In the real world, building circuits often start with a “schematic” for the circuit. Just like a blueprint tells a builder exactly how to build a house, a schematic tells an engineer how to build a circuit.
A schematic for a circuit might look something like this:
Looks pretty complicated, huh?
But, if you stick with me through this section, I think you’ll find that, even a complex looking schematic like that, is really not too difficult to understand.
Let’s take this step by step…
Schematics use symbols to represent all the common parts you will find in a circuit. The symbols don’t always look like the parts they represent, so I’ve provided a little cheat-sheet right here:
You may see some slight variations between how some of these things are drawn, but if you use the chart above as your reference, you should be able to figure out these basic components in any schematic.
You’ll also notice that in some cases, the symbols may contain (+) and (-) signs that indicate which side of the part is which – for example, which side of the battery is the positive (+) side and which is the negative (-) side. And in some cases, you’ll see a number next to a part indicating what size of that part to use – for example, a resistor may have something like “100 Ω” next to it, indicating that the resistor should be “100 ohm” in size.
In a schematic, you’ll notice that all the parts are attached to each other by lines. These lines represent the wires through which electric current in the circuit flows.
Wiring a circuit is as easy as attaching the wires to the parts (and to each other) as indicated in the schematic. If you can replicate the drawing using real wires and parts, your circuit will work as expected.
Let’s use the following schematic as an example:
Does this look familiar? It should – this is a schematic of the circuit we built in the previous project.
You probably noticed in our schematic above that the entire circuit made a loop. This makes sense given our definition of a circuit as needing to be a closed loop for it to work (remember, in an open circuit, electricity can’t flow). Conceptually, we can think of the electricity as flowing through the circuit starting at the (+) terminal of the battery and finishing at the (-) terminal of the battery, the ground.
A good way to think about current flow through a circuit is to assume that the flow starts at a particular power source and finishes at the ground of that power source. And in many schematics – like the one we’ve been using above – the circuit will appear as a closed loop, starting at one side of the power source and completing at the other side. And while this is how circuits work, it’s not always convenient to draw circuits in a closed loop. Especially in complex schematics, where having to draw everything in a loop can make the drawings very difficult to read.
Which is why schematics will often separate the two sides of the power source, with the power source at the top of the schematic and the ground at the bottom. This convention makes the schematic much simpler to read (and to write when you're the one creating the schematic).
Let’s take a look at how our schematic above would typically be drawn in the real world:
The circuit that this schematic represents is exactly the same as the circuit we presented above (the one in the shape of a loop). At the top of the schematic is the power source – indicated by the “3.3V”. Just like in our previous schematic drawing, the power source is connected to the Resistor, which is then connected to the LED.
In this schematic diagram, the other lead of the LED doesn’t loop back to the power source, but instead connects to a symbol that represents “ground.” In the real world, the circuit would connect back to the (-) side of the power source (which is ground), but in the diagram, the power and ground are separated to make the diagram simpler and easier to read.