To create a NOT logic circuit, how are switches wired?

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

Answer: (B)

To invert a control signal with switches, you use a normally closed switch in the path. A NC switch is conducting when the input is not energized, so the output is on by default. When the input activates, the switch opens and the circuit is broken, so the output goes off. This creates output = NOT input: high when the input is low, and low when the input is high. That’s why a single normally closed switch is the simplest and most direct way to realize a NOT function.

Using a normally open switch would cause the output to follow the input instead of invert it, since it closes only when the input is active. Two switches in series—whether NC or NO—adds unnecessary complexity and doesn’t provide the clean, simplest inversion that a single NC switch gives.