Name Exercise 2 The Protoboard: Bus Connections Purpose The purpose of this exercise is to allow you to become familiar with the protoboard bus strips and to learn the protoboard s electrical characteristics. Objectives After completing this exercise, you should be able to: 1. Measure circuit continuity with the ohmmeter. 2. Determine the patterns of conductive paths in the protoboard. 3. Insert test points into the protoboard for measurements. 4. Confirm the connectivity of the bus lines used in the protoboard. Lab Preparation Review Section 4.5 of Fundamentals of Electronics: DC/AC Circuits. Materials Discussion 1 trainer with hookup wires as needed 1 VOM with test leads A protoboard is used extensively as you proceed through the exercises in this lab manual. A protoboard allows you to assemble (breadboard) electrical circuits quickly and to perform tests and measurements on the circuits easily. This exercise provides further experience with the ohmmeter, a specialized test instrument that can measure resistance. The ohmmeter can be used as a continuity tester to check the conductivity of the material put between its leads. When the test leads are touched together or placed across a material with good conductivity, the needle indicator moves to the right end of the scale. This indicates 0. When the test leads are held apart or placed across a nonconductive material, the needle indicator does not move, indicating an open circuit (infinite ohms) and no continuity.
Figure 2.1 Figure 2.2 The protoboard, as shown in Figures 2.1 and 2.2, is a rectangular plastic block with many small holes in the top. The holes are arranged in rows, and their purpose is to enable you to insert wires into them to make electrical contact. In the plastic material are metallic conductors. Underneath the holes, the metallic conductors interconnect the holes in patterns to make groups of connection points, around which you can build electrical circuits. The metallic conductors receive wires and gently clip onto them to hold them in place. Along the edges of the two long sides of the protoboard are the bus lines (strips). There are two pairs of bus strips at each side of the board that extend the length of the board. The bus strips are a part of the connection scheme on the board and are usually wired to a power source when using the board to test electrical circuits. The long buses conveniently distribute electrical power down the length of the board. When you get into more advanced circuits, the bus strip is a very convenient feature. Figure 2.3 shows the bus-strip pairs of the protoboard. The bus-strip pairs are labeled A and B at the top, C and D at the bottom. All holes in each bus strip are connected together in the respective buses. Thus, all holes in bus line A are connected together, as are all holes in bus lines B, C, and D. Each bus strip is isolated from each of the others. Although the conductor is shown in Figure 2.3, when you actually look at the protoboard the conductor is hidden by the plastic body and cannot be seen. That is why it
is very important to know exactly how the buses are connected electrically because the internal metallic conductor is not visible. Figure 2.3 Procedure Figure 2.4 1. Prepare six short lengths of hookup wire as shown in Figure 2.4, by cutting the wires to length and stripping about a quarter inch of insulation from each end. 2. Insert two of the test wires into bus line A. Place one of the wires at the end of the bus and place the second wire into the same bus several holes away from the first wire. Check to ensure that both wires are installed in bus line A and that neither has been accidentally placed into bus line B. The two bus lines are close together.
3. Prepare the ohmmeter for use to verify continuity between the two test wires in bus line A. Use the R 1 scale, plug the test leads into the meter, and zero the meter. Zeroing is done by shorting the test leads (simulating zero resistance) and adjusting the zero-ohms adjustment until the pointer on the meter scale is positioned exactly over the 0 mark. 4. Connect the ohmmeter test leads to the test wires that are installed in bus line A. Connect one lead to each test wire. Look at the ohmmeter indication. Does the ohmmeter show a value of 0? 5. Insert a third test wire into bus line A several holes down the line from the second test wire. Look carefully to see that the third wire has been inserted into bus A and not inadvertently put in bus B. Connect the ohmmeter to test wires 1 and 3. What value does the ohmmeter indicate? 6. Now connect the ohmmeter leads to test wires 2 and 3. What value does the ohmmeter indicate? Does the ohmmeter still indicate zero? 7. Now insert a fourth wire into bus line A at the opposite end of the bus away from the test wire 1 as seen in Figure 2.5. Use the ohmmeter to measure continuity between all four of the test wires (1 to 2, 1 to 3 and 1 to 4). Does the ohmmeter show 0 for each of the tests? Figure 2.5 8. Insert the fifth test wire into bus B at the same end of the bus as test wire 1, as shown in Figure 2.6. Test wire 5 and test wire 1 will now be beside each other but in their separate buses. If bus line A and bus line B are really isolated electrically from each other, there should be no continuity, or electrical connection, between any wires in bus A and any test wires in bus B. Figure 2.6 9. Connect the ohmmeter test leads to test wires 1 and 5. What does the ohmmeter indicate?
10. Change the ohmmeter scale to its highest range, which is R 10,000. Before using the analog ohmmeter on this new scale it will be necessary to re-zero it. Do this as before by shorting the ohmmeter test leads together and adjusting the zero-ohms knob for exactly 0 on the meter dial. After calibration, it will give accurate measurements. Remember that it is necessary to re-zero the analog ohmmeter each time you change its measurement scale. If the re-zeroing is not done, then any measurements made with the ohmmeter will not be accurately indicated on the ohmmeter scale. Connect the ohmmeter leads again, this time to test wires 1 and 5 on the protoboard board. What does the ohmmeter indicate? 11. Insert test wire 6 into bus B at the far end of the bus away from test wire 5. Test wires 4 and 6 will now be adjacent to each other but in separate buses. Test for continuity with the ohmmeter between all test wires on bus A and the test wires on bus B, checking to see if there is any connection between bus A and bus B. What does the ohmmeter indicate? 12. Set the analog ohmmeter back to its low-resistance mode (don t forget to re-zero this meter) and test for continuity between test wires 5 and 6. These are the test wires inserted into opposite ends of bus B. What does the ohmmeter indicate? 13. Repeat the same measurements that you just performed on buses A and B on the buses for buses C and D. Are they the same electrically? Questions 1. Why is the protoboard sometimes referred to as a breadboard? 2. What is the purpose of the bus strip? 3. What do you think is the purpose of the vertical columns of contacts?