by Earl Boysen
B. When a switch is open, does current flow? _____
C. For a transistor to turn OFF and act like an open switch, how much base current is needed? _____
Answers
A. At different voltages, the supply voltage and ground voltage.
B. No.
C. The transistor is OFF when there is no base current.
12 You can be sure that there is no base current in the circuit shown in Figure 4.6 by opening the mechanical switch.
Figure 4.6
To ensure that the transistor remains OFF when the base is not connected to the supply voltage, you add a resistor (labeled R2 in Figure 4.7) to the circuit. The base of the transistor connects to ground or 0 volts through this resistor. Therefore, no base current can possibly flow.
Figure 4.7
Questions
A. Why doesn't current flow from the supply voltage to the base-emitter junction? _____
B. How much current flows from collector to base? _____
C. Why doesn't current flow from collector to base through R2 ground? _____
D. Why is the transistor base at 0 volts when R2 is installed? _____
Answers
A. There is no current path from the supply voltage through the base-emitter junction. Thus, there is no base current flowing.
B. None at all.
C. The internal construction of the transistor prevents this, because the collector-to-base junction is basically a reverse-biased diode.
D. Because there is no current through R2, there is no voltage drop across R2 and, therefore, the transistor base is at ground (0 volts).
13 Because no current is flowing through R2, you can use a wide range of resistance values. In practice, the values you find for R2 are between 1 kΩ and 1 MΩ.
Question
Which of the following resistor values would you use to keep a transistor turned off? 1 ohm, 2 kΩ, 10 kΩ, 20 kΩ, 50 kΩ, 100 kΩ, 250 kΩ, and 500 kΩ. _____
Answer
They would all be suitable except the 1 ohm because the rest are all above 1 kΩ and below 1 MΩ.
14 Figure 4.8 shows a circuit using both R1 and R2. Note that the circuit includes a two-position switch that you can use to turn the transistor ON or OFF.
Figure 4.8
Questions
A. As shown in Figure 4.8, is the transistor ON or OFF? _____
B. Which position, A or B, can cause the collector current to be 0 amperes? _____
Answers
A. ON—the base-to-emitter diode is forward-biased. Therefore, base current can flow.
B. Position B—the base is tied to ground. Therefore, no base current can flow, and the transistor is OFF.
Why Transistors Are Used as Switches
15 You can use the transistor as a switch (as you saw in the previous problems) to perform simple operations such as turning a lamp current on and off. Although often used between a mechanical switch and a lamp, there are other uses for the transistor.
Following are a few other examples that demonstrate the advantages of using a transistor in a circuit as a switch:
Example 1—Suppose you must put a lamp in a dangerous environment, such as a radioactive chamber. Obviously, the switch to operate the lamp must be placed somewhere safe. You can simply use a switch outside the chamber to turn the transistor switch ON or OFF.
Example 2—If a switch controls equipment that requires large amounts of current, then that current must flow through the wires that run between the switch and the lamp. Because the transistor switch can be turned ON or OFF using low voltages and currents, you can connect a mechanical switch to the transistor switch using small, low-voltage wire and, thereby, control the larger current flow. If the mechanical switch is any distance from the equipment you're controlling, using low-voltage wire can save you time and money.
Example 3—A major problem with switching high current in wires is that the current induces interference in adjacent wires. This can be disastrous in communications equipment such as radio transceivers. To avoid this, you can use a transistor to control the larger current from a remote location, reducing the current needed at the switch located in the radio transceiver.
Example 4—In mobile devices (such as a radio-controlled airplane), using transistor switches minimizes the power, weight, and bulk required.
Example 5—When you use a sensor to activate devices, the sensor provides a low current to the transistor, which then acts as a switch controlling the larger current needed to power the equipment. An everyday example is a sensor that detects a light beam across a doorway. When the beam is blocked by a person or object passing through, the sensor stops generating a current, switching a transistor OFF, which activates a buzzer.
Question
What features mentioned in these examples make using transistors as switches desirable? _____
Answer
The switching action of a transistor can be directly controlled by an electrical signal, as well as by a mechanical switch in the base circuit. This provides a lot of flexibility for the design and allows for simple electrical control. Other factors include safety, reduction of interference, remote switching control, and lower design costs.
16 The following examples of transistor switching demonstrate some other reasons for using transistors:
Example 1—You can control the ON and OFF times of a transistor accurately, whereas mechanical devices are not accurate. This is important in applications such as photography, where it is necessary to expose a film or illuminate an object for a precise period of time. In these types of uses, transistors are much more accurate and controllable than any other device.
Example 2—A transistor can be switched ON and OFF millions of times a second and will last for many years. In fact, transistors are one of the longest lasting and most reliable components known, whereas mechanical switches usually fail after a few thousand operations.
Example 3—The signals generated by most industrial control devices are digital. These control signals can be simply a high or low voltage, which is ideally suited to turning transistor switches ON or OFF.
Example 4—Modern manufacturing techniques enable the miniaturization of transistors to such a great extent that many of them (even hundreds of millions) can be fabricated into a single silicon chip. Silicon chips on which transistors (and other electronic components) have been fabricated are called integrated circuits (ICs). ICs are little, flat, black plastic components built into almost every mass-produced electronic device and are the reason that electronic devices continue to get smaller and lighter.
Question
What other features, besides the ones mentioned in the previous problem, are demonstrated in the examples given here? _____
Answer
Transistors can be accurately controlled, have high-speed operation, are reliable, have a long life, are small, have low power consumption, can be manufactured in large numbers at low cost, and are extremely small.
17 At this point, consider the idea of using one transistor to turn another one ON and OFF, and of using the second transistor to operate a lamp or other load. (This idea is explored in the next section of this chapter.)
If you must switch many high-current loads, then you can use one switch that controls several transistors simultaneously.
Questions
A. With the extra switches added, is the current that flows through the main switch more or less than the current that flows through the load? _____
B. What effect do you think the extra transistor has on the following?
1. Safety _____
2. Convenience to the operator _____
3. Efficiency and smoothness of operation _____
Answers
A. Less current flows through the main switch than through the load.
B.
1. It increases safety and allows the operator to stay isolated from dangerous situations.
2. Switches can be placed conveniently close together on a panel, or in
the best place for an operator, rather than the switch position dictating the operator position.
3. One switch can start many things, such as in a master lighting panel in a television studio or theater.
18 This problem reviews your understanding of the concepts presented in problems 15, 16, and 17.
Question
Indicate which of the following are good reasons for using a transistor as a switch:
A. To switch equipment in a dangerous or inaccessible area on and off
B. To switch low currents or voltages
C. To lessen the electrical noise that might be introduced into communication and other circuits
D. To increase the number of control switches
E. To use a faster, more reliable device than a mechanical switch _____
Answer
A, C, and E.
Project 4.1: The Transistor Switch
Objective
The objective of this project is to demonstrate how light can switch a transistor ON or OFF to control a device.
General Instructions
This project uses two breadboarded circuits. The circuit shown on the left side of Figure 4.9 is used to generate infrared light. Another circuit, shown on the right side of Figure 4.9, switches on a buzzer when the infrared light is blocked by an object.
Figure 4.9
The infrared light in this project is generated by a light-emitting diode (LED). In an LED, a current runs through a PN junction that generates light. This same process occurs with all diodes. Infrared LEDs are simply diodes with a transparent case that enables the infrared light to show through. LEDs also have a PN junction made with semiconductor material that produces a large amount of infrared light. Figure 4.10 shows a typical LED and its schematic symbol, the symbol for a diode with arrows pointing outward, indicating that light is generated.
Figure 4.10
In this project, a photodiode detects the infrared light. When light strikes a PN junction in a photodiode (or any diode), a current is generated. Infrared photodiodes also have a transparent case and junction material that produces a large current when it absorbs infrared light. Figure 4.11 shows a typical photodiode and its schematic symbol consisting of the symbol for a diode with arrows pointing inward, indicating that light is absorbed.
Figure 4.11
When the circuits are set up, the buzzer sounds whenever the infrared light is blocked from the photodiode.
Parts List
One 9-volt battery.
One 6-volt battery pack (4 AA batteries).
Two battery snap connectors.
One 100-ohm, 0.5-watt resistor.
One 1 k, 0.25-watt resistor.
One 10 k, 0.25-watt resistor.
Two breadboards.
Two terminal blocks.
One piezoelectric buzzer with a minimum operating voltage of 3 volts DC. Using a buzzer with pins (such as part # SE9-2202AS by Shogyo International) enables you to insert the buzzer directly into the breadboard. If you use a buzzer with wire leads (such as part # PK-27N26WQ by Mallory), you need another terminal block.
One infrared LED.
One infrared photodiode.
One PN2222 transistor. Figure 4.12 shows the pinout diagram for the PN2222.
Figure 4.12
Step-by-Step Instructions
Set up the circuits shown in Figure 4.9. If you have some experience in building circuits, this schematic (along with the previous parts list) should provide all the information you need to build the circuit. If you need a bit more help building the circuit, look at the photos of the completed circuit in the “Expected Results” section.
Carefully check your circuit against the diagram, especially the connection of the long and short leads to the LED and photodiode. The LED is connected so that it is forward-biased, whereas the photodiode is connected so that it is reverse-biased, as indicated by the direction of the schematic symbols in the circuit diagrams.
1. Align the rounded top of the LED toward the rounded top of the photodiode with the circuit boards a few feet apart from each other. (If you use a typical LED and photodiode, you must bend their leads to align them.) Note that the rounded top of both the LED and photodiode shown in Figures 4.10 and 4.11 contain a lens to emit or absorb light. Some LEDs and photodiodes have lenses on the side, instead of on the top. If it isn't obvious where the lens is in your components, check the manufacturer's data sheet.
2. Turn on the power switch. When the power switch is on, the buzzer should sound whenever the photodiode does not sense infrared light.
3. Bring the circuits close enough together so that the buzzer shuts off.
4. Block the infrared light; the buzzer should turn on.
Expected Results
Figure 4.13 shows the breadboarded buzzer circuit for this project.
Figure 4.13
Figure 4.14 shows the breadboarded LED circuit for this project.
Figure 4.14
Figure 4.15 shows the test setup for this project with the rounded top of the LED and photodiode aligned toward each other.
Figure 4.15
The photodiode is connected to the base of a transistor. Therefore, current generated by the photodiode turns the transistor ON. When the transistor is ON, VC is about 0 volts, turning off the buzzer. When the infrared light is blocked, the photodiode stops generating current, which turns OFF the transistor, increasing VC, which turns on the buzzer.
These circuits work with the LED and photodiode about 7 inches apart. With more complicated photo detectors that have circuitry to amplify the detected signal, this technique can work over several feet. One common application of this technique is a buzzer that sounds when a shopper enters a store, blocking the light, setting off a sound, and alerting the sales staff.
19 Many types of electronic circuits contain multiple switching transistors. In this type of circuit, one transistor is used to switch others ON and OFF. To illustrate how this works, again consider the lamp as the load and the mechanical switch as the actuating element. Figure 4.16 shows a circuit that uses two transistors to turn a lamp on or off.
Figure 4.16
When the switch is in position A, the base-emitter junction of Q1 is forward-biased. Therefore, base current (IB1) flows through R1 and through the base-emitter diode of Q1, turning the transistor ON. This causes the collector current (IC1) to flow through Q1 to ground, and the collector voltage drops to 0 volts, just as if Q1 were a closed switch. Because the base of Q2 is connected to the collector of Q1, the voltage on the base of Q2 also drops to 0 volts. This ensures that Q2 is turned OFF and the lamp remains unlit.
Now, flip the switch to position B, as shown in Figure 4.17. The base of Q1 is tied to ground, or 0 volts, turning Q1 OFF. Therefore, no collector current can flow through Q1. A positive voltage is applied to the base of Q2, and the emitter-base junction of Q2 is forward-biased. This enables current to flow through R3 and the emitter-base junction of Q2, which turns Q2 ON, allowing collector current (IC2) to flow, and the lamp is illuminated.
Figure 4.17
Now that you have read the descriptions of how the circuit works, answer the following questions. First assume that the switch is in position A, as shown in Figure 4.16.
Questions
A. What effect does IB1 have on transistor Q1? _____
B. What effect does turning Q1 ON have on the following?
1. Collector current IC1 _____
2. Collector voltage VC1 _____
C. What effect does the change to VC1 covered in the previous question have on the following?
1. The base voltage of Q2 _____
2. Transistor Q2 (that is, is it ON or OFF) _____
D. Where does the current through R3 go? _____
E. In this circuit is the lamp on or off? _____
Answers
A. IB1, along with a portion of VS (0.7 volts if the transistor is silicon), turns Q1 ON.
B. (1) IC1 flows; (2) VC1 drops to 0 volts.
C. (
1) base of Q2 drops to 0 volts; (2) Q2 is OFF.
D. IC1 flows through Q1 to ground.
E. Off.
20 Now, assume that the switch is in the B position, as shown in Figure 4.17, and answer these questions.
Questions
A. How much base current IB1 flows into Q1? _____
B. Is Q1 ON or OFF? _____
C. What current flows through R3? _____
D. Is Q2 ON or OFF? _____
E. Is the lamp on or off? _____
Answers
A. None
B. OFF
C. IB2
D. ON
E. On
21 Refer to the circuit in Figures 4.16 and 4.17. Now, answer these questions assuming the supply voltage is 10 volts.
Questions
A. Is the current through R3 ever divided between Q1 and Q2? Explain. _____
B. What is the collector voltage of Q2 with the switch in each position? _____
C. What is the collector voltage of Q1 with the switch in each position? _____
Answers
A. No. If Q1 is ON, all the current flows through it to ground as collector current. If Q1 is OFF, all the current flows through the base of Q2 as base current.
B. In position A, 10 volts because it is OFF.
In position B, 0 volts because it is ON.
C. In position A, 0 volts because it is ON.
In position B, the collector voltage of Q1 equals the voltage drop across the forward-biased base-emitter junction of Q2, because the base of Q2 is in parallel with the collector of Q1. The voltage drop across the forward-biased base-emitter junction does not rise to 10 volts, but can rise only to 0.7 volt if Q2 is made of silicon.