Figure 1. An electric circuit. Click on the switch to open or close it. |
In order for electricity to perform work, such as lighting a lamp, there must be a complete path from the power source through the parts of an electrical appliance and back to the power source.
In the schematic diagram of Figure 1, the switch is initially shown in the open or off position. There is a path from the battery (the power source) to the switch, and from the switch to the lamp and back to the battery. However, there is no path through the switch. Because the switch is off, the path is interrupted and no current flows. Click on the switch to close it.
When the switch is closed, the path is completed, current flows, and the lamp lights. The necessity for a complete path is why we call this a circuit. In case you are wondering about real-world applications, the diagram in Figure 1 is a good representation of how a flashlight works.
Figure 2. An electric circuit with two switches in series is similar to the Boolean AND function. |
You can experiment with this circuit by clicking on the switches. The lamp lights only when both switches are closed.
Notice that this circuit is like the Boolean AND function. Both the left switch and the right one must be closed before the circuit is completed and the lamp lights.
Consider the circuit in Figure 3 There are still two switches, but they are arranged differently.
Figure 3. When the two switches are in parallel, the circuit is similar to the Boolean OR function. |
Experiment by closing first the upper switch by itself, then the lower switch by itself. Trace the path of the circuit when one of the switches is closed.
What do you expect happens when both switches are closed? Try it and see. The lamp lights because electricity is flowing through both possible paths.
When switches are wired this way, they are said to be in parallel. A circuit with two switches wired in parallel is like the Boolean OR function. The circuit is completed when one of the switches or the other is closed, or when both of them are closed.
In the circuit in Figure 4 the switch is represented by a pushbutton.
Figure 4. The switch is replaced by a pushbutton and the lamp by an LED. |
In many cases, we can abstract a circuit still further. Every electric circuit requires a connection to a power source and a return to the power source.
Figure 5. |
If you have downloaded Digital Works, you can "wire up" the circuit in Figure 5. Digital Works omits the power source and return wires as we have done here.
In the next section we will see that transistors can function as switches and can be wired in series or parallel to compute Boolean functions.
Table of Contents Next: Transistors and Gates
Originally published: 2000-07-20
Copyright © 2012 by Bob Brown. Some rights reserved.
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
Last updated: 2012-10-21 16:34