Hands-On Approach to Teaching Digital Circuits

Use this example to investigate the design of a four-bit binary counter with a 555 timer IC generating the digital clock signal (Figure 1).

Figure 1. Example Circuit

For this exercise, apply a constant voltage source and a DC current source to the base. By performing a DC analysis simulation, for which you sweep the current source to the base, you can see the relationship of  plotted against. Observe this relationship with Multisim simulation.

Begin by drawing your schematic in the Multisim environment. 

1. Select Start»All Programs»National Instruments»Circuit Design Suite 11.0»Multisim 11.0 to open Multisim.
2. Multisim opens showcasing the default capture and simulation environment.

Figure 2. Multisim Environment

There are two fundamental tasks in the design of a circuit: the placement of components and the wiring of these components to create a complete design.

3. To select a component, go to Place»Component …
4. The Select a Component dialog appears (also known as the Component Browser).

Figure 3. Select a Component Dialog

The component browser organizes the database components into three logical levels. The Master Database contains all shipping components in a read-only format. The Corporate Database is where to save custom components to be shared with colleagues (via a network collection and so on). Finally, the User Database is where custom components are saved that can be used only by the specific designer.

Additional Points

• The components (or parts) are organized into Groups and Families to intuitively and logically group common parts together and make searching easier and more effective.
• The component selection box shows the Component name, Symbol, the functional description, Model, and Footprint all in a single pop-up.

To invoke simulation, you need a power source and a ground somewhere in your circuit to correctly reference voltages and currents in your circuit simulation.  

5. To place a Ground, go to the Sources group and highlight the POWER_SOURCES family.   
6. Highlight the GROUND component (as shown in Figure 4).
7. Click OK.
8. The component selection window temporarily closes and the ground symbol is "ghosted" to the mouse pointer.
9. Move the mouse to the appropriate place on the schematic and left-click once to place the component.
10. Now place two more GROUND components on the schematic.

Figure 4. Placing a Ground Symbol

To place a VCC power supply:

11. Go to the Sources group again and highlight the POWER_SOURCES family (if not already highlighted from the previous selection).
12. Select the component called VCC.
13. Place the VCC source on the schematic.

Additional Points

• Without a power and ground, your simulation cannot run.
• If you need multiple components, you can repeat the placement steps as shown or place one component and use copy <Ctrl-C> and paste <Ctrl-V> to place additional components as needed.
• By default, the component selection box keeps returning as a pop-up until you have completed placing your components. Close the window to return to the schematic entry window (Close button). You can change this in the global preferences dialog box.

Now place the remaining circuit components using the techniques discussed in the previous steps.

14. Select Place»Component.
15. To place the 555_VIRTUAL component, select the Mixed group and MIXED_VIRTUAL family. Highlight the 555_VIRTUAL component, click OK and place.
16. Select Place»Component.
17. Place the four-bit binary counter 7493N found by selecting the TTL group and the 74STD family. Type in 7493N in the component.

Now place the resistors and capacitor in the design.

18. Select the Basic group and the Resistor family.
19. In the Component field, type 10 k to select a 10 kΩ resistor.
20. You can rotate a part before placement by using the <Ctrl-R> shortcut on the keyboard when the component is ghosted to the mouse. Once rotated, place the component.
21. Place a 100 kΩ resistor.
22. Now select the Capacitor family and in the component field, type in 100 n to select a 100 nF capacitor. Place the component in your schematic.

 At this point, your schematic should look something like Figure 5:

Figure 5. Components Placed

Multisim is a modeless wiring environment. This means that Multisim determines the functionality of the mouse tool by the position of the mouse. You do not have to return to the menu to choose between placement, wiring, and editing tools.

23. To begin wiring, move the mouse close to a pin of a component.
24. The mouse appears as a crosshair rather than the default Windows mouse.
25. Place an initial wire junction by clicking on the pin/terminal of the component (see Figure 6 for an example).

Figure 6. Wiring the Circuit

26. Complete the wire by moving the mouse to another terminal or just double-click to anchor the termination point of the wire to a floating location somewhere in the schematic window.
27. Continue with wiring the rest of the circuit as shown in Figure 7. Do not worry about the labeled numbers on the wires, which can be referred to as nets. 

Figure 7. Wire the Components as Seen Above

You are now ready to run an interactive Multisim simulation; however, you need a way to visualize the data. Multisim provides simulation-driven instruments such as oscilloscopes, wattmeters, and more to help you visualize the simulated measurements.

Find instruments on the right menu bar -- they are indicated by the following icons.

Figure 8. Instrument Toolbar

28. Select the four-channel oscilloscope instrument from the menu (fifth icon from the left) and place it onto the schematic as you would any other Multisim component.
29. Wire the Channel A, Channel B, Channel C, and Channel D terminals of the oscilloscope to pins 12, 9, 8, and 11 of the 7493N four-bit counter (as shown in Figure 9).
30. Change the color of the input channel net (connected to channel A) by right-clicking on the wire.
31. Select the Color Segment option.
32. Select a shade of Orange and click on the OK button.
33. Repeat steps 30-32 to change the color of the segments for the different channels of the oscilloscope. This is important because this determines the color of the signal displayed on the instrument.
34. Also place a Ground at this point to the oscilloscope and provide a trigger by connecting the Trigger pin (labeled T) to the clock signal from the 555_VIRTUAL timer.

When you are done, the circuit looks like this:

Figure 9. Oscilloscope Connected to the Four-Bit Counter 

It is simple to begin running and visualizing an interactive simulation in Multisim.

35. Press the green run button on the simulation toolbar as seen in Figure 10.
36. Double-click on the oscilloscope on the schematic. You can now visualize the simulation measurements in the oscilloscope interface (Figure 11). Click on the Reverse button of the oscilloscope to change the background color from black to white.
37. You can now stop the simulation by pressing the red stop button in the simulation toolbar (Figure 10).

Figure 10. Simulation Toolbar

Figure 11. Visualizing Simulation in the Oscilloscope

You have just successfully built, simulated, and analyzed a digital circuit using Multisim.

Now that you have explored how a four-bit digital counter works, are you able to design an eight-bit decimal counter with two seven-segment displays using a 555 timer IC to generate the clock signal?

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