode the answer.

There always have been solutions like XML for structured data. But XML is hard to decode, complicated an takes up a lot of space. And then there is JSON. JSON is an easy and efficient way to transfer data. A complex data structure can for example represented as
{
"name": "Jack (\"Bee\") Nimble",
"format": {
"type": "rect",
"width": 1920,
"height": 1080,
"interlace": false,
"frame rate": 24
}
}

aJson is an Arduino library to enable JSON processing with Arduino. It easily enables you to decode, create, manipulate and encode JSON directly from and to data structures. By this you don’t have to bother with data encoding and decoding – this will hand aJson for you. It is based on the cJSON implementation, reduced in size and removing one or two features.

Using aJson

To use aJson get it from the git repository or download the latest version. Unpack the zip file (or the git files) into an Folder ‘aJson’ in the Arduino library directory ‘libraries\’. Due to the early stage of this library you can encounter problems. Simply submit a ticket and it will fix it as soon as possible. On github you will also find a simple Arduino sketch to test the library, or as a starting point.

To parse the above structure with aJson you simply convert it to a object tree:

 aJsonObject* jsonObject = aJson.parse(json_string);

(assuming you got the JSON string in the variable json_string – as a char*). This is an object. We’re in C. We don’t have objects. But we do have structs. Therefore the objects are translated into structs, with all the drawbacks it brings. Now we can e.g. retrieve the value for name:

 aJsonObject* name = aJson.getObjectItem(root, "name");

The value of name can be retrieved via:

Serial.println(name->value.valuestring);

Note that the aJsonObject has a union ‘value’ which holds all possible value types as overlays – you can get only useful data for the type which you have at hand. You can get the type as

 name->type

which can be either aJson_False, aJson_True, aJson_NULL, aJson_Number, aJson_String, aJson_Array or aJson_Object. For aJson_Number you can use value.number.valueint or value.number.valuedouble, for aJson_String you can use value.valuestring, for True or False, you can use value.valuebool.

If you want to change the name, you can do so by

aJson.getObjectItem(jsonObject,"name")->value.valuestring="a new name";

To render the object back to a string you can simply call

 char *json_String=aJson.print(jsonObject);

Finished? Delete the root (this takes care of everything else).

 aJson.delete(root);

This deletes the objects and all values referenced by it. So take care for your string, you directly assigned. They are deleted too.  If you want to see how you’d build this struct in code?

aJsonObject *root,*fmt;
root=aJson.createObject();
aJson.addItemToObject(root, "name", aJson.createString("Jack (\"Bee\") Nimble"));
aJson.addItemToObject(root, "format", fmt = aJson.createObject());
aJson.addStringToObject(fmt,"type",        "rect");
aJson.addNumberToObject(fmt,"width",        1920);
aJson.addNumberToObject(fmt,"height",        1080);
aJson.addFalseToObject (fmt,"interlace");
aJson.addNumberToObject(fmt,"frame rate",    24);

The whole library (nicely provided by cJSON) is optimized for easy usage. You can create and modify the object as easy as possible.

Limitations of aJson

Unfortunately everything comes for a price and complex data structures and text processing is not really the primary domain for Arduino. So some simplifications had to be made:

• aJson is not really suitable for ATmega168 based Arduinos – there is just not enough RAM – and the library itself eats up toe 80% of the flash memory.
• aJson cannot handle Unicode.
• Lists and Objects can only handle up to 256 entries.
• All strings are case sensitive, this is no real problem for values. But if you want to find a name you have keep in mind that it works case sensitive.

A lot of these limitations will be gone in the future versions of aJson.

Further Development of aJson

Currently aJson is considered to be in a beta phase. It is usable, but it can get better. The most important task is to further reduce the memory footprint. Therefore the API may change a bit in next versions. When the library is better tested and more evolved it will be package as real Arduino library.

I hope that many people will use and test the library. So go ahead and download aJson!

A short German sumary, English version after the click:

Auf zur Palo Altonale! Zusammen mit Tinkerlog, Palo Altona und Interactive Matter bietet die Good School einen Kurs zu Physical Computing & Tinkering in Hamburg an. Ziel des Workshop ist es Kreativen in 2 Tagen die Umsetzung von interaktiven Physical-Computing-Projekten auf Basis von Arduino zu vermitteln.Wer immer schon mal interaktive, elektronische Spielereien selber basteln wollte ist bei uns bestens aufgehoben.

Wir geben nicht nur eine Einführung in die Programmierung mit Arduino sondern helfen jedem ein eigenes kleines Projekt umszusetzen! Jeder Teilnehmer erhält einen Arduino mit Netzwerkverbindung, notwendige Bauteile und Sensoren. Grundkentnisse sind eigentlich keine erforderlich (OK, Computer bedienen istsollte drin sein). Wer dann schon mal eine Programmiersprache gesehen hat (ActionScript, JavaScript, oder was auch immer) kann mitmachen. Wer denkt er könne keinen Arduino programmieren wird eines Besseren belehrt.

Der Workshop findet am 29. & 30.10.2010 statt. Weitere Informationen gibt es bei der Good School oder Facebook. Wer Interesse hat innerhalb des Workshops spezielle Themen umzusetzen will (MIDI, Robotik, …) meldet sich frühzeitig bei der Good School – dann können wir darauf gerne eingehen.

And now for the English description …

Tinkerlog, Palo Altona and Interactive Matter will help Good School to offer a Arduino workshop in Hamburg. The idea of the workshop is to bring creative professional in touch with physical computing. Every participant will build a physical project interacting with the internet. The basic concept of the workshop is pretty simple. Every participant gets an Arduino with a network connection and some physical interface sensors.  We will give a short introduction into the Arduino platform and how to program it. Everybody who had a look at ActionScript or any other programming language will be able to programm his Arduino. Don’t be shy, since you think you cannot program – nobody will be left behind. The most important part of the workshop will be free tinkering. You can design and realize your project. We will help you with any question, programming or sensors that you need! We will bring a plethora of stuff and experience! If you are in Hamburg on the 29. & 30.10.2010 contact the Good School and register! Uhmm, the course will be held in German.

Related posts:

• Palo Altona – Hamburg Tinker Drinkup
• Palo Altona will build Hamburg’s first Makerbot

Developing software – or better firmware – for the Atmel AVR can be quite easy or quite complicated. A lot of people like to just use vi, some source files and a make file. Here at Interactive Matter we are a tad lazy and want a fully fledged IDE, with code completion, one click building, no make files and buttons to flash the AVR. The easiest was is to achieve this with Open Source Software, using avr-gcc, avrdude and avr-eclipse.

To help anybody who wants to use this very convenient package Interactive Matter offers a complete guide on how to install and use avr-gcc, avrdude, Eclipse and the avr-eclipse plugin.

You all know the Blinken Button Kit (aka Space Invaders Button). One of the biggest flaws of this kit is that it is not really a beginners kit. But with the help of Jeff (really big thanks for that) I was able to convert it to a perfect SMT beginners kit. The design was drastically simplified to make it easy to solder, while maintaining the original functionality.

The original Blinken Buton design has undergone some changes to make the desing more SMT beginners-friendly:

• Massively reduced number of components (removing the Arduino comatibility).
• Very clear component marking.
• All resistors and capacitors use a 1206 packaging for ease of assembly.
• Enhanced LED-Matrix footprint for easier testing.

The most important change was to improve the component marking and using hand-solder-friendly 1206 components. It now has a clear marking which component goes where and for the ATMega orientation:

The whole kit can be soldered by SMT novice and is easily testable – so if you was looking for a SMT beginners kit this is the one for you!

The LED matrix footprint has clear markings which pin is an Anode (square pins) or Cathode (round pins) to enhance testability. It is easy to ensure that you have a  working design before you solder the LED matrix to reduce the probabilty of errors.

Related posts:

• Blinken Button – The LED Matrix Button Kit
• Space Invaders Button

Palo Altona is happy to announce that we will build the first MakerBot Cupcake CNC in Hamburg (at least according to Google). Good School approached Alex of Tinkerlog, me and Palo Altona in general, if we want to help to build their MakerBot. And yes we are happy to help.

The weekend project will be documented over at Palo Altona (mostly in German). Some updates will be posted here too.

If this weekend is successful the good boys and girls at Good School will have a nice functional MakerBot Cupcake CNC (probably the first in Hamburg) and I am looking forward to it!

Related posts:

• Palo Altona – Hamburg Tinker Drinkup
• Palo Altonale – Learn Tinkering at the Good School

SkatePOV brings POV to your skate board. It is a minified version of MiniPOV2 attachable to your skateboard. Draw texts while skating!

The LEDs pulse to write the text. By moving the skateboard (aka skating) the blinking is converted into a matrix and the blinking becomes the text. This is the first project in a series to bring more effects to your skate board.

Construction

The schematics is drastically reduced to the original version, the LEDs have been reduced to white 0603 LEDs. and layd out in a very small PCB, small enough to not touch the ground or come anywhere where it is physically endangered (at least on my long-board). And yes, the silkscreen is one of my major fails. I do not know how often I checked the PCB but never spotted the real big mistake – no matter.

On the back there is a small NCP1402 step up converter to power it from two AAA batteries, with some too big capacitors. The ATTiny2313 is used in the very small and convenient MLF Package (a lot of pins on a very small space).

The PCB is mounted vertically on the battery holder. The whole device is attached with Velcro Tape to the skateboard to make it removable (a anti theft prevention?).

The Effect

In the end it gives a very nice (and in this version boring) text while skating around. You cannot see it good enough in the first picture, since it was a bit too dark while I made this photos, here is a better version:

But it was really worth doing it in SMT. What did we learn from this? Doing a daylight POV is much harder than I thought. It works after sunset and of course in the night, but there is much more power needed to be daylight compatible. More powerful LEDs, with a much smaller viewing angle is needed. But it was fun and pimping skateboards with light is a topic too long neglected. I think there is more to come. So stay tuned.

Related posts:

• Low Voltage RGB LED
• How was your day, darling?
• µTVBG – World Smallest TV-B-Gone clone

You may know that I played around a bit with the BMP085 barometric pressure sensor. Now I am proud to announce that the BMP085 barometric pressure sensor is available as breakout board in the Interactive Matter Shop.

The BMP085 Breakout Board is a limited offer. I just got some at hand and supplies last as long as supplies last and are gone after they are gone. They come completely pre-assembled and tested.

Related posts:

• Arduino & Barometric Pressure Sensor BMP085
• Interactive Matter goes Kit
• Filtering Sensor Data with a Kalman Filter

Interactive Matter proudly present the first Interactive Matter SMT Kit
The Blinken Button! Head over to the shop to grab yours!

The Blinken Button is a kit for a 8×8 LED Matrix button (the jacket one). It is based on the Space Invaders Button you may already know. Get all the information on the product page.

History

The Blinken Button is a direct descendant of the Space Invaders Button. It is optimized to be an entry level SMT kit. It did not really work out to be total entry level, since all the electronics is behind the display, which makes it a great gift or fashion accessory, but if you made a mistake in soldering – there is no way to fix it (easily). So very intensive testing is needed. But if this is not your first kit and you already did some mistakes, you can handle it.
It is basically Arduino compatible. But it comes without the Arduino code out of the box. This is something that will be enabled quite soon.

The need for a name

Coming up with a name was the most complicated thing. I positively did not wanted to use the name Space Invaders Button. I do not want to fight with Atari or whoever got the name now.
Using then name 64 Pixels Button was something I tried and it really made it onto the PCB – but Alex had concerns that it interferes with his 64 Pixels project. And he was right.
So the name was Blinken Button. It is so for three reasons: First my girlfriend calls it like that anyway, second it is basically just some Blinkenlights and third it is a homage to the CCC Blinkenlights, I really enjoyed some years ago.

The Future

It will not stop here.

• First the Arduino compatibility will be explored. This will most probably work out with the current version with an software update.
• There is an USB version planned, with much more control for users who do not have an AVR programmer.

Thats all for now. So go over to the shop , join Interactive Matter in the grand shop opening and grab yours.

Related posts:

• Blinken Buttons for Beginners – a SMT Beginners Kit
• Space Invaders Button
• 64 Pixels Roundup

Die deutsche Version gibt es nach dem Klick (und dann weiter unten).

This time it is all non-technical (technically not – but that starts to get complicated). To further encourage tinkering in Hamburg, Alex and I have established a regularly tinker drinkup every Thursday (nearly) at Saal II.

Currently it is a very cosy drinkup, just a few guests. Hopefully this will change. We post announcements with very short notice on Twitter via #palo_altona. If you want to join to discuss some tinker stuff or present your Arduino or tinker projects, just tell Alex or me and we will tell when the next #palo_altona takes place.

Update: There is now an official Palo Altona site for further references.

Und nun zur deutschen Version:

Um auch in Hamburg einen regelmäßige Austauch zu Arduino, Tinkern und Open Source Hardware zu etablieren haben Alex und ich ‘Palo Altona’ gegründet. Ein regelmäßiges, manchmal auch unregelmäßiges, Treffen, jeden Donnerstag, im Saal II in der Schanze. Gäste sind gerne willkommen!

Die Koordination erfolgt recht kurzfristig (leider bisher meist erst am selben Tag) über Twitter (#palo_atona). Meldet euch bei Alex oder mir und wir geben Bescheid ob und wann das nächste Treffen ist. Wir freuen uns über jeden Gast.

Update: Es gibt inzwischen eine offizielle Palo Altona Site für weitere Informationen & Koordinierung.

Related posts:

• Palo Altona will build Hamburg’s first Makerbot
• Palo Altonale – Learn Tinkering at the Good School

This is definitively the last post for this year. So I wish everybody Merry Christmas or whatever you want to celebrate and a pleasant & successful new year!

Even I stopped tinkering and went on to hacking cookies. Have fun & take care!

Related posts:

• Last year in electronics