When I started writing the curriculum for this website, I started by reviewing what is already out there. I was hoping to find out if there really was a need or if the community was already been well served. I found that there's so much information that it can be hard to work through. So while I found all of the information is available, and much for free, organizing it into a class is really hard to do. So I decided to take on this project and develop a set of curriculum to help students new to electronics work through the basics through to advanced level of electronics, circuit design, digital logic, microcontrollers, and robotics.
While doing research, I reviewed a number of classes, curriculum, books, and websites. To my surprise, I ran into numerous that had major errors. This tells me that the authors do not have a background in electronics, engineering, or software, but are simple authors and teachers. I guess that should be expected, but I find it troubling.
Teaching Current
One of the errors that really bothers me is in regards to the current flow. A typical approach is explain how to use a resistor to protect an LED. They move forward and build an array of LEDs, each with a resistor. They then explain how the current flows out of the battery's positive terminal to the resister, then the LED, then back to the battery.
In circuit design you have to talk about current flowing like this, but it is not actually how it flows. The way you can test this is to simply move a 220 ohm resistor and position it between the negative rail and the battery. It works exactly the same as having it sit between each LED and the positive terminal of the battery. What this proves is that the current is not flowing the way we talk about it flowing. It's a common beginner level misunderstanding. But we have come up with a model like this so we can talk about it and do circuit analysis.
While we talk about current flowing like this, we should then point out that it is not truly what is going on, but a model to help us work with it, and explain how this effects or circuit design. For a student learning circuit design, it is more troubling because it will really set them back in their understanding of circuit analysis.
Here's a great video on current flow from Khan Academy.
Now, one resistor per LED maybe be in fact what you need, as if you turn on more and more LEDs, the total power available will drop and the LEDs will become more dim as you turn on more of the LEDs. Using one resistor per LED resolve this. But if you are only turning on one or two LEDs at a time, a single LED on your ground will suffice. If you are just building a breadboard prototype, often using one LED on your negative rail is probably your better choice, as you don't need to cram as many components on your breadboard. But maybe in a final circuit board design you'll use one resistor per LED. My point is, you really need to understand how and why this works, so you can make the correct decisions.