Which arrangement describes NAND logic using switches?

• A. Two normally opened in parallel
• B. Two normally closed in parallel
• C. Two normally closed in series
• D. Two normally opened in series

Answer: (B)

NAND with switches is achieved by using two normally closed switches placed in parallel. A normally closed (conducting by default) switch opens when its input is activated. With two such switches in parallel, current can flow through either path as long as at least one switch remains closed.

If neither input is active, both switches stay closed and current flows, so the output is on. If exactly one input is active, that one opens, but the other switch remains closed, so there is still a path for current and the output stays on. Only when both inputs are active do both switches open, removing every path for current and turning the output off. This pattern exactly matches NAND: the output is off only when both inputs are high; otherwise it is on.

Other arrangements don’t produce this inverted AND behavior. For example, two normally opened switches would require both to close to pass current (an AND-like behavior without inversion), and series or other configurations don’t provide the same condition where the output stays on unless both inputs are active.