Fabricating an Amplifier

This laboratory provides information that can be used to: (1) construct a basic amplifier that incorporates an op amp, and (2) modify the circuit to create a more powerful amplifier using power transistors. The op amp is a basic building block of analog electronics. Therefore this laboratory provides an initial introduction to the field of electronics.  The age of electronics began with the invention of the vacuum tube at the beginning of the twentieth century. Since then, it has changed almost every facet of our lives.

1. Dawn of the Age of Electronics

A telephone consists of a microphone and a speaker. The microphone converts sound waves into an electrical signal. A speaker converts an electrical signal back into sound waves. In the early days of the phone system, it was only possible to transmit a phone signal a few hundred miles. Invention of the amplifier made it possible to transmit the signal longer distances.

The amplifier was made possible by invention of the vacuum tube. AT&T purchased patent rights from Lee de Forrest. In 1915 Alexander Graham Bell made the first coast to coast phone call from New York City to his former assistant, Thomas Watson, in San Francisco.

Amplifiers consist of two primary elements: (1) a voltage into the amplifier, and (2) a voltage out of the amplifier.  The secondary circuit has its own power source, thereby allowing the signal to travel an additional distance.

Figure 1. An amplifier increases the amplitude of a signal.

Edison laid the foundation for invention of the vacuum tube. In the process of inventing the incandescent light bulb, Edison found that as a filament in a glass bulb was heated, some of the electrons in the filament escaped into the vacuum of the tube. Edison found that the free electrons were attracted to a second electrode in the bulb, creating a flow of current. The negative electrode that is heated to free the electrons is called a cathode. The positive electrode that attracts the electrons is called an anode. This phenomenon was called the Edison effect. Edison patented the effect, but did not see an immediate use for it.

Figure 2. When the cathode is heated, electrons flow from the cathode to the anode.

Lee De Forest added a wire grid between the two electrodes.  De Forest found that the magnitude of the current between the two electrodes could be controlled by a voltage through the wire grid. A negative voltage relative to the cathode through the wire grid restricts electrons from reaching the anode. This allows a fluctuating voltage applied to the grid to modify the current from the cathode to the anode of the tube. This is the basis of the vacuum tube amplifier. The current in the cathode-anode circuit is provided by a secondary battery.

In the process of making this discovery, De Forest invented the means to create an electronic amplifier. The electronic amplifier made the long distance phone system possible, as well as many other electronic inventions that followed, including commercial radio, television, and the computer.

Figure 3. A fluctuating voltage applied to a control grid (green) modifies the current from the cathode to the anode of the vacuum tube.

Vibrations in the air (sound waves) move the diaphragm of a microphone. This movement causes electrical fluctuations (green) at the control grid. The voltage fluctuations affects the flow of electrons in the speaker circuit (blue). These fluctuations result in variations in the current in the speaker circuit. Note that the source of power in this circuit is a high voltage battery. The net effect from sound detection to sound production is amplification. That is, the speaker output follows the microphone input and is louder.

Figure 4. The lower voltage signal generated by the microphone is amplified by the vacuum tube.

The increase in amplification, also known as the gain in amplification, is determined by the ratio of the smaller battery on the input side to the larger battery on the output side. For example, if the battery on the left-side of the figure is 20 volts and the larger battery on the right-hand side is a 200volt battery, a tenfold increase in amplification would be achieved.

The vacuum tube was eventually replaced by the solid state transistor, which provided the same functionality in a more compact, reliable form. An integrated circuit combines thousands of transistors in a single device. The operational amplifier (op amp) is an integrated circuit designed for use as an amplifier. Today’s op amps that are the successor of the vacuum tube are still a basic building block of electronics.

2. Components of a Music System

Most music systems consist of three major components:  (1) a source of music such as a CD player, and iPod, or a smart phone, (2) an amplifier, and (3) a speaker. A stereo music system has two speakers.The role of an amplifier in this system is to amplify the sound (i.e., makes it louder).  In some cases, it may be possible to connect the output of a music source (such a smart phone) to a speaker and hear a discernable sound. However, the volume of the sound will be increased if an amplifier is placed between the music player and the speaker.

A cable is often used to connect the output of a music player to the input of amplifier. If the connector at the end of a monophonic cable is taken apart, two wires will be seen. One wire conducts the voltage that carries the audio signal. The other wire is the ground wire. By convention, the wire that carries the audio signal (shown in red) is connected to the tip of the connector and the ground wire (shown in blue) is connected to the base of the audio connector.

Figure 5. The wiring of an audio connector

The audio connector with these two wires is connected to the amplifier. This conducts the audio signal from the music player to the amplifier.

Figure 6. The audio input of a Dayton Audio DTA-1 amplifier

In the case of a commercial amplifier, there is typically an audio connector labeled audio input. There are also output connectors that are used to attach a cable to a speaker (or to left and right speakers in the case of a stereo amplifier).

3. Origins of the Concept of Ground

In order to understand the symbols in modern electric circuits, it is necessary to take a step back and explore the orgins of a concept known as ground.  The first telegraph systems could send the electrical signal a distance of about twenty miles. Initially two wires were used: (1) One wire went from one terminal of the battery to the telegraph sounder. (2) A second wire completed the return path from the telegraph sounder to the second terminal of the battery, completing the circuit.

Figure 7. A two-wire telegraph circuit

About the time that Morse developed the first commercially successful telegraph system, it was discovered that electrical signals could be transmitted through the ground. A metal stake was driven into the earth, and connected to one terminal of the battery via a short cable. A second metal stake was driven into the ground near the telegraph sounder. This allowed the return path of the telegraph circuit to travel via the earth, saving the expense of a second wire.

Figure 8. A telegraph circuit with a return path via ground

Three short dashes that represent the sharp end of a ground stake are now the traditional symbol for ground.  Today’s circuit boards are not connected to a stake driven into the ground. However, the term ground over time came to refer to any common return path to the negative terminal of a power supply

Figure 9. The symbol for ground

4. An Introduction to Resistors

Resistors are found in almost all circuits. Resistors resist the flow of electricity in a circuit. Electrical resistance is measured in ohms, named in honor of Ohm, who is also known for Ohm’s Law:  “Voltage = Current / Resistance.” Controlling the voltage in an a circuit plays an essential role in design and operation of an electrical device. Changing the resistance of a circuit also changes the voltage (as Ohm discovered) which makes resistors useful in circuit design. The value of a resistor is indicated by four bands of color on the resistor. The first two colors of the resistors shown below are brown and black.

Figure 10. A 1,000 ohm resistor (left) and a 10,000 ohm resistor (right)

The third band of color is very similar. However, close examination reveals the third band of Resistor 1 is red, while the third band of Resistor 2 is orange (a slightly lighter shade).

Decoding Resistors

The value of the resistors, measured in ohms, is indicated by the color code. The left-hand resistor in Figure 10 (above) has a code of brown, black, red, and gold. The right-hand resistor has a code of brown, black, orange, and gold.  Each band of color indicates a different value.  <unfortunately, in the photo, the colors red and orange are very difficult to differentiate, so the resistors appear to be the same>

Figure 11. The value of resistors is indicated by the color code

With the aid of the chart in Figure 12, we can determine that the left-hand resistor has a total value of 1,000 ohms and the right-hand resistor has a total value of 10,000 ohms.

Figure 12. A color code chart for resistors

5. An Introduction to Use of Breadboards for Prototyping Circuits

Early electronics experimenters fastened components to a wooden breadboard. For convenience, a strip of wire along one edge of the breadboard was often used as a common return path for several components in the circuit. By analogy with the telegraph ground return path, this common point of connection became known as a ground strip. When schematic diagrams of circuits were drawn, experimenters used the ground symbol to indicate that components were connected via a common return path. This saved the effort of drawing a line between the components. This shorthand convention that has its origins in the past is helpful in understanding today’s circuit diagrams.

A modern breadboard looks like the example shown in Figure 13.  The blue line shows the connecting strip that serves as a ground return path.

Figure 13. A modern breadboard for prototyping electronic circuits

Connecting wires can be inserted between any of the holes in the breadboard to connect components. However, in order to use the breadboard, it is necessary to understand the way in which the holes are connected. Metal strips connect each of the holes in the breadboard in the manner shown in Figure 9.

Figure 14. The structure of a breadboard

The red and blue strips in the diagram are connected with horizontal copper strips beneath the plastic breadboard. By convention, the strip shown in blue is used as a common ground (i.e., the common return path to the negative terminal of a power source). The strip shown in red is used as a common source of power for components. The strips shown in gray are connected with vertical copper strips. An understanding of the way in which the breadboard is organized is necessary in order to use it for prototyping electrical circuits.

6. A Basic Op Amp Circuit

Modern solid state components make it possible to create a functional amplifier with just three components: one op amp integrated circuit and two resistors.

Figure 15. A resistor and an operational amplifier (op amp)

Decoding the Functions of Op Amp Pins

An integrated circuit such as an op amp includes specifications that describe the function associated with each pin on the chip. The specifications for an LM 741 op amp are shown below. The input from a music source is connected to Pin 2 and the output to a speaker is connected to Pin 6.  The op amp is supplied with power via + 9 volts connected to pin 7 and – 9 volts connected to pin 4.  Pin 3 is connected to ground. The remaining pins will not be used for the application in this lab.

Figure 16. Specifications describing the functions of the pins of an op amp

In a circuit diagram a triangle is used to represent the op amp and a resistor is represented by a zig-zag line. An input resistor (Rin) is placed on the input side of the op amp. A feedback resistor (Rf) bridges the input and the output of the op amp. When the pins on the chip are translated into a diagram that depicts, that shows the relationship of the op amp to the resistors, the results might look like Figure 15.

Figure 17. Essential elements of a basic operational op amp circuit

The Amplification Function

The developer who created the LM 741 integrated circuit designed it so that the input and the output of the op amp balance. In other words, LM 741 op amp is designed so that the chip will attempt to make the value of the input and output pins remain equal.  Consequently, the increase in amplification of the basic op amp circuit shown in Figure 6 is determined by the ratio of the input resistor (measured in ohms) to the value of the feedback resistor. In other words:

In the case of a 1,000 ohm input resistor and a 10,000 ohm feedback resistor, a ten-fold gain in voltage out would be achieved.  The reason for this result is a function of Ohm’s Law. Ohms Law specifies that current in an electrical circuit is equal to resistance divided by voltage. Therefore, if the current remains the same, if the resistance changes, the voltage also must change. Cosequently adjusting the value of the feedback resistor with respect to the input resister causes the output voltage to change.

In order to achieve this effect, the chip must have a power supply. The design for the basic op amp circuit uses two nine-volt batteries.  One battery supplies power for the voltage range between 0 and + 9 volts. The second battery supplies power for the voltage range between 0 and – 9 volts.

Figure 3. The basic op amp amplifier circuit uses two batteries.

Pins 4 and 7 on the LM 741 are the power inputs. Pin 7 is connected to the positive terminal of one battery. Pin 4 is connected to the negative terminal of a second battery. The negative terminal of the first battery and the positive terminal of the second battery are connected to a common tie point that becomes ground. This establishes a common referent point of zero volts.

Figure 18. An amplifier increases the volume of the sound produced by a speaker.

Clip leads will be used to connect the output of the prototype amplifier that is being fabricated to the two wires of the speaker constructed in the previous lab (i.e., the wires wrapped around the speaker solenoid). One speaker wire will be connected to pin 6 of the op amp chip. The other wire will be connected to ground. (Note that for the sake of simplicity the power supply inputs are not shown in Figure 18.)

7. Building the Basic Op Amp Circuit

The op amp should be inserted into the breadboard so that the pins straddle the center line. This will connect each of the pins of the op amp to access a different vertical row.

Pin one is denoted by a shallow circle or indentation on the surface of the plastic case of the op amp (shown in the lower left-hand corner in this instance.) The numbering of the pins wraps around the case in a counter-clockwise direction (so that pin 8 is in the upper left-hand corner in this instance)

Establishing Power and Ground

The LM741 op amp is a dual power supply op amp. The design specifications require one power source with a positive voltage and a second power source with a negative voltage. The positive voltage will be supplied by connecting the positive terminal of a nine-volt battery to the top strip of connectors indicated by the red line beneath it. The negative voltage will be supplied by connecting the negative terminal of a second battery to the bottom strip of connectors with the red line beneath it.

Figure 19. Prototype circuit with positive and negative voltages supplies

In the nineteenth century a row of connectors that conducted the output of an electrical generator across a copper bar or strip was known as a power bus or power rail. The same terminology continues to be used today, and can be applied to the two strips of connectors carrying the positive and negative voltages. This provides a convenient way to connect all of the components to power, rather than connecting multiple wires directly to the terminals of the batteries.

The remaining two connectors of both batteries are then connected to the bottom strip of horizontal connectors with blue line above it. This will be used as a ground strip. This will provide a common return path to complete the circuit for several of the components. This initial setup establishing rows of power and ground connectors provides an important context for construction of the other elements of the circuit.

The steps described above are outlined in the checklist that follows:

1. Positive Power Supply (top horizontal red row of connectors)

The positive terminal of the top battery is connected to the horizontal row of connectors with the red line beneath them to create a positive 9 volt power supply.

2. Negative Power Supply (bottom horizontal red row of connectors)

The negative terminal of the bottom battery is connected to the horizontal row of connectors with the red line beneath them to create a positive 9 volt power supply.

3. Common Ground (bottom horizontal blue row of connectors)

The negative terminal of the top battery and the positive terminal of the bottom battery are then connected to a horizontal row of connectors with a blue line above it. This creates a common ground path. The ground path will have a voltage of zero.

Once the batteries are connected to the breadboard, Pin 7 of the op amp is connected to the positive nine-volt power supply and Pin 4 is connected to the negative nine-volt power supply. The designation of the pins used to supply power to the chip are arbitrarily chose by the designer. The specification of the voltages used for the power supply is, within limits, also somewhat arbitrary. In this case, the ready availability of nine-volt batteries may have been a consideration.

One implication of this choice is that the audio signal will not be able to go above plus nine volts or below minus nine volts without distorting. An attempt to amplify the volume more than that with this op amp will result in audible distortion that can be easily heard in the music played.

Figure 20. Connections to the positive voltage (Pin 7) and the negative voltage (Pin 4)

Once the power supplies are connected to the op amp, Pin 3 is connected to ground. This gives a terminal of each battery and the op amp chip a common ground path.

Figure 21. Connection to ground (Pin 3)

The connections to power and ground are part of the basic setup that will take place for any integrated circuit (IC) chip. All solid state electronics chips need power in some form and also need a connection to a ground path. Although a strip of copper beneath the connectors provides that function rather than a stake driven into the earth, the concept is still the same.

Connecting the Music Source and a Speaker

Once the basic setup of connections to power and ground is completed, the remainder of the connections can be made. These consist of: (1) connecting the music player to the op amp via the input resistor, (2) installing the output resistor, and (3) connecting the op amp to a speaker.

Step 1. Connect the Music Player to the Op Amp

The music player is connected to ground and to one end of the input resistor. The other end of the input resistor is connected to Pin 2 of the op amp. The specification of Pin 2 as the input to the op amp is another arbitrary decision made by the chip designer. This information is included in the specifications provided with the chip.

It is important to remember that the horizontal strips at the top and bottom (indicated by the blue and red lines next to them) connect the holes above them horizontally. The vertical strips on each side of the op amp chip connect all of the holes in that vertical column. Therefore, if one end of the input resistor is placed in the second hole beneath Pin 2 of the chip, the copper strip underneath the holes will connect the end of the resistor to the metal end of the pin.

Figure 22. Music player input to op amp (Pin 2 via the input resistor and Ground)

Similarly, the top connecting wire from the music player (shown in green) is connected to the top connecting hole in the fifth column of holes on the bottom half of the board. Since the other end of the input resistor is also connected to this column of holes, the copper strip underneath the vertical column of holes will connect the input from the music player to one end of the input resistor.

Step 2. Connect the Feedback Resistor between the Input and the Output

The output resistor connects the input and the output of the op amp. The input pin is Pin 2. The output pin is Pin 6. (These designations are arbitrarily made by the chip designer, and made be found in the specifications that accompany the chip.)

The designer of the chip also incorporated a key feature into the chip that determines how it is used. The power on the input side must equal the power on the output side. Ohms Law states that voltage is equal to current (measured in amps) times resistance (measured in ohms). Therefore, if the resistance increases and current remains the same, the voltage on the output side must increase in order to maintain a power output that is equal to the power on the input side of the op amp.

The feedback resistor provides the means through which the chip compares the power on the output side and the power on the input side. It then uses this information to adjust the voltage on the output side to ensure that the power levels on each side remain equal.

Figure 23. Music player input to op amp (Pin 2 via the input resistor and Ground)

Place one end of the feedback resistor into the vertical column of holes beneath Pin 2 (input). Place the other end of the feedback resistor beneath the vertical column of holes above Pin 6 (output).

Step 3. Connect the Op Amp to a Speaker

The op amp can now be connected to a speaker. Place one end of the speaker wire into one of the holes in the vertical column of holes above Pin 6 (output).

Figure 24. Pin 6 and ground are connected to the speaker

Place the other end of the speaker wire into one of the holes in the ground strip.

8. Testing the Basic Op Amp Circuit

To test the speaker, play some music on the music player (i.e. smart phone or iPod, etc.). Experience indicates that in a class of students who complete a circuit of this complexity, several are likely to place some of the connecting wires into incorrect locations. That is especially true if this is a first experience with a breadboard. The holes are close together and all look alike, so it is easy to get off by a row or a column.

If one of the connections in your prototype circuit is inserted into an incorrect location, you can conduct some basic trouble shooting steps. First, review the checklist below:

1. Power Supply: Pin 7 is connected to positive voltage (+9V) and Pin 4 is connected to negative voltage (-9V).
2. Ground: Pin 3 is connected to ground.
3. Feedback Resistor: The feedback resistor connects Pin 2 to Pin 6.
4. Input (Music Player): One input from the music player is connected to ground. The other input from the music player is connected to the input resistor, which – in turn – is connected to Pin 2.
5. Output (Speaker): Pin 6 and ground are connected to the speaker.

If all of the connectors appear to be in the right location, ask a friend to double-check your work. Often another person can look at the connections with a set of fresh eyes to spot the problem.

A continuity tester and / or a multimeter may also be helpful. A continuity tester typically has a bulb that lights up when a circuit is complete. This will allow you to determine if there is a lose wire that is in the correct  hole, but is not making a solid electrical connection. A multimeter has (1) an Ohm meter that is used to test resistance, (2) amp meter, and (3) a voltage meter. The Ohm meter can be used in a manner similar to a continuity tester. The voltage meter can be used to verify that the voltages supplied by the batteries are correct.

9. An Introduction to Transistors

Op amps are a basic building block of modern analog electronics. The LM 741 op amp is widely used as an introduction to op amps, and lays the foundation for a number of other applications. The basic op amp circuit described provides a good initial introduction, and can move a linear motor or produce a faint sound from a speaker. This is because the LM 741 op amp only draws about 30 to 40 milliamps. Therefore the circuit will need to be enhanced. Power transistors provide a common way of achieving this. The addition of power transistors will allow the op amp to handle a larger current. This will allow the combined amplifier circuit to handle several amps.

The transistor was developed at Bell Labs. It was designed to replace the vacuum tube and therefore provides the same functionality in a smaller, more compact format. The transistor consists of three main elements – a base, a collector, and an emitter. In this application, these elements will be used in the following ways:

1. Collector. The output of the op amp will be fed into the collector of the transistor.
2. Base. The collector will be connected to a voltage supply to provide power to the transistor.
3. Emitter. The output of the emitter will be connected to the input of the speaker.

When the transistor shown on the left in figure below is laid flat on the table (so that the letters on the front can be read), the three pins of the transistor (read left to right) are as follows:

1. Pin 1 (left): Collector
2. Pin 2 (middle): Base
3. Pin 3 (right): Emitter.

A typical schematic depiction of a transistor in a circuit diagram is shown on the left in the figure below.

Figure 25. A transistor has a collector, base, and emitter

There are two main types of transistors:  NPN (Negative-Positive-Negative) and PNP (Positive-Negative-Positive). Many amplifier circuits use a matched pair of transistors consisting a NPN transistor paired with a complementary PNP transistor. The NPN transistor conducts the positive half of the signal while the PNP transistor conducts the negative half of the signal. This allows the amplifier to drive the speaker in both a positive direction and a negative direction.

You will recall that in the case of the linear motor, a positive current was used to drive the armature in one direction while a negative current was used to drive the motor in the opposite direction. Since a speaker is essentially a linear motor with cone attached to focus and direct the sound, the same consideration applies to operation of a speaker.

10. Enhancing the Basic Circuit with Power Transistors

A complementary pair of TIP 122 and TIP 127 power transistors will be used to enhance the previous op amp circuit, as shown in the figure below.

Figure 26. Amplifier circuit with transistors

The signal in the enhanced circuit is routed from the op amp through the transistors to the speaker. The feedback resistor is adjusted so that it bridges the input of the op amp (Pin 2) and the input to the speaker.

Adding Power Transistors to the Amplifier Circuit

To modify the previous circuit, begin by temporarily removing the feedback resistor and the output connections to the speaker from the circuit.

1. Feedback Resistor: Remove the feedback resistor that connected Pin 2 to Pin 6.
2. Output (Speaker): Remove the connection between Pin 6 and the speaker.

Once you have completed Steps 1 and 2 (above), identify three unused rows of vertical connections and insert the pins of the TIP 122 power transistor into the breadboard. Then identify three additional unused rows of vertical connections and insert the TIP 127 power transistor into the breadboard. Then make the following connections:

1. Connect the Transistor Collectors to the Op Amp Output:

Connect Pin 1 (collector) of each of the two transistors to Pin 6 (output) of the op amp.

2. Transistor Power Supply
3. Connect Pin 2 (base) of the TIP 122 transistor to the positive (+9V) power supply.
4. Then connect Pin 2 (base) of the TIP 127 transistor to the negative (-9V) power supply.

3. Output to Speaker
4. Connect Pin 3 (emitter) of each transistor to a common output point on the breadboard.
5. Then connect the output of the two transistor to one wire of the speaker.
6. The other wire of the speaker should already be connected to ground.

4. Feedback Resistor
5. Connect one end of the feedback resistor to Pin 2 of the op amp.
6. Connect the other end of the feedback resistor to the common output point (that, in turn, is connected to the speaker) described in Step 3 (above).

The breadboard layout in Figure 14 depicts one way in which the transistors might be incorporated into the modified circuit.

Figure 27. The op amp output provides the input to the transistors

The completed working amplifier can then be combined with the speaker that you created in the previous lab. The illustration below shows the amplifier combined with the initial Make to Learn speaker constructed with the linear motor and a paper cone.

Figure 28. The amplifier combined with a Make to Learn speaker

11. Evaluating the Amplifier

In the same way that a speaker can be evaluated for fidelity, the amplifier can also be evaluated for fidelity. One way of doing this is by comparing the voltage at the input of the amplifier with the voltage at the output amplifier. Ideally the amplification should be the same amount across all frequencies. Table 1 below illustrates an ideal amplifier that has a flat frequency response. The amount of amplification is the same for every frequency.

The actual results obtained for amplifier circuit described above (with an LM 741 op amp and two power transistors) yield the result shown in Table 2.

The frequency response is fairly flat, although the output falls off at the 8000 Hz.

The frequency response of a solid state amplifier will almost always be better than the speakers in a high fidelity stereo system. The frequency response curves for a speaker with a paper cone for two different amplifiers is shown in Table 3.

The frequency response curve for the two amplifiers in the chart below demonstrates how closely the performance of the two amplifier track one another with the exception of a slight divergence at 500 Hz.

Figure 29. Output (in decibels) of a speaker with a paper cone with two amplifiers:
(1) a Dayton DTA-1 amplifier (blue)  and (2) a fabricated amplifier with an LM 741 op amp (red).

Once you have evaluated the circuit with the 10,000 ohm feedback resistor, replace it with a different resistor and evaluate its effect on the amount of amplification achieved. The amplified signal cannot plus nine volts on the positive side or minus nine volts on the negative side without distorting the signal. This is because once the signal reaches nine volts, the battery that supplies power has reached his limit.

Figure 30. A signal with peak clipping (left)

At that point the signal flattens out. This can clearly be seen in a program or app that displays the waveform, but also can easily heard, especially for a pure tone produced by a tone generator.

12. Fabricating a Circuit Board

A breadboard provides a way to test a circuit and modify it as needed in order to achieve the desired performance. The wires on a breadboard can easily come loose (and do). Therefore a breadboard is a prototyping tool and not a permanent solution. The connections can be physically soldered together to create a more permanent circuit. The illustration below depicts one way in which the components might be configured.

Figure 31. Op amp circuit with power transistors

This solution has the advantage of being inexpensive. A soldering iron can be acquired for about ten dollars. Soldering is a skill that can be acquired in about half an hour, and is often useful for putting together circuits with a small number of components such as this one.

Designing a Prototype Circuit Board

If a large number of circuits are needed, a circuit board could be designed. Computer-assisted design (CAD) software can be used to automatically route the connections among the component to find the most efficient path.

Figure 32. Design for a prototype circuit board

The prototype circuit board designed with the aid of a computer can then be fabricated with a milling machine. A rotating drill bit in the milling machine removes all of the copper except for the path for the desired copper traces that make connections between the pins of the components.

Figure 33. A prototype circuit board

The components are inserted into the circuit board and soldered to the copper traces to make electrical connections along the desired circuit paths. The digital milling machine took an hour to remove the excess copper from the prototype circuit board in the figure above. For that reason, this method is not used to fabricate large numbers (hundreds or thousands) of circuit boards. However, this prototyping method is useful for verifying that the circuit board design works as expected before committing to the expense of a mass manufactured order.

Figure 34. A digital milling machine used to create a prototype circuit board

Once the prototype circuit board is used to identify any potential problems with the board, and make corrections as needed, an order can be placed for a large quantity of the board. Manufacturing a board such as this one would likely involve an initial setup fee of about $500. After that, the circuit boards with the components installed could be manufactured for a cost about five dollars per board.