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I’ve read a lot of computer programming books in my day, and I’ve written a few too. In a computer programming book, the first complete computer program usually shown is a program called Hello World, a program that simply displays the text “Hello World!” on a screen, and then quits. It’s pretty much the simplest possible computer program that can be written. It doesn’t do anything useful, but it’s a great starting point for learning how to write computer programs.

Figure 5-1 shows a schematic diagram that is the electronic equivalent of the Hello World program. This diagram is about the simplest schematic diagram possible that actually does something: It lights a lamp, thus announcing to the world that a circuit is indeed working.

FIGURE 5-1: A simple schematic of a circuit that lights a lamp.

This diagram contains two symbols representing the two components in the circuit: a 1.5 V battery and an incandescent lamp. The lines that connect the two components represent conductors, which could be actual wires or traces of copper in a printed circuit board.

In the circuit depicted in this schematic, the positive side of the battery is connected to one lead from the lamp, and the other lead from the lamp is connected to the negative side of the battery. Once these connections are made, current will flow from the battery to the lamp, through the lamp’s filament to produce light, and then back to the battery.

Schematic diagrams always depict conventional current flow, which, as you learn in Chapter 2 of this minibook, means that current flows from positive to negative. Thus, the current flows from the positive terminal of the battery through the lamp and then back to the negative terminal of the battery.

In reality, conventional current flow is the opposite of the actual flow of electrons through the circuit. The negative side of the battery has an excess of negatively charged particles (extra electrons) whereas the positive side has an excess of positively charged particles (missing electrons). Thus, the electric charge flows through the conductor from the negative side of the battery, through the lamp, and back to the positive side. (For more about the difference between real current flow and conventional current flow, see Chapter 2 of this minibook.)

As it passes through the lamp, the resistance of the lamp’s filament causes the current to heat the filament, which in turn causes the filament to emit visible light.