What It's All About
 
Anyone who understands basic electronic theory can explain the following diagram, no matter what language he speaks.
 


Engineers have discovered interesting ways to control the flow of electricity for many useful purposes. The way they share these ideas with each other is by using symbols. In the drawing above (called a SCHEMATIC) the symbol next to the B is a battery, the symbol next to the S is a switch, and DS is a display or a lamp. Anyone who knows how to interpret the schematic can build the  circuit it represents by connecting a battery, a switch and a lamp together with conductors. 

Maybe you have already recognized the circuit above as a simple flashlight
 

flashlight picture
but...
The same circuit could also be built a different way. You could take the battery out of a smoke detector, pry the doorbell button out of the jamb, and clip one of the light sockets out of your car, twist the wires together and tape it all down to a piece of cardboard!
 

home-made circuit board

While the creation above works, it is not easy to manufacture, nor is it a very reliable solution.
Most circuits are constructed by designing a board that the components can be mounted to. The components have metal "feet" (called leads) that are conductive, they sit on a metal "footprint" that is also conductive, and they are welded in place with a material called solder that is also conductive.

Soldering components onto a circuit board accomplishes two things;
 

  • it provides the mechanical attachment that supports the component, and

  • it provides an electrical connection between the component and the board.

Instead of twisting wires together, the board itself has conductors built into it (called traces and planes) that connect the footprints together, and creating these boards with footprints and conductors is what circuit board designers do.
That's enough about the physical board for now, but notice the difference in the new picture below:
 

circuit board

In the schematic, components are identified by a  reference designator (the letters B, S and DS), but there is no information about what type or what size the components should be. Consider the fact that there are many different types of batteries available, but nothing in the schematic above suggests which battery would be best. To be useful, a schematic must also provide some attributes to insure that an appropriate component is selected. Symbols are also assigned numbers to differentiate between components of the same type. For example, the reference designators for two resistors would be R1 and R2. Other attributes that are often added to the symbols are Values, Power Ratings and Tolerances. CAD systems may even assign hidden attributes like part numbers or simulation data. The next schematic is a more typical example of what schematics look like.

 

insert real schematic here

That's as much as we need to know about schematics for now. We'll cover more of the details later on...
Here's a chart of the most common SYMBOLS