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Before we start into a discussion of digital electronics, it would help to start with a discussion of electricity and basic electronic circuits. You may already know this information from a Physics class, but a review will still serve as a good starting point.

What is electricity?

"Electricity" is a form of energy created by the flow of an electrical charge through a conductor. The atoms making up all matter contain protons and electrons; an imbalance between the protons and electrons in a substance cause that matter to have an "electrical charge," positive if the protons outnumber the electrons, and negative if the electrons outnumber the protons. A conductor is any substance that allows the transfer of electrons without changing its own nature; metals such as copper and gold are excellent electrical conductors. When electrons flow from a negatively charged source to a positively charged destination through a conductor, this forms an electrical circuit. This is a transfer of energy, and a circuit can put this energy to work. A simple example is electricity flowing through a light bulb; the filament of the bulb is a material which converts some of the electrical energy passing through it into light and heat.

This inbalance between electrons and protons can be created in a lot of ways. An obvious example is a battery, which is a manufactured object whose materials have a considerable electrical charge from one end to the other. Placing a battery into a circuit which is "on" allows the electrons to flow, both powering the circuit and gradually depleting the battery. When the battery is "dead" it no longer contains enough electrical charge to perform any useful work. The steady flow of electrons in the case of a battery is an example of "direct current" (DC).

Another electrical power source with which we are all familiar is the wall outlet in our homes. Distant electrical generating stations create huge amounts of electrical charge, using other forms of energy (usually steam or water) to turn magnetic coils. The electrical power generated is so great that it can travel many miles through the power lines to many individual homes and businesses. The nature of this generated power is such that the imbalance between electrons and protons is constantly cycling. If you measured the imbalance at one instant you would see that one conductor had a positive charge with respect to the other, but if you measured it at another moment you would observe a negative charge betwen the two. The charge changes continuously in a cycle from positive to negative to positive again, based on the rate at which the magnetic coils change their relationship in the generator. Electric supply in the United States cycles through both charges sixty times each second. This sort of electrical source is called "alternating current" (AC). Alternating current is a good way to move electricity over long distances, and for some applications (such as light bulbs) the direction of electron flow doesn't matter. However for other applications, including most electronics, the direction of electron flow must be constant. In these applications, an alternating current source must first be "rectified" into direct current.

There is a large and interesting area of study in electricity and electronics related to power supplies and power transmission. However, in the field of digital electronics we pretty much ignore all of this. We simply need there to be a direct current power supply of particular characteristics as a starting point.

Properties of electricity

When we describe our needs for electrical supply in a circuit, there are two properties we need to care about. The first of these is voltage. Voltage is simply a measure of the magnitude of the electrical charge between two points. It is always a relative measure; a single point has no inherent voltage. The standard unit for measuring this charge is a "volt" (V); the charge is described as some number of volts. For example, measuring the tip of a flashlight battery relative to the base of the same battery gives us one and a half volts (1.5V). If you instead measured the base of the battery relative to the tip, you would instead see negative one and a half volts (-1.5V). If you measured the charge between the two slots in a wall outlet you would see a constantly changing voltage. (Since such a measurement is not a useful way to describe alternating current, an AC source is generally measured according to the average absolute voltage difference. In mathematical terms this is determining the area within the sine wave that describes the actual voltage. The average "root mean square" voltage of your wall outlet is then described as around 115V RMS, or more commonly 115VAC. Note that at the peak of the wave, the DC voltage is actually higher than 115V.)

The other important property of electricity we should consider is current. This basically describes the rate of flow of electrons through a circuit. When a battery is not connected to anything there is no transfer of electrons, so the current is zero. When that battery is connected to a light bulb, there is some rate at which electrons flow through the circuit. This flow is measured against a standard unit called an "ampere" (usually called "amp" for short, and written as the letter A). As you might guess, a faster flow of electrons causes the battery to be depleted sooner. It is generally a goal of electric circuits to use the minimum current to get a job done, in order to not deplete your power supply. In digital electronics the current used is generally small, and is more often described in terms of milliamps (mA, or thousandths of an ampere).

If you connected a wire directly from one end of a battery to the other, the electrons would flow quite rapidly, limited only by how much current the wire could support. The wire would get warm, and the battery would quickly be spent, but no useful work would have been done. This is called a "short circuit," and as you might guess, the current involved is quite high. This rapid flow of electrons can be dangerous, both to the circuit and to the power supply. The electric supply in your home uses fuses or circuit breakers to protect against short circuits; these stop letting electricity flow if the current exceeds some rated number of amps.


Electricity can be put to a number of uses, such as creating light or heat, or driving motors in appliances. But there are a number of effects that occur in various materials when electricity flows through them. Electronics is the study that focuses on using these electrical effects for some purpose.

As just one example, a coil of wire that is passing an electrical current becomes a magnet. The strength of the magnet is related to the voltage in the coil. If you attach a regular magnet to a paper cone, and place the coil close to the magnet, varying the amount of voltage going through the coil causes the magnet to be more or less attracted to the coil, which in turn causes the paper cone to move. If you vary the attraction rapidly enough, the movement of air caused by the paper cone's movements becomes a sound you can hear. This is a speaker. The speaker's air displacement (sound) is directly related to variations in the voltage provided. If you have a voltage source which, somehow, changes in the right ways over time, you produce complex sounds (voice, music and so on).

The signal for the speaker might come from an amplifier, which is simply a circuit which causes the same relative variations in voltage to occur on a wider voltage range. That is, take a signal with fluctuations that will represent sound, and effectively "multiply" that signal to a wider range of voltages. This is accomplished with a variety of electronic components, each of which have some basic principles of what effect they each have on the flow of electricity through the circuit. A volume control causes still another mathematical effect, where the range of the amplification is increased or decreased proportionally to the flow of electricity through the component within the knob.

...And so on. Electronics applies a great deal of analysis and mathematics, combined with the understanding of how certain components effect changes in an electrical circuit, to accomplish desired effects. Looked at in terms of a total system (such as, for example, a television), the electronics are almost insanely complicated! But each component is functioning according to a set of known principles, allowing circuits of increasing complexity to be built from combining them.

One of the things that makes electronics so complicated is that every change effected by a component has some small element of error or uncertainty. For example, a component called a resistor causes a reduction in the voltage or current of a circuit by a known amount, but that rated amount is subject to a certain degree of precision (such as a 1% tolerance). This means that the mathematical effect could be as much as 1% different from the expected result. As the complexity of a circuit increases with the addition of more components, the range of possible mathematical outcomes increases. Further, the components can be effected by changes in temperature or other external factors. So the design of electronic circuits must consider these effects and still perform the desired function.

If you think electronics sounds like a complex and challenging field, you would be absolutely correct! Fortunately, however, digital electronics makes a number of simplifications. These allow us to think very little about these issues, as you will see in the next section.

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