# Computing stuff tied to the physical world

## Circuits and schematics

The world seems to be split into 10 groups of people:

• Either you think in bits and bytes, wondering how on earth those circuits work …
• … or you dream in silicon and electrons, and get lost in all those software trends.

Well, ok, probably there’s an 11th group … :o)

• The 99% out there who have no affinity with either bits, bytes, silicon, or electrons.

Software is trivial. Just a few bits and bytes, interacting in very basic logical ways, right?

Electronics is equally simple. Just a dozen or so types of components, wired together in a certain way to create an electrical circuit. Its behaviour is defined by the laws of physics.

Ah, but the tricky bit is the complexity of those combinations, and their sheer number!

### Schematics

There are only a limited number of electronic symbols, as summarised on Wikipedia:

(well… there are some more, but these are the most common ones)

By drawing them out and drawing lines between them to represent wires, we get a clean overview of the essence of every electronic circuit. Here is the minimal ARM circuit used in the Getting Started series – also known as the “Bare ARM Blinker”:

A connector with 6 pins, a red LED, a 0.1 µF capacitor, and the LPC810 microcontroller. Note the essential convention that lines which crossc are not connected together, unless there is a “junction” dot. For clarity, these dots are normally placed in all the places where three or more connections come together, in this case only the top/bottom of the capacitor.

Such a schematic clearly and unambiguously defines how things are connected together.

Note that the LPC810 chip actually looks very different, and has its pins laid out differently. The point is not resemblance (though that helps reduce the cognitive effort to understand a design), but formal accuracy. Schematics are the lingua franca of the electrical engineer.

You could draw a schematic by hand, or in your favourite diagramming tool, but don’t …

### Design tools

There are many Electronic Design Automation tools to help with the task of drawing and editing schematics. In the hobby world, EAGLE and KiCAD are probably the most used.

While these tools are well-suited to making schematic drawings, the real value of using them is a different one: they are based on building the circuit out of a large library of “parts” which contain many common components in ready-made form. You pick parts from several libraries, place them on the canvas, and draw connecting wires as needed.

When the part you’re using doesn’t have a definition, and can’t be found in any of the libraries available on the web somewhere, you can create your own, and add it to your own library for re-use and sharing. It’s also easy to make variants of parts.

Building up a good collection of parts and parts libraries is, eh, part of the whole process.

Another aspect of a schematic is annotation – you can add information about each part, such as its name and value, and – depending on the program used – in some cases a lot more. Not to mention that you can always add some notes to clarify the design, special details, as well as the author and creation date. Perhaps also a release / version number.

The result could be something like this, which could then be published as PDF document:

So in a way, a part appears to be little more than a template. It gets you going fast when “populating” a schematic, but wait … it does in fact considerably more!

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