Table of Contents
This page is about schematics, specifically schematic diagrams of electrical or electronic circuits.
Here's a video introduction that may be useful: https://vimeo.com/90280008
The basic idea in a schematic is that symbols stand in for components, with lines representing wires or connections between them. Many of the symbols you'll see are described here
Components
The schematic describes the electrical part of what we're going to design or talk about - what it does, while abstracting away the physical details - what it looks like. For example, these are all resistors:
One thing they all have in common is that they have a property called resistance, which measures how much they resist or impede the flow of current. For many of the design decisions and calculations about a circuit, that's all we need to know. So we use the same schematic symbol for all of them, in part to emphasize that we can reason about them all in exactly the same way.
When we go from the schematic to PC board layout, we bring back the physical details:
- what is the exact part number that we need to by in order to build our circuit?
- what shape is the resistor, switch or other component?
- what size is it?
- where do we put physically it in relation to the other components?
Often, answering these questions leads us to other properties of the component, that help us choose between the many different ones that use the same schematic symbol.
One last thing before we move on from components: there can sometimes be more than one schematic symbol for a component for no good reason other than the history and tradition of the drawing style. Again using resistors as an example, the traditional symbol is a jagged line, while the European style is a long, skinny box:
No reason for the difference here, just tradition. You can pick the style you like, but should be able to read and understand other styles.
Connections
When lines cross, there's no connection between the wires they represent. Connection is shown as a dot, which is called a node.
The crossing on the left shows a connection between the two wires. The crossing on the right has no connection.
There's another way to show connections, by name. If we draw this:
(picture TODO - see the sparkfun link for now)
the fact that those two wires are both labeled “pwm3” means that they are connected, and also connected to any other wire labeled “pwm3” on the schematic.
Connection by name can be a huge help in keeping a complex schematic uncluttered. Here's an old schematic, before the named-wire convention was common. It also shows an old style of wire-crossing, that emphasizes the one wire “jumping over” another.
On the other hand, connections by signal name can also hinder reading, by causing a long, slow search for labels every time we want to understand how one component affects something else. Good schematics use both techniques, as appropriate. And when choosing signal names, make them good and descriptive, but not too long.