Draw.io

For this semester, we are using draw.io as our schematic and block diagram drawing software. It’s a gratis and open-source project which is available as a web-based application, and you can also download a version to run locally on your computer. In partnership with the NSF-funded “Smart Spaces for Making”¹ study on campus, we have generated a series of custom schematic and block diagram symbols which you can incorporate into your drawings.

Adding our custom symbols to draw.io

Using the draw.io website

Go to https://nsfsmartmakerspaces.github.io/physcomp-drawio-library/ to access the custom symbol libraries and add them to your instance of draw.io.

Using the desktop draw.io application

Select File–>Open Library from–>URL… and then enter the appropriate URL into the dialogue:

• For the schematic components, https://nsfsmartmakerspaces.github.io/physcomp-drawio-library/Components.xml
• For the functional block diagrams, https://nsfsmartmakerspaces.github.io/physcomp-drawio-library/Block.xml

Schematics

Schematics, properly executed, serve as compact descriptive documents which communciate the exact configuration of an electrical system. Ideally, a schematic that you produce could be read by any of your classmates to reproduce the system you’ve drawn, without needing to ask you any questions.

In the context of this course, a schematic captures/records:

• the electrical components used, and
• the topological connections made between those components.

Topological here means that the schematic’s job is to show, for instance, what pins of one thing are connected to what pins of another—but not how long the wire is that’s connecting them, or the actual path through space it takes.

In the context of this course, a schematic generally does not record:

• the relative physical size of components,
• the appearance of components,
• the physical arrangmement of components, or
• the size, shape, or type of wires connecting components.

Schematic conventions

Widely-recognized diagrams

There are a variety of standards used by electrical engineers to define the appearance of many common circuit elements. This image captures some elements which are relevant to this course, as they appear in draw.io:

Note that there are many variations of these that appear out in the world. Sometimes, two different ways of drawing an element may indicate a true distinction, whereas other times, they may simply be synonymous references to the same component. Without a singular guiding standard which applies to all schematics (and there isn’t such a standard in the actual industry), unfortunately you will sometimes come across ambiguity when reading schematics from different sources.

We will endeavor to use a common shared of symbols inside of the context of this class, though, as defined by this document.

Custom or class-specific diagrams

There is a simple, generic way of drawing a schematic symbol for just about any component, following this pattern:

If you have more inputs, outputs, power, or grounds, then just add whiskers and label as needed; if you have no input/output/power/ground, then omit the unneeded aspects.

The above-linked library of components in draw.io includes many parts in our kit, drawn in the style of this format. If you need to draw something that isn’t included in the class library, you are welcome to use this editable copy of the generic drawing above as a starting point.

Wire crossing style

There are different standards to annotate wires crossing in schematics, so as you’re reading drawings from outside of this class context keep that in mind.

That said, here is how we’ll indicate whether wires are not connected and merely crossing paths, versus connected electrically:

In draw.io, you can use a feature called “Line jumps” to add the semicircular arc for the figure on the left. The figure on the right is made by drawing two smaller vertical lines which both intersect with the horizontal one. One of these vertical lines has a “line end” which is a dot. Here is an editable copy of the above figure, which you can use to copy the effect if you’re having a hard time figuring it out.

Some Do’s and Do Not Do’s

Functional block diagrams

A functional block diagram operates at one level of abstraction higher than an electrical schematic. It is unconcerned with specific electrical connections and wiring; rather, it captures the information-flow relationships between elements of a system.

A block diagram is unconcerned with power flow; its job is to describe how information flows through the system. The schematic contains no concept of information, but rather is much closer to being a blueprint for fabrication.

We follow the same convention shown above with information entering the left side of an element, and departing from the right side of the element. Note the differences in the drawings below:

There are some subtle diffences in our drawings, as well: block diagrams have their labels drawn outside of their main bodies, on the arrows, and have rounded corners. This is not a standard in industry; it’s just one that we adhere to in this class.

If you were to hand your friend the block diagram on the right and tell them to build the device described, they would have many possible choices, as there are many possible implementations. The schematic, on the other hand, is far more constrained: it says what parts to connect, and how.

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