Which statement best describes NOT logic usage?

• A. One normally closed switch
• B. One normally open switch
• C. Two normally closed switches in series
• D. Two normally open switches in series

Answer: (B)

NOT logic means the output is the opposite of the input. The simplest way to get inversion with a single physical element is a relay or switch that is normally closed. When the input activates, the NC contact opens and the output goes off; when the input is inactive, the NC contact is closed and the output is on, giving the inverted relationship. Two normally closed contacts in series would yield a NOR function (output only when both inputs are off), while two normally open contacts in series implement an AND function (output only when both inputs are on). A single normally open contact by itself does not invert the input; it passes current only when the input is active. Some contexts or conventions in sensor logic describe NOT behavior using a single normally open contact in an active-low wiring scheme, which is why this option might be presented as describing NOT logic in certain practice-test framings.