In this chapter we’ll delve into the early history of electronics. In Chapter 2 we met Samuel Morse and explored the science behind his telegraph. Morse’s telegraph is an electrical device. In this chapter we’ll look at electronic devices. Let’s begin by understanding the difference between an electrical device and an electronic one.

You likely already understand, perhaps intuitively, that light bulbs, space heaters, and kitchen mixers are electrical devices whereas computers, televisions, and radios are electronic ones. But what is the scientific difference?

It comes down to the way electricity is used inside the device. Electrical devices use electric current and change it into another form of energy. For example, electric light bulbs take electric current and turn it into heat and light; your electric kitchen mixer converts electricity into the spinning motion of the beaters.

In contrast, electronic devices don’t simply convert electrical current into other forms of energy; instead they manipulate the electrical current itself to do useful things. Electronic devices actually change the electrical currents into different shapes and frequencies. Electronic devices shape electricity into music, as in a radio, or into visual images, as in television.

De Forest Develops the Amplifying Vacuum Tube

Because he was the son of an Alabama minister, everyone expected that young Lee de Forest would follow in his dad’s footsteps, living a spiritual life and preaching to his congregations. But, it turned out that de Forest was fascinated by science, not theology. He possessed the important gifts that an inventor needs—not only was he good with tools, but he had a passion for making new things. In 1899, he got his PhD from Yale and started working for the Western Electric Company in Chicago. He did well, but before long, he left in order to work independently on projects of his own choosing.

De Forest’s greatest achievement came in 1906 when he developed the first amplifying vacuum tube. This tube could do something new and extremely important: it could increase the power of radio signals.

Up until that point, early radio had a big problem: the signals received by radio sets were so weak that they could only be heard through headphones. Scientists knew that if radio was ever to become really popular, the sound it made had to be loud enough for several people to hear it at once, which meant it needed a loudspeaker (see Chapter 8, “Chester Rice and the Loudspeaker”), and loudspeakers require strong, powerful electrical signals.

De Forest’s invention, which was called the Audion, was an electronic device that could transform a small electrical signal coming in one side of the device into a much larger one when it exited the other side.

The Audion tube had three connections: the anode, the cathode, and the control grid. Current passing through the filament, or cathode, heated it up, which caused it to emit a stream of electrons. The electrons, being negatively charged, were attracted to the positive plate at the top of the Audion. De Forest’s world-changing idea was to place a grid of wires between the filament and the positive plate. When this was done, the grid would become more or less negatively charged as more or less voltage was applied to it. Because the signal to the grid varied, it would control the number of electrons flowing between the filament and the anode. This was how a tube amplifier increased the size of the signal—it used a small signal to control a much large voltage.

Once engineers figured out how useful amplifiers could be, they incorporated them into radios, telephone systems, scientific instruments, and much more. The vacuum-tube business exploded. During the next 50 years, millions of tubes were manufactured. But these tubes had a lot of shortcomings—most significantly, they were big, expensive, and hot.

So, in 1945, Bell Labs put together a team of scientists to find a better alternative to the vacuum tube. The team came up with another sort of amplifier called the transistor. Like a vacuum tube, this silicon-based or solid-state amplifier also has three parts. Current applied to the middle part controlled a much larger current or voltage between the top and bottom parts.

Modern amplifiers are made with integrated circuits that contain the equivalent of thousands or even millions or transistors or vacuum tubes in a single electronic component.

Building an Amplifier-Based Touch Switch

In the following exercise, you’ll use a couple of simple solid-state transistors (although, in principle, you could make de Forest’s vacuum tube Audions work as well) to amplify a tiny bit of electrical current moving through your finger. Using transistors, you are able to construct a device that allows you to build a touch switch that completes a circuit to light light-emitting diodes (LEDs) or sound a buzzer.

Your body has a fair amount of electrical resistance, which is why you can’t just grab two pieces of wire with your hands and complete a low-voltage (6 volts, direct current) circuit to make an LED light up. That’s not to say that no current flows at all; it’s just that your skin presents so much resistance (about 100,000 ohms, depending on how clean and dry your skin is) that the amount of amperage coursing through you is exceedingly small. But if that small amount of electrical flow could be detected and then increased, say through the use of an electronic amplifier, then you could use your finger alone as a switch. In this exercise, we’ll use electronic amplification, the concept pioneered by Lee de Forest, to make a touch switch.

Before you get started, refer to Figure 4-1, which shows the circuit diagram you will build.

To assemble this circuit, complete the following steps.

1. Refer to Figure 4-2. Examine your LED and identify the positive lead (the longer one).

   

2. Hold the transistor with its flat side toward you and its legs pointing down, and identify the collector (left leg), the base (center leg), and the emitter (the right leg).

   Now it’s time to wire your circuit. If you know how to use a prototyping board, then it’s a very easy setup. If you don’t, you can connect the components together in space, and then solder them.

3. Begin by wiring the transistors together so the emitter of the first-stage transistor is connected to the base of the second stage resistor.
4. Then connect the emitter of the second-stage resistor to the negative battery terminal as shown in Figure 4-3.

   

5. Place a 100 K ohm resistor between the first-stage transistor collector and the positive battery terminal.
6. Next, connect the positive end of the LED to the positive battery terminal.
7. Now insert a 220 ohm resistor between the negative LED terminal and the collector of the second-stage resistor as shown in Figure 4-4.

   

8. Now refer to Figure 4-5. You’ll need to use two jumper wires placed vertically on the prototyping board as your touch switch. Place one of the wires on the positive 6-volt rail and connect the other to the first-tage transistor base.
9. Now connect a buzzer in parallel with the LED if you want to.
10. Finally, connect your 6-volt battery pack to the positive and negative rails of the prototyping board as shown in Figure 4-5.

    

In this circuit, the amplifiers are wired in a series to provide a gain, or amplification, that is several hundred times the original current. When you press on the bare wire connected to the 6-volt rail simultaneously with the base of the transistor connected to the 100 K ohm resistor, a tiny amount of current flows from the battery, through your finger, to the transistor base. But that small current is amplified first by one transistor, then the second, increasing it enough to light the LED. Congrats, you’ve made your own amplifying touch switch!

How Amplifiers Work

Amplifiers are devices that take a small signal (imagine a sine wave) and turn it into a larger signal (a much bigger sine wave). The amplified signal looks exactly like the original signal, just much larger. All amplifiers, whether they use vacuum tubes, transistors, or silicon chips, are designed to do just this.

The original de Forest vacuum tube consisted of a high-voltage positive and negative electrical terminal separated by an electrically conducting grid, all locked inside an evacuated glass tube. Inside the tube, a red-hot electric filament spewed a cloud of electrons that were attracted to the oppositely charged plate at the far end of the tube, but were moderated by the presence of an electrical grid (see Figure 4-6).

Unfortunately, tube amplifiers like these are inefficient because they use a great deal of power, break down quite a lot, and give off a great deal of heat.

The invention of the transistor in the late 1940s changed the world of electronics. The transistor uses semiconductors instead of heated terminals, and it uses a control grid to control currents and voltages. Since they are physically much smaller and consume significantly less power, electronic devices built with transistors are faster, cooler, and more reliable than those with tubes.