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All this is really saying that as time passes, the output of the oscillator alternates between 0 and 1 on a regular basis. For that reason, an oscillator is sometimes often referred to as a clock because by counting the number of oscillations you can tell time (kind of).

How fast will the oscillator run? That is, how quickly will the metal contact of the relay vibrate back and forth? How many times a second? That obviously depends on how the relay is built. One can easily imagine a big, sturdy relay that clunks back and forth slowly and a small, light relay that buzzes rapidly.

A cycle of an oscillator is defined as the interval during which the output of the oscillator changes and then comes back again to where it started:

The time required for one cycle is called the period of the oscillator. Let's assume that we're looking at a particular oscillator that has a period of 0.05 second. We can then label the horizontal axis in seconds beginning from some arbitrary time we denote as 0:

The frequency of the oscillator is 1 divided by the period. In this example, if the period of the oscillator is 0.05 second, the frequency of the oscillator is 1 ÷ 0.05, or 20 cycles per second. Twenty times per second, the output of the oscillator changes and changes back.

Cycles per second is a fairly self-explanatory term, much like miles per hour or pounds per square inch or calories per serving. But cycles per second isn't used much any more. In commemoration of Heinrich Rudolph Hertz (1857–1894), who was the first person to transmit and receive radio waves, the word hertz is now used instead. This usage started first in Germany in the 1920s and then expanded into other countries over the decades.

Thus, we can say that our oscillator has a frequency of 20 hertz, or (to abbreviate) 20 Hz.

Of course, we just guessed at the actual speed of one particular oscillator. By the end of this chapter, we'll be able to build something that lets us actually measure the speed of an oscillator.

To begin this endeavor, let's look at a pair of NOR gates wired a particular way. You'll recall that the output of a NOR gate is a voltage only if both inputs aren't voltages:

NOR

0

1

0

1

0

1

0

0

Here's a circuit with two NOR gates, two switches, and a lightbulb:

Notice the oddly contorted wiring: The output of the NOR gate on the left is an input to the NOR gate on the right, and the output of the right NOR gate is an input to the left NOR gate. This is a type of feedback. Indeed, just as in the oscillator, an output circles back to become an input. This idiosyncrasy will be a characteristic of most of the circuits in this chapter.

At the outset, the only current flowing in this circuit is from the output of the left NOR gate. That's because both inputs to that gate are 0. Now close the upper switch. The output from the left NOR gate becomes 0, which means the output from the right NOR gate becomes 1 and the lightbulb goes on:

The magic occurs when you now open the upper switch. Because the output of a NOR gate is 0 if either input is 1, the output of the left NOR gate remains the same and the light remains lit:

Now this is odd, wouldn't you say? Both switches are open—the same as in the first drawing—yet now the lightbulb is on. This situation is certainly different from anything we've seen before. Usually the output of a circuit is dependent solely upon the inputs. That doesn't seem to be the case here. Moreover, at this point you can close and open that upper switch and the light remains lit. That switch has no further effect on the circuit because the output of the left NOR gate remains 0.

Now close the lower switch. Because one of the inputs to the right NOR gate is now 1, the output becomes 0 and the lightbulb goes out. The output of the left NOR gate becomes 1:

Now you can open the bottom switch and the lightbulb stays off:

We're back where we started. At this time, you can close and open the bottom switch with no further effect on the lightbulb. In summary

Closing