How are switches wired to create a NAND logic circuit?

• A. Two normally opened in series
• B. One normally opened in series with a normally closed
• C. Two normally closed circuits in parallel
• D. Two normally closed in series

Answer: (C)

Understanding how to realize a NAND function with switches comes down to how the output should behave: it stays high unless both inputs are high. Using two normally closed contacts in parallel achieves this perfectly. In the resting state, both contacts are closed, so current can flow and the output is high. If either input is activated (raising that contact), that particular path opens, but as long as at least one contact remains closed, the output stays high. Only when both inputs are activated do both contacts open, breaking the circuit and making the output low. This exactly matches NAND behavior.

Other configurations would not produce the same truth table. For example, two normally opened contacts in series would require both inputs to be active to conduct (an AND behavior). Two normally closed contacts in series would cut the current if any input is active, which also doesn’t align with NAND.