Chapter 6

Working with Transistors

IN THIS CHAPTER

Learning about the miracle device of electronics

Comprehending the basic operation of a transistor

Surveying the many different types of transistors

Using a transistor as an amplifier

Using a transistor as a switch

Building some simple transistor circuits

I was born just a few years after the beginning of the transistor age. As a young boy, I sometimes took the back off of our old black-and-white television set (when my parents weren’t home) and marveled at the neat electronic stuff inside. Besides the huge picture tube, the most interesting gadgets in there were the little glass tubes the size of my thumb. As I remember it, there were hundreds of them, each one glowing with a comfortable warm orangish glow.

By the time I was old enough to start studying electronics, that old TV set was replaced by a new color set that wasn’t nearly as interesting to look at inside. The big picture tube was still there, but all the little glowing thumb-sized tubes had been replaced by little silver cans smaller than a thimble.

Transistors had taken over, and I hated them. Tubes were much more interesting than transistors. They got hot and glowed. You could see little wire structures inside them — little towers with meshes and grids and who knows what else. Transistors just looked like little thimbles. It was as if someone had built a television set out of stuff they found in my mom’s sewing drawer.

I was quite certain that the spaceships on my favorite TV shows like Star Trek, Lost in Space, and Land of the Giants were all run on tubes, not transistors. It wasn’t until I found out that real spaceships like Gemini and Apollo were filled with transistors and had nary a tube that I decided maybe transistors were okay. If they were good enough for NASA, they were good enough for me.

In the 40 years since my dad bought that first color television set, the little thimble-sized transistors have given way to transistors that are literally millions of times smaller. Nowadays, we can put 100 million transistors on a single piece of silicon crystal about the size of your fingernail.

In this chapter, you take a look at what transistors are and how you can put them to use in your own circuits. Along the way, you build a few simple transistor circuits to learn how they work.

What’s the Big Deal about Transistors?

Here’s an interesting thing about the transistor: When it was invented in 1947, it didn’t really do anything that hadn’t already been done before. It just did it in a radically different way.

The basic idea behind a transistor is that it lets you control the flow of current through one channel by varying the intensity of a much smaller current that’s flowing through a second channel.

Think of a transistor as an electronic lever. A lever is a device that lets you lift a large load by exerting a small amount of effort. In essence, a lever amplifies your effort. That’s what a transistor does: It lets you use a small current to control a much larger current.

Figure 6-1 shows several of the many different kinds of transistors that are available today. As you can see, transistors come in a variety of different sizes and shapes. One thing all of these transistors have in common is that they each have three leads.

FIGURE 6-1: Transistors come in many shapes and sizes.

Why were transistors invented?

Devices that performed the function of transistors had been around for 30–40 years prior to the invention of the transistor. They were called vacuum tubes. A vacuum tube consisted of a vacuum chamber made from glass or metal, a heating element that heated the space inside the chamber, and electrodes that protruded into the chamber. (I use past tense here because although vacuum tubes still exist, they really aren’t used all that often.)

One specific type of vacuum tube was called a triode; it had three electrodes. In a triode, a large current flowing through two of the electrodes (called the anode and the cathode) could be regulated by placing a wire grid (called the control grid) between the cathode and the anode. Applying a small current to this grid slowed down the flow of electrons between the cathode and the anode.

It didn’t take long to figure out that you could use a fluctuating signal such as a radio or audio wave on the control grid. When you did that, the current on the anode followed the fluctuations of the control grid current, but with much larger variations. Thus, the triode was an electronic lever: Small variations in current at the control grid were amplified to create large variations in current at the anode.

The vacuum tube triode was patented in 1907 and was the key invention that enabled the development of radio, television, and computers. But vacuum tubes had many serious limitations: They were expensive to manufacture, big (the small ones were about the size of your thumb), required a lot of power to operate, generated a lot of heat, and lasted only a few years before they burned themselves out.

The transistor changed all that. A transistor performs the same function as a vacuum tube triode, but using semiconductor junctions instead of heated electrodes in a vacuum chamber. Although the transistor didn’t do anything that the vacuum tube triode didn’t already do, it did it in a radically different way that had huge advantages over the vacuum tube. The earliest transistors were small, required very little power to operate, generated much less heat, and lasted much longer than vacuum tubes.

Looking inside a transistor

There are many different kinds of transistors. The most basic kind is called a bipolar transistor. Bipolar transistors are the easiest to understand, and they’re the ones you’re most likely to work with as a hobbyist. As a result, most of this chapter focuses on bipolar transistors. I describe some of the other types of transistors later in this chapter. But for now — indeed throughout this entire book — you can assume that whenever I use the term transistor by itself, I’m referring to a bipolar transistor.

Now let’s peer inside a transistor to see how it works.

In the previous chapter, you learn that a diode is the simplest kind of semiconductor, made from a single p-n junction, which is simply a junction of two different types of semiconductors, one that’s missing a few electrons and thus has a positive charge (p-type semiconductor) and the other with a few extra electrons, thus having a negative charge (n-type semiconductor).

By itself, a p-n junction works as a one-way gate for current. In other words, a p-n junction allows current to flow in one direction but not the other. A diode is simply a p-n junction with leads attached to both ends.

A transistor is like a diode with a third layer of either p-type or n-type semiconductors on one end. Thus, a transistor has three regions rather than two. The interface between each of the regions forms a p-n junction. So another way to think of a transistor is as a semiconductor with two p-n junctions.

Figure 6-2 shows the structure of two common types of transistors along with their schematic diagram symbols. The details shown in this figure are explained in the following paragraphs.

FIGURE 6-2: NPN and PNP transistors.

One way to make a transistor is with a p-type semiconductor sandwiched between two n-type semiconductors. This type of transistor is called an NPN transistor because it has three regions: n-type, p-type, and n-type. It’s shown in the top part of Figure 6-2.

The other way to make a transistor is just the opposite, with an n-type semiconductor sandwiched between two p-type semiconductors. This type is called a PNP transistor because its three regions are p-type, n-type, and p-type. It’s shown in the bottom part of Figure 6-2.

Each of the three regions of semiconductor material in a transistor has a lead attached to it, and each of these leads is given a name:

• Collector: This lead is attached to the largest of the semiconductor regions. Current flows through the collector to the emitter as controlled by the base.
• Emitter: Attached to the second largest of the semiconductor regions. When the base voltage allows, current flows through the collector to the emitter.
• Base: Attached to the middle semiconductor region. This region serves as the gatekeeper that determines how much current is allowed to flow through the collector-emitter circuit. When voltage is applied to the base, current is allowed to flow.

These two current paths are important in a transistor:

• Collector-emitter: The main current that flows through the transistor. Voltage placed across the collector and emitter is often referred to as V_ce, and current flowing through the collector-emitter path is called I_ce.
• Base-emitter: The current path that controls the flow of current through the collector-emitter path. Voltage across the base-emitter path is referred to as V_BE and is also sometimes called bias voltage. Current through the base-emitter path is called I_BE.

Here are a few additional points to ponder concerning transistors before we move on to more details:

• In an NPN transistor, the emitter is the negative side of the transistor. The collector and base are the positive sides.
• In a PNP transistor, the emitter is the positive side of the transistor. The collector and base are the negative sides.

• Most circuits that you can build with an NPN transistor can also be built with a PNP transistor. But if you do, you must remember to flip the power connections.

• In a schematic diagram, transistors are usually represented by the letter Q.

• Yes, there are reasons why the terms collector, emitter, and base were chosen for three leads of a transistor. Unfortunately, those reasons have to do with the internal operation of the transistor at a level that’s deeper than you really need to go for this book. So please take my word for it: The guys who invented the transistor didn’t choose the terms collector, emitter, and base just to confuse you.

Examining transistor specifications

Transistors are more complicated devices than resistors, capacitors, inductors, and diodes. Whereas those components have just a few specifications to wrangle with, such as ohms of resistance and maximum watts of power dissipation, transistors have a bevy of specifications.

You can find the complete specifications for any transistor by looking up its data sheet on the Internet; just plug the part number into your favorite search engine. The data sheet gives you dozens of interesting facts about the transistor you’re interested in, with charts and graphs only a rocket scientist could love.

If you happen to be a rocket scientist and you’re thinking about using the transistor in a missile, by all means please pay attention to every detail in the data sheet. But if you’re just trying to do a little on-the-side circuit design, you need to pay attention to only the most important specifications — these in particular:

• Current gain (H_FE): This is a measure of the amplifying ability of the transistor. It refers to the ratio of the base current to the collector current. Typical values range from 50 to 200. The higher this number, the more the transistor is able to amplify an incoming signal.
• Collector-emitter voltage (V_CEO): The maximum voltage across the collector and the emitter. This is usually 30 V or more, which is well above the voltage levels you work with in most hobby circuits.
• Emitter-base voltage (V_BEO): The maximum voltage across the emitter and the base. This is usually a relatively small number, such as 6 V. Most circuits are designed to apply only small voltages to the base, so this limit isn’t usually a concern.
• Collector-base voltage (V_CBO): The maximum voltage across the collector and the base. This is usually 50 V or more.
• Collector current (I_CE): The maximum current that can flow through the collector-emitter path. Most circuits use a resister to limit this current flow; the value of the resistor must be calculated using Ohm’s law to keep the collector current below the limit. If you exceed this limit for long, the transistor may be damaged.
• Total power dissipation (P_D): This is the total power that can be dissipated by the device. For most small transistors, the power rating is on the order of a few hundred milliwatts (mW).

You need to worry about these specifications only if you’re designing your own circuits. If you’re building a circuit you found in a book or on the Internet, all you need to know is the transistor part number specified in the circuit’s schematic.

CONSIDERING FIELD-EFFECT TRANSISTORS

Bipolar transistors aren’t the only kids on the semiconductor block. Another variety of transistor, called a field-effect transistor (FET), has become extremely popular in recent years, especially as the building blocks for integrated circuits (also known as ICs). Field-effect transistors can be made much smaller than bipolar transistors, and they use much less current.

Field-effect transistors behave much like bipolar transistors, but they have a nomenclature all their own: Instead of base, emitter, and collector, the terminals in a field-effect transistor are called the gate, drain, and source. Internally, a field-effect transistor is very different from a bipolar transistor. Instead of using a pair of p-n junctions, a field-effect transistor consists of a single piece of n- or p-type semiconductor with a special substance placed on it that can control the current flow through the semiconductor.

There are a dozen or so different types of field-effect transistors in existence, but the most commonly used are called MOSFET (for metal-oxide-semiconductor field-effect transistor) and JFET (for junction field-effect transistor).

Be warned that field-effect transistors are quite susceptible to accidental static discharge. If you touch one and hear a little pop as static in your skin discharges through the FET, you may as well throw it away. You should always take precautions against static discharge whenever you handle a field-effect transistor or an integrated circuit that contains field-effect transistors.

Amplifying with a Transistor

The most common way to use a transistor as an amplifier is shown in Figure 6-3. This type of circuit is sometimes called a common-emitter circuit because, as you can see, the emitter is connected to ground, which means that both the input signal and the output signal share the emitter connection.

FIGURE 6-3: A basic transistor amplifier circuit.

There are two other ways to use a transistor as an amplifier, called common-base and common-collector. As you might guess, they involve connecting the base and the collector to ground, respectively. Common-emitter circuits are used more often than common-base or common-collector, so that’s what I show you in this chapter.

The circuit in Figure 6-3 uses a pair of resistors as a voltage divider to control exactly how much voltage is placed across the base and emitter of the transistor. The AC signal from the input is then superimposed on this bias voltage to vary the bias current. Then, the amplified output is taken from the collector and emitter. Variations in the bias current are amplified in the output current.

You might remember from Chapter 2 of this minibook that a voltage divider is simply a pair of resistors. The voltage across both resistors equals the sum of the voltages across each resistor individually. You can divide the voltage any way you want by picking the correct values for the resistors. If the resistors are identical, the voltage divider cuts the voltage in half. Otherwise, you can use a simple formula to determine the ratio at which the voltage is divided. (If you want to review this formula, feel free to refer to Chapter 2 of this minibook.)

If you look at the schematic diagram in Figure 6-3 and squint your eyes just a bit, you might see that there are actually two voltage dividers in the circuit. The first is the combination of resistors R1 and R2, which provide the bias voltage to the transistor’s base. The second is the combination of resistors R3 and R4, which provide the voltage for the output.

In reality, there’s a third resistor in the output voltage divider: the collector-emitter path in the transistor itself. In fact, one common way to explain how a transistor works is to think of the collector-emitter path as a potentiometer (a variable resistor), whose knob is turned by the bias voltage. For a more detailed explanation of this, see the sidebar titled “The magic pot.”

This second voltage divider is a variable voltage divider: The ratio of the resistances changes based on the bias voltage, which means the voltage at the collector varies as well. The amplification occurs because very small variations in an input signal are reflected in much larger variations in the output signal.

I used the term reflected in the preceding paragraph on purpose. In a common-emitter amplifier circuit, the amplified output is a reflection of the input signal. In other words, positive voltage variations in the input appear as negative variations in the output. Another way to put this is to say that the output signal is inverted — which is just a fancy way of saying it’s upside down.

Because this is tricky stuff, look at this circuit more closely:

• The input arrives at the left side of the circuit in the form of a signal, which usually has both a DC and an AC component. In other words, the voltage fluctuates but never goes negative.
• One side of the input is connected to ground, to which the battery’s negative terminal is also connected. The transistor’s emitter is also connected to ground (through a resistor), as is one side of the output.
• The purpose of C1 is to block the DC component of the input signal. Only pure AC gets past the capacitor. Without this capacitor, any DC voltage in the input signal would be added to the bias voltage applied to the transistor, which could spoil the transistor’s ability to faithfully amplify the AC part of the input signal.
• R1 and R2 form a voltage divider that determines how much DC voltage is applied to the transistor base. The AC portion of the signal that gets past C1 is combined with this DC voltage, which causes the transistor’s base current to vary with the voltage.
• R3, R4, and the variable resistance of the collector-emitter circuit form a voltage divider on the output side of the amplifier. Amplification occurs because the full power supply voltage is applied across the output circuit. The varying resistance of the collector-emitter path reflects the small AC input signal on the much larger output signal.
• C2 blocks the DC component of the output signal so that only pure AC is passed on to the next stage of the amplifier circuit.

The trick in designing transistor amplifiers is picking the right values for all the resistors and capacitors. That involves more than a little bit of math and engineering knowledge, and is just beyond the scope of this book. Most hobbyists can get along with published circuits that you can find in kits or on the Internet. But if you really want to know how to calculate these values for yourself, you can find excellent tutorials on the subject on the Internet. Just search for common emitter and you’ll find what you’re looking for.

THE MAGIC POT

My high-school electronics instructor explained that a transistor works like a “magic potentiometer,” or as we kids liked to call it, the “magic pot.” We used to joke that Puff the Magic Dragon would have enjoyed electronics class because of the magic pot. (Hey, it was the seventies!)

The basic idea is this: A transistor works like a combination of a diode and a variable resistor, also called a potentiometer or pot. But this isn’t just an ordinary pot; it’s a magic pot whose knob is mysteriously connected to the diode by invisible rays, kind of like this:

When forward voltage is applied to the diode, the knob of the magic pot turns much like the needle on a voltmeter. This changes the resistance of the potentiometer, which in turn changes the amount of current that can flow through the collector-emitter path.

Note that a magic potentiometer is wired so that when bias voltage increases, resistance decreases. When bias voltage decreases, resistance increases.

Besides being connected to the diode by invisible rays, the magic pot is magic in one more way: Its maximum resistance is infinite. Real-world potentiometers have a finite maximum resistance, such as or , but the magic pot has infinite maximum resistance.

With this knowledge of the magic pot’s properties in mind, you can visualize how a transistor works. There are three positions that the magic knob can be in, which correspond to these three operating modes for a transistor.

• Infinite resistance: When there’s no bias voltage, the magic pot’s knob is spun all the way in one direction, providing infinite resistance. Thus no current flows through the transistor. (Actually, remember that the base of the transistor is like a diode, which means that a certain amount of forward voltage is required before current begins to flow through the base. The magic pot stays at its infinite setting until that voltage — usually about 0.7 V — is reached.) This state is called cut-off because current is cut off. No amps for you!
• Some resistance: As the bias voltage moves past 0.7 V, the diode begins to conduct, and the invisible rays start turning the knob on the magic pot. Thus current begins to flow. How much current flows depends on how far the bias voltage has caused the knob to turn.
• No resistance: Eventually, the bias voltage turns the knob to its stopping point, and there’s no resistance at all. Current flows unrestricted through the collector-emitter circuit. You can continue to increase the bias voltage, but you can’t lower the resistance below zero! This state is called saturation.

Using a Transistor as a Switch

One of the most common uses for transistors is as simple switches. In short, a transistor conducts current across the collector-emitter path only when a voltage is applied to the base. When no base voltage is present, the switch is off. When base voltage is present, the switch is on.

In an ideal switch, the transistor should be in only one of two states: off or on. The transistor is off when there’s no bias voltage or when the bias voltage is less than 0.7 V. The switch is on when the base is saturated so that collector current can flow without restriction.

Figure 6-4 shows a schematic diagram for a circuit that uses an NPN transistor as a switch that turns an LED on or off.

FIGURE 6-4: Switching an LED with an NPN transistor.

Look at this circuit component by component:

• LED: This is a standard 5 mm red LED. This type of LED has a voltage drop of 1.8 V and is rated at a maximum current of 20 mA.
• R1: This resistor limits the current flowing into the base of the transistor. You can use Ohm’s law to calculate the current at the base. Because the base-emitter junction drops about 0.7 V (the same as a diode), the voltage across R1 is 5.3 V. Dividing 5.3 by 100,000 gives the current at 0.000053 A, or 0.053 mA. Thus, the 12.7 mA collector current (I_CE) is controlled by a much smaller base current (I_BE).
• R2: This resistor limits the current through the LED to prevent the LED from burning out. You can use Ohm’s law to calculate the amount of current that the resistor will allow to flow. Because the supply voltage is , and the LED drops 1.8 V, the voltage across R1 will be . Dividing the voltage by the resistance gives you the current in amperes, approximately 0.127 A. Multiply by 1,000 to get the current in mA: 12.7 mA, well below the 20 mA limit.
• Q1: This is a common NPN transistor. I used a 2N2222A transistor, but just about any NPN transistor will work. R2 and the LED are connected to the collector, and the emitter is connected to ground. When the transistor is turned on, current flows through the collector and emitter, thus lighting the LED. When the transistor is turned off, the transistor acts as an insulator, and the LED doesn’t light.
• SW1: This switch controls whether current is allowed to flow to the base. Closing this switch turns on the transistor, which causes current to flow through the LED. Thus, closing this switch turns on the LED even though the switch isn’t placed directly within the LED circuit.

You might be wondering why you’d need or want to bother with a transistor in this circuit. After all, couldn’t you just put the switch in the LED circuit and do away with the transistor and the second resistor? Of course you could, but that would defeat the principle that this circuit illustrates: that a transistor allows you to use a small current to control a much larger one. If the entire purpose of the circuit is to turn an LED on or off, by all means omit the transistor and the extra resistor. But as you work with more advanced circuits, you’ll find plenty of cases when the output from one stage of a circuit is very small and you need that tiny amount of current to switch on a much larger current. In that case, the transistor circuit shown here is just what you need.

An LED Driver Circuit

In Project 18, you build the circuit shown in the preceding section. The circuit uses a transistor to switch on an LED using a current that’s much smaller than the LED current. The schematic for this project is the same as the schematic shown in Figure 6-4.

Figure 6-5 shows the completed project.

FIGURE 6-5: The circuit for the LED driver circuit.

If you feel like experimenting a bit after you complete this project, here are couple of suggestions:

• Try replacing R1 with larger resistor values, such as , , or . At what point is the base current no longer sufficient to light the LED?
• Measure the base current and the collector current, as follows:
  • To measure the base current, remove the R1 lead in hole F1 and touch your multimeter probes to the lead you removed and the 2X terminal on the switch.
  • To measure the collector current, remove the R1 lead at J15 and touch the multimeter probes to the lead you removed and the LED1 anode.

Project 18: A Transistor LED Driver

In this project, you build a circuit that uses a transistor to drive an LED. The transistor allows a small amount of current to control a larger amount of current that passes through an LED. The circuit includes two LEDs: one on the base and one on the emitter. Both LEDs light up when the push button is pressed, but the LED on the emitter circuit glows brighter than the one on the base circuit, demonstrating the transistor’s current gain.

To complete this project, you’ll need wire cutters, wire strippers, and a multimeter.

Parts

• Four AA batteries
• One four AA battery holder (RadioShack 2700391)
• One small solderless breadboard (RadioShack 2760003)
• One normally open DIP breadboard push button
• One NPN switching transistor, 2N2222A or similar (RadioShack 2761617)
• One 5mm red LED (RadioShack 2760209)
• One resistor (orange-orange-brown)
• One resistor (brown-black-yellow)
• Two -inch jumper wires

Steps

Throughout these steps, use the negative (–) bus strip on the bottom of the board for ground and the positive (+) bus strip on the top of the board as the positive voltage.

1. Mount the push button on the breadboard so that it straddles holes G3, I3, G5, and I5.

   Orient the button so that when it’s pressed, the circuit is closed between G3 and G5.

2. Mount the resistor in holes E5 and F5.
3. Mount the resistor in hole F10 and any nearby hole in the positive bus at the top of the breadboard.

4. Mount the transistor.

   The following table shows the connections for each of the three transistor leads:

   Lead	Hole
   Collector	B4
   Base	B5
   Emitter	B6

5. Mount the LED.

   Mount LED1 so that the cathode (the shorter lead) is in hole A6 and the anode (the longer lead) is in hole A10.

6. Mount the first jumper wire with one end in hole J3 and the other end in any nearby hole in the positive bus at the top of the breadboard.
7. Mount the second jumper wire with one end in hole A4 and the other end in any nearby hole in the negative bus at the bottom of the breadboard.

8. Connect the battery holder.

   The red wire should go in the any hole of the positive bus at the top of the breadboard; the black wire should go in any hole of the negative bus at the bottom.

9. Insert the four AA batteries into the battery holder.

10. Press the push button.

    The LED lights up. When you release the button, the LED goes out.

11. Congratulate yourself!

    You’ve built your first transistor circuit!

Looking at a Simple NOT Gate Circuit

A gate is a basic component of digital electronics, which you learn all about in Book 5. Gate circuits are built from transistor switches that are either ON or OFF. There are a total of 16 different kinds of gates, and you learn about all 16 of them in Book 5, Chapter 2. For now, though, I want to introduce you to one of the simplest of all gate circuits, called a NOT gate, which simply takes an input that can be either ON or OFF and converts it to an output that’s the opposite of the input. In other words, if the input is ON, the output is OFF. If the input is OFF, the output is ON.

Figure 6-6 shows the schematic diagram for a circuit that uses a single transistor to implement a NOT gate. Here’s how the circuit works:

• The input is controlled by a single-pole switch. When the switch is closed, the input is ON. When the switch is open, the input is OFF.
• The input is sent through the R1and LED1 to bias the transistor. Thus, when the input is ON, LED1 lights up, and the transistor is turned on, which enables the collector-emitter path to conduct. When the input is OFF, LED1 is dark, the transistor turns off, and no current flows through the collector.
• LED2 is connected directly between the +6 V power supply and ground, through a current-limiting resistor, of course, to keep the LED from burning itself out.
• The anode of LED2 is connected to the transistor’s collector.
• When the transistor is off, current flows through R2 and LED2 and the LED lights up, indicating an ON output. But when the transistor turns on, a short circuit is created through the transistor’s collector and emitter. This causes the current to bypass LED2, so the LED goes dark to indicate an OFF output condition.

FIGURE 6-6: The schematic diagram for the NOT gate circuit.

Building a NOT Gate

Project 19 shows you how to build the one-transistor NOT gate circuit on a solderless breadboard. The completed project is shown in Figure 6-7.

FIGURE 6-7: The transistor NOT gate.

Project 19: A NOT Gate

In this project, you build a simple NOT gate circuit that uses a transistor to invert an input signal. That is, if the input signal is on, the output if off; if the input signal is off, the output is on. One LED is used to indicate the status of the input, and a second LED is used to indicate the status of the output. The schematic for this circuit is the same as the schematic shown in Figure 6-6.

The only tools you’ll need complete this project are wire cutters and wire strippers.

Parts

• Four AA batteries
• One four AA battery holder (RadioShack 2700391)
• One small solderless breadboard (RadioShack 2760003)
• One normally open DIP breadboard push button
• One NPN switching transistor, 2N2222A or similar (RadioShack 2761617)
• Two 5mm red LEDs (RadioShack 276209)
• One resistor (brown-black-red)
• One resistor (orange-orange-brown)
• Four short jumper wires

Steps

Throughout these steps, use the negative (–) bus strip on the bottom of the board for ground and the positive (+) bus strip on the top of the board as the positive voltage.

1. Mount the push button on the breadboard so that it straddles holes G1, I1, G3, and I3.

   Orient the button so that when it’s pressed, the circuit is closed between G1 and G3.

2. Insert the jumper wires.

   Insert the four jumper wires as follows:

   From	To
   J3	Any hole in the positive bus on the top of the breadboard
   H12	H16
   F10	Any hole in the negative bus on the bottom of the breadboard
   A16	Any hole in the negative bus on the bottom of the breadboard

3. Insert the resistors.

   Insert the two resistors as follows:

   Resistor	From	To
   	F1	F6
   	I15	Any hole on the positive bus on the top of the breadboard

4. Insert the transistor.

   The following table shows the connections for each of the three transistor leads:

   Lead	Hole
   Emitter	G10
   Base	G11
   Collector	G12

5. Insert LED1.

   Insert the cathode (the shorter of the two leads) in hole I11 and the anode (the longer of the two leads) in hole I6.

6. Insert LED2.

   Insert the cathode (the short lead) in hole E16 and the anode (the long lead) in F16.

7. Connect the battery holder.

   Insert the red lead in any hole of the positive bus at the top of the breadboard and the black lead in any hole of the negative bus at the bottom of the breadboard.

8. Insert the four AA batteries into the battery holder.

   When the batteries are connected, LED2 lights up to indicate that the button is not pressed.

9. Press the push button.

   LED2 goes dark and LED1 lights up to show that the button is pressed.

10. Release the push button.

    The LEDs return to their original status.

Oscillating with a Transistor

An oscillator is an electronic circuit that generates repeated waveforms. The exact waveform generated depends on the type of circuit used to create the oscillator. Some circuits generate sine waves, some generate square waves, and others generate other types of waves. Oscillators are essential ingredients in many different types of electronic devices, including radios and computers.

One of the most commonly used oscillator circuits is made from a pair of transistors that are rigged up to alternately turn on and off. This type of circuit is called a multivibrator. If the circuit is designed to continuously cycle between the two transistors, it’s called an astable multivibrator because the circuit never reaches a point of stability — that is, it never decides which of the two transistors should be on, so it just keeps flipping back and forth between the two. Astable multivibrators are great for producing square waves.

Figure 6-8 is a generalized schematic diagram for an astable multivibrator made from a pair of NPN transistors.

FIGURE 6-8: An astable multivibrator.

Examine this circuit to see how it works. When you first power it on, only one of the transistors turns on. You might think that they would both turn on, because the bases of both transistors are connected to +V, but it doesn’t happen that way: One of them goes first. For the sake of discussion, assume that Q1 is the lucky one.

When Q1 comes on, current flows through R1 into the collector and on through the transistor to ground. Meanwhile, C1 starts to charge through R2, developing a positive voltage on its right plate. Because this right plate is connected to the base of Q2, positive voltage also develops on the base of Q2.

When C1 is charged sufficiently, the voltage at the base of Q2 causes Q2 to start conducting. Now the current flows through the collector of Q2 via R4, and C2 starts charging through R3. Because the right-hand plate of C2 is bombarded with positive charge, the voltage on the left plate of C2 goes negative, which drops the voltage on the base of Q1. This causes Q1 to turn off.

C1 discharges while C2 charges. Eventually, the voltage on the left plate of C2 reaches the point where Q1 turns back on, and the whole cycle repeats.

Don’t worry if this all seems confusing. It is. If the details seem baffling, just think of the big picture: The dueling capacitors alternately charge and discharge, turning the two transistors on and off, which in turn allows current to flow through their collector circuits. Back and forth it goes, like an amazing rally at Wimbledon that no one ever wins … the players just keep lobbing the ball back and forth forever, until their batteries run out.

Here are a few other interesting things to know about astable multivibrators:

• The time that each half of the multivibrator is on is determined by the RC time constant formed by the capacitor charging circuits. Thus, you can vary the speed at which the circuit oscillates by adjusting the capacitor and resistor values. For more information about calculating resistor and capacitor time constants, refer to Chapter 3 of this minibook.
• You can, if you want, create an astable multivibrator from PNP transistors simply by switching the ground with the +V voltage source.
• Output from the multivibrator circuit can be taken directly from the collector of either transistor. For example, you could place an LED or a speaker in series with R1 or R4 to see or hear the oscillator in action. You can see an example of this in the next section.

• Alternatively, you can use a third transistor to couple the multivibrator with an output load, as shown in Figure 6-9. Just connect the emitter of one of the multivibrator transistors to the base of the third transistor and connect the load to the collector, as shown in the figure.

  This arrangement has two advantages. First, the load itself interferes with the multivibrator circuit if you take it directly from the collector of Q1 or Q2. By using a third transistor, you isolate the load from the multivibrator circuit. And second, the output is much closer to a true square wave when the coupling transistor is used; without it, the output isn’t a clean square wave because of the effects of the capacitor charging.

FIGURE 6-9: Using a transistor to couple an output load to an astable multivibrator.

Building an LED Flasher

In Project 20, you build a circuit that uses an astable multivibrator to alternately flash two LEDs. LED flasher circuits are a favorite of electronic hobbyists because flashing LEDs have all sorts of fun uses. For example, you can add creepy blinking eyes to a jack-o’-lantern for Halloween, or you can add blinking warning lights to your model railroad layout. Figure 6-10 shows the finished LED flasher project.

FIGURE 6-10: The LED flasher project.

The LED flasher circuit is simply an astable multivibrator similar to the one shown in Figure 6-8. The only differences are that I’ve added LEDs to the collector circuit of each transistor and filled in the resistor and capacitor values. With the values I selected for this project, the lights alternate quickly, a bit faster than once per second.

If you feel like experimenting a bit after you complete this project, here are a couple of suggestions:

• Try replacing R2 and R3 with resistors of different values, such as and . What effect does this have on the flasher?
• Try adding a potentiometer in series with R2 or R3. This allows you to vary the flash rate by turning the potentiometer knob.
• Try changing the capacitors.

Project 20: An LED Flasher

In this project, you build a circuit that uses a two-transistor astable multivibrator to flash two LEDs in sequence. You’ll need a small Phillips-head screwdriver, wire cutters, and wire strippers to build this project.

Parts

• Four AA batteries
• One four AA battery holder (RadioShack 2700391)
• One small solderless breadboard (RadioShack 2760003)
• Two NPN switching transistors, 2N2222A or similar (RadioShack 2761617)
• Two 5mm red LEDs (RadioShack 2760209)
• Two electrolytic capacitors (RadioShack 2721044)
• Two resistors (brown-black-orange)
• Two resistors (yellow-violet-brown)
• Two -inch jumper wires
• One 1-inch jumper wire
• One -inch jumper wire

Steps

Throughout these steps, use the negative (–) bus strip at the bottom of the board for ground and the positive (+) bus strip on the top of the board for the positive voltage.

1. Insert the jumper wires.

   Insert the four jumper wires as follows:

   Length	From	To
   inch	F10	Any hole in the negative bus at the bottom of the breadboard
   inch	F17	Any hole in the negative bus at the bottom of the breadboard
   1 inch	F11	F20
   inch	H13	H18

2. Insert the capacitors.

   Insert the two electrolytic capacitors as follows:

   Capacitor	–	+
   C1	I13	J12
   C2	I20	J19

   Be sure to observe the correct polarity of the capacitors. The negative lead is marked with minus signs.

3. Insert the transistors.

   The following table shows the connections for each of the three transistor leads of the two transistors:

   Lead	Q1	Q2
   Emitter	G10	G17
   Base	G11	G18
   Collector	G12	G19

4. Insert the LEDs.

   LED	Cathode (Short)	Anode (Long)
   LED1	J7	Any hole in the positive bus at the top of the breadboard
   LED2	J24	Any hole in the positive bus at the top of the breadboard

5. Insert the resistors.

   Resistor	From	To
   R1 (470)	I7	I12
   R2 (100K)	J11	Any hole in the positive bus at the top of the breadboard
   R3 (100K)	J18	Any hole in the positive bus at the top of the breadboard
   R4 (470)	H19	H24

6. Connect the battery holder to the breadboard.

   Attach the red lead from the battery holder to any hole in the positive bus strip at the top of the solderless breadboard.

   Attach the black lead from the battery holder to any hole in the negative bus strip at the bottom of the breadboard.

7. Insert the four AA batteries into the battery holder.

8. Enjoy the show!

   The LEDs alternately flash as long as you leave the batteries in.

Wrapping Up Our Exploration of Discrete Components

That about does it for our foray into transistors — and in fact, that wraps up our survey of the most common types of discrete electronic components (that is, components that contain just a single part in a single case). If you’ve worked your way through all the chapters in this minibook, you now know the basics of resistors, capacitors, inductors, diodes, and transistors.

In a way, that’s all there is. What remains are clever ways to combine these components into different kinds of circuits that you can put to different kinds of uses.

In Book 3, you learn how to use integrated circuits, which enable you to replace a whole circuitful of discrete components with a single tiny chip. Integrated circuits are simply a way to combine a bunch of semiconductors, resistors, and capacitors into a single tiny package.

On you go!