What is a circuit?

A circuit is a circular path in which electrical current flows. In Chapter 2, Basic Electronics we described electrical current as the flow of electrons. The circuit is the path in which these electrons flow, powering the components that make up the circuit. The point where the electrons enter the circuit, and the current starts, is called the source. The point where the electrons exit the circuit is called the return. Everything between the source and the return, the components and wires that make up the remainder of the circuit, is called the load.

In a closed circuit, the current has a complete path from the source to the return allowing the current to flow through the circuit. The following diagram shows a closed circuit where a switch is closed allowing the current to flow through the circuit. In this circuit, we are powering a light bulb. This makes the light bulb the load:

An open circuit is one that has a break in the circuit that prevents the current flow. The following diagram shows an open circuit where the switch is open, thereby preventing the current flow:

The last two examples show how a typical light switch works in your house. When you turn the switch on, the circuit is closed, which turns the light on; however, when you turn the switch off, the circuit is open, which turns the light off.

Most multimeters have a continuity mode that can be used to test if you have an open circuit or not. If your multimeter has a continuity mode, you should see a symbol that looks somewhat like this:

If you set your multimeter to continuity mode and touch the two leads together the meter will emit a tone. This tone lets you know that you have a closed circuit. To test if your circuit is closed or open, put the multimeter into continuity mode and place the two leads from the multimeter at different points in your circuit, and if there is a connection between the two points the meter will emit the tone.

The most dangerous circuit is a short circuit. A short circuit is one that allows the current to flow through the circuit with little to no resistance. This is usually a circuit with no load.

The following diagram shows what a short circuit looks like:

While the preceding diagrams look nice, it would be almost impossible to show complex circuits with diagrams like these. It is also hard for someone else to understand these diagrams without an explanation, because there are no standard symbols defined. The standard way to represent a circuit is with a schematic diagram.

A schematic diagram, when referring to electronic circuits, is a diagram that contains representations of the elements in the circuit. Each type of electrical component has a standard symbol that is used when representing it on a schematic diagram. These diagrams will show how each component is connected giving anyone that reads the diagram an understanding of how the circuit works.

All the circuit diagrams in this book will be created using the very popular, and free, Fritzing application, which can be downloaded here: http://fritzing.org/home/. At the time this book was written, the latest version of Fritzing is 0.9.3b and is the version used to create all of the diagrams in this book and the diagrams that come with the downloadable code. Let's look at what Fritzing is before we look at the types of diagrams it can produce.