5.1 Circuit Requirements

We need the source because, without a source of power, the charge won’t move! If we have a circuit, but no electrical power, it will just sit there. In our circuits, batteries will usually provide the power we need. They will do this by providing a (relatively) constant voltage to our circuits.

We need the wires because unless we provide a complete pathway from a higher voltage (the source) to a lower voltage (the destination), the electricity won’t be able to move. If we want the charge to move, we have to make a pathway from the higher voltage to the lower voltage. Without this pathway, we have what is known as an open circuit . No electricity flows in an open circuit.

The components in the pathway from the source to the destination do the “electrical stuff” that we want to accomplish, whether that is turning on a light, running a motor, running a computer, or whatever else it is we want to do. In order for them to take advantage of the electrical power, they have to be in the pathway in which the current is moving. Think of this like a watermill—those structures that sit near a river and use the power of the water moving through the river to turn a wheel and therefore operate whatever is inside the house. In order for that to work, the water must flow through the watermill-connected structure. If the watermill is not built next to the river or if the river runs dry, the watermill doesn’t work. Similarly, if the components are not in the flow of moving electric charge, they will not operate.

However, in addition to the wires and components, we must also have resistance (though sometimes this is added by the components themselves). Without resistance, the current would be too high. If you had zero resistance, the current would be so high that it would immediately drain your battery and likely destroy all of your components that you have connected. You can actually see this using Ohm’s law. If we have a 10-volt source with no resistance, the current is given by the equation I = V/R = 10/0 ≈ ∞. Dividing by zero gives you, essentially, infinite current. Now, wires and batteries themselves have some amount of resistance, so the current wouldn’t be infinite, but it would be very, very large and would quickly drain your battery and destroy any sensitive components you had connected. Therefore, every pathway from the positive side of the battery to the negative must have some measurable amount of resistance. When a pathway from positive to negative occurs without resistance, this is known as a short circuit.

In other words, to accomplish real tasks with electricity, we must control its flow. If it doesn’t flow (as in an open circuit), it can’t do anything. If it flows without resistance (as in a short circuit), it does damage rather than work. Therefore, the goal of electronics is to provide a controlled route for electric charge to follow so that the power of electricity does the things we want it to do on its way from the source to the destination.

5.2 Basic Components

As we have discussed, batteries provide a relatively constant amount of voltage between the positive and negative terminals. A 9-volt battery, therefore, is supposed to have a 9-volt difference between the positive and negative terminals. However, there is an actual range that 9-volt batteries have. A 9-volt battery, depending on the battery chemistry, will deliver between 7 and 10 volts. Additionally, when the battery is worn out, its voltage will decrease as well.

Therefore, in order to make the projects operate more uniformly across different power sources, our projects will often include a regulator, which will take the battery’s voltage (which can vary from 7 to 10 volts) and return a constant 5-volt output. So keep in mind that if the project does not have a regulator attached, the actual voltages in the circuit may vary depending on the battery. If the project does have a regulator attached, it should be reliably yielding a 5-volt difference from the positive to the negative. We will treat the regulator just like a battery, but one that is more stable and reliable than a battery.

A resistor is a device that, as its name implies, adds resistance to a circuit. Resistors have colors that indicate how much resistance they add to the circuit. You don’t need to know the color codes yet, but if you are curious, you can see the resistor datasheet in Appendix E. So, if we want to add 100 Ω to our circuit, we just find a resistor with a value of 100 Ω. Resistors are not the only devices that add resistance to a circuit, but they are usually what are used when you want to add a fixed amount of resistance. Resistors have two terminals (connecting points), but they both function identically—unlike other components, there is no backward or forward for a resistor. You can put them in your circuit either way, and they will function just fine.

Of the components in this section, the LED is probably the strangest. LED stands for “light-emitting diode.” A diode is a component that only allows current to flow in one direction. It blocks the flow of electricity in the other direction. However, more importantly, LEDs emit light when current passes through them. However, LEDs do not resist current, so they must be used with a resistor to limit the amount of current flowing through them (most of them will break at 20–30 milliamps). Also, since LEDs only allow current to flow one way, they have to be wired in the correct direction. To find out which way to wire an LED, look at the legs of the LED—one is longer than the other. To allow current flow, the longer leg of the LED should be on the more positive side of the circuit.

Most of your components (especially your resistors) come with very long legs. You can feel free to bend or cut these legs however you please to better fit in your circuit. However, on LEDs (and any other component where leg length matters), be sure to keep the longer legs longer so you don’t get confused about which way it should be inserted in your circuit.

5.3 Creating Your First Circuit

If you already have resistors, you can use a multimeter to determine the resistance, or you can use the color chart in Appendix E. Most electronics enthusiasts I know don’t actually use the color codes, because they are too small. If you have better vision than we do, then the color chart may work great. Otherwise, use a multimeter to check the resistance, or, better yet, keep your resistors organized so it is easy to find the value you want. If you don’t have resistors, it is even easier—just buy a resistor with the resistance you are looking for!

To make this circuit, take one leg of the resistor and twist it together with the short leg of the LED. It should look like Figure 5-1.

Now, take the long leg of the LED and touch it to the positive terminal of the battery. Nothing happens—why not? Nothing happens because, even though we have connected the wires to the positive side of the battery, the electricity has nowhere to go to, so the current won’t move. We have an open circuit because there is not a complete path from positive to negative.

Now, touch the long leg of the LED to the battery and, at the same time, touch the unattached end of the resistor to the battery. The LED should give a nice glow of its color. Congratulations! You have built your first circuit!

Even though we can’t see the electricity moving, I hope you understand how it will flow through the circuit. We can trace the current flow from the positive terminal of the battery through the LED. The resistor limits the amount of current flowing through the circuit and therefore through our LED (the resistor can actually go on either side of the LED; it will limit the flow no matter which side it is on). Without the resistor, the battery would easily go over the 30-milliamp rating of our LED, and it would short out and no longer work. If you connected it without a resistor, you might see it turn on for a moment and then very quickly turn off, and then it would never work again. If you have an extra LED, you can try this out if you want. It is not dangerous—it will just cost you the price of an LED.

If your LED is backward, no current will flow at all. It won’t hurt the LED, but it won’t turn on unless it is oriented in the right direction.

5.4 Adding Wires

We are not going to physically add wires to our circuit at this time, but I did want to make a note on wires. Changing the lengths of wires will not affect our circuits in any way. On advanced projects (usually projects with extremely high precision or extremely long wires), the length of a wire will sometimes have an effect on such circuits. We are not doing any high-precision circuits, and our wire lengths are all less than a meter. Therefore, for the electronics we are doing, we can totally ignore wire length.

Therefore, if we connected our components using wires rather than directly wrapping their legs around each other directly, it would have no effect on the circuit at all. What is important is not the wires but the connections—what components are connected together and how are they connected. The length of the wire used to connect them is not important. You can connect them with a really long wire or directly touch the legs of each component together with no wires. It doesn’t matter because the result is the same—the components are connected.

5.5 Drawing Circuits

So far, we have only described circuits in words or by showing you pictures. This, however, is a lousy way of describing circuits. In complicated circuits, trying to trace the wires in a photograph is difficult. If you wanted to draw a circuit that you wanted built, you would have to be an artist to render it correctly. Likewise, reading through text describing a circuit takes a long time, and it is easy to get lost when discussing large circuits.

Therefore, in order to communicate information about how a circuit is put together in a way that is easy to read and write, engineers have developed a way of drawing circuits called circuit diagrams or electronic schematics (often just called diagrams or schematics). In a circuit diagram, each component is represented by an easy-to-draw symbol that helps you remember what the component does. Figure 5-2 shows the symbols for the components we have used so far. Note that everybody draws the symbols slightly differently and some components have more than one symbol. However, these are the symbols we will use in this book. For more symbols, see Appendix B.

Then, the components are connected together using lines to represent the wires and connections between the components.

Therefore, we can redraw our original circuit using these symbols as you see in Figure 5-3.

Notice that our components are laid out on the diagram with wires connecting them. Remember that it doesn’t matter if we have very long wires or very short wires or if the components are directly placed end to end—the resulting circuits will operate identically. Also notice that each component is labeled (R1 and D1) because, as we make more complicated circuits, it is important to be able to refer back to them.

It does not matter in a diagram which way you have your components turned, how long or short your wires are, or what the general spacing looks like. When you actually wire it, all of those things will change. The important part of a circuit diagram is to convey to the reader what the parts are, how they are connected, and what the circuit does in the way that is easiest to read.

For instance, all of the circuits in Figure 5-4 are equivalent to the circuit in Figure 5-3, they are just drawn differently. All of them have the positive terminal of a battery connected to a resistor, with the other leg of the resistor connected to the positive end of an LED, with the other leg of the LED connected to the negative side of the battery. Since the connections are identical, the circuits are identical. Schematics don’t care how long the lines are that you draw or how the components are oriented or even where the components are drawn on the page. The only thing important in the schematic is how the components are connected.

For consistency, I like to draw all of my batteries to the left of the drawing with the positive side on top. By keeping the battery positive-side-up, components with higher voltage are usually closer to the top, and components with lower voltages are usually closer to the bottom, with the ground (i.e., 0 volt) coming back into the negative terminal. I also try to make my wire lines as simple as possible in order to make following them easier.

By keeping some amount of consistency, it is easier to look at a drawing and see what is happening.

5.6 Drawing the Ground

Remember that for electricity to move, every circuit must be fully connected from the positive side to the negative side. That means that in larger circuits, there are numerous connections that come from the positive or go back to the ground/negative. Because of this, a special symbol has been adopted to refer to the ground point in a circuit. This symbol, the ground symbol, has three lines, each shorter than the next. Points on a circuit that have this symbol connected to them are connected to each other (usually they are all connected to the negative side of the battery).

Therefore, the circuit in Figure 5-5 is the same circuit as before, just drawn using the ground symbol. Since points with the ground symbol are all connected together, using this symbol on both the negative terminal and the negative side of the LED means that they are wired together.

This doesn’t help us a lot for this circuit (and, in fact, it makes it a little less easy to read). However, in complex circuits, it is much easier to write the ground symbol than trying to have 20 lines drawn back to the negative terminal.

Additionally, the same is true with the positive side of the battery. Many components require a direct connection to a specific voltage to work correctly. These are usually marked with just a disconnected wire with the end of the wire marking what voltage it requires. We make less use of that symbol in this book than the ground symbol, but it does come in handy sometimes.

So, using both the voltage source and ground symbols, we could rewrite the same circuit again in the manner shown in Figure 5-6. This circuit, again, is not wired any differently than before. We are just drawing it differently. For this circuit, it doesn’t matter, but in more complex circuits, if we need a specific voltage at a specific location, this symbol tells us to put it there.

5.7 Review

5.8 Apply What You Have Learned