Friday, December 10, 2010


I love Radio Shack.  Maybe not as much as I used to, but still, I'm very fond of the store.  When I was a kid, when asking for my battery-of-the-month, the polyester-clad Radio Shack salesman would happily give me a thick catalog from the stack behind the counter.  I would spend hours poring through the gossamer pages, examining everything, from resistors to a MOOG synthesizer IC (Which I bought, but never built anything with - I'd love to find another).  True to their name, they sold CB radios, shortwave radios, weather radios, and HAM equipment. 

In high school, I took an electronics class from two math teachers who also taught computer science.  Sadler and Crandell were like a midget wrestling tag team, gleefully alternating between the duties of instruction and running labs.  I enjoyed the class immensely and to pass, I was required to assemble some kind of circuit.  Mr. Crandell preferred a finished product, inside a project case of some sort with integral power and all necessary control and indication hardware.  Kits were acceptable, but I chose to make mine from scratch. 

Finding electronic circuits to construct was a vastly different process in pre-Internet 1979.  I found a circuit for a light activated alarm in a library book, imagining it to be installed inside a dark locker, triggered by some unsuspecting interloper opening the door.   I think I saw a tear of joy in the corner of Mr.Crandell's eye when I asked him to help me with etching a printed circuit board.  The circuit wasn't very complicated, and the circuit board only needed about six components.  I was horrible at soldering, and I ruined the board I had made, delaminating the conductive traces from the board matrix, a murder victim of excessive heat.  Radio Shack to the rescue.

One of the frequently-eyeballed items in the phonebook catalog was a general-purpose prototyping board, about two by three inches.  It was designed around the Dual Inline Package (DIP) integrated circuits ("computer chips") that had been developed in the sixties.  Thanks to Moore's Law, ever more complex circuits could be squeezed into ever smaller chips, and the right circuit board made working with the chips trivial.  As much as I wanted to build a computer (or a synthesizer!), I had to spend some time with the basics first.  My alarm circuit was very simple, and I really just needed some way to mount and connect the components.  Item 276-150 would fit the bill just fine. 

An aluminum box (from Radio Shack) housed the alarm, which used a CdS (Cadmium Sulfide) light sensitive cell to trigger the alarm, and mercury switches to turn it off.  There were no external controls and it could only be turned off by orienting it in a certain non-obvious way.  It was kinda cool. 

I'm reminded of my high school light alarm project because I have been building an Arduino-derived circuit on the trusty 276-150 general purpose printed circuit board.  An important part of my open source project is documenting the circuit and how it is assembled.  Developers of Arduino-compatible platforms must release a schematic and a CAD drawing of the circuit board .  The CAD program frequently used for this is an amazingly capable (and inscrutable) suite of software tools.  Once the schematic has been designed, an intelligent process generates as much or as little of the resulting printed circuit board as the designer / engineer desires. 
I'm kind of going about this process in reverse - I'm starting with an existing PC board and imposing it upon an Arduino-compatible circuit.  This circuit board is literally available in every Radio Shack, in every strip mall, everywhere.   If you buy the lovely teal Arduino board from the Italians, you'll pay about $30 (plus shipping - you can't simply run down the street to a corner store to buy Arduino).  If you buy a kit, you'll be out about half that.  My hope is that you can build my version for $6-8, depending on what you need to buy, and what features you want.  I am essentially  writing a recipe for building my vision of the Arduino-compatible.  With an open source distribution, just like as in  the kitchen, you, the user, can follow the recipe slavishly, borrow ideas from it, or evolve it into your own personal vision.

My goal for this riff on the Arduino motif is to make a microcontroller from locally-sourced (or scavenged) components as much as possible.  The user can opt for absolute minimal support for the ATmega368 chip, on-board regulated power supply, a variety of interfaces, and even a "shield" ( a "daughterboard" that stacks on top of the controller) for custom development.   Lacking the skills to use the CAD software (yet), I am working on distribution documents that describe the circuit, show assembly on the PC board, and I'm doing it all in Microsoft Paint -  that freebie drawing program that comes with Windows.  In the process, I have become intimately acquainted with the 276-150. 

The 276-150 general purpose printed circuit board measures 1.875 inches by 2.875 inches.  A single-sided PC board, there is a printed mask on top that diagrams the copper foil pads underneath.  A mounting hole in each corner allows the business-like use of standoffs to securely mount the finished circuit. Twenty five rows of holes intersect seventeen columns spaced 0.01 inch apart.  One side of the DIP pattern has three columns of discreet, unconnected pads, while opposite, there are only two columns.  The asymmetry slightly offends my OCD, but I leverage the extra room for the on-board regulated five volt power supply.  In all, there are 417 holes in the board.

I am drawing inspiration and direction from several different sources (to be acknowledged at release), and coming up with a name for my Arduino-compatible project was a significant creative challenge.  I settled on 417duino, to honor the noble phenolic rectangle that saved my grade in high school. 

Two bucks at a Radio Shack near you.

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