How does a limit switch function in a control system?

• A. It senses temperature and opens circuit when overheated.
• B. It acts as a timer delaying a circuit.
• C. It is a physical device that changes its contact state when a moving part reaches a set position, providing a position signal.
• D. It provides visual indication of position with a lamp.

Answer: (C)

A limit switch detects when a moving part has reached a specific position and then changes its electrical state to send a signal to the control system. It’s a physical device with contacts and an actuator (like a plunger, lever, or roller) that is moved by the machine part. When the part reaches the set position, the actuator moves the contacts from their resting state to the opposite state (normally open closes, or normally closed opens), producing a discrete signal that the controller can act on. This makes it useful for end-of-travel sensing, position feedback, and safety interlocks in automation.

In contrast, a temperature-sensing device would respond to heat, a timer delays actions rather than signaling position, and a lamp is simply a visual indicator. A limit switch’s defining feature is that its contact state changes due to the physical position of a moving part, providing a clear position signal to the control system.