d. Since we've completely determined how the circuit works, we can replace it with a symbol.
In Figure 3-4 for example, we can predict the output resulting from any given input combination
because we know how the circuit works.
On a separate piece of paper, complete the fourth column of the truth table as a final check to
assure yourself that you understand the OR gate. Remember that in positive logic each high is a binary
1 and each low is a binary 0. If you have any difficulty, refer back to the schematic in Figure 3-3 and
imagine a third diode in parallel with the other two. The third diode provides the third input connection
represented by input C, Figure 3-4. The correct solution is shown on Figure 3-17.
Figure 3-4. Positive OR gate and truth table
A transistorized negative logic OR gate is similar to the circuit in Figure 3-3 but is designed to
follow the rules of negative logic. The difference compares to the difference between negative and
positive logic diode circuits.
Learning Event 2:
IDENTIFY A TWO SWITCH AS AN AND GATE
A mechanical equivalent of an AND circuit can be shown just as easily as the first OR circuit we
discussed. All we need is two switches and a light bulb. This time, however, we connect the switches in
series instead of in parallel, as in Figure 3-5.
a. Once again we'll consider the light from the bulb as the output. When we explain how to
light the lamp, we will have stated the definition of an AND gate.