Mechanical Limit Switch

A mechanical limit switch is one of the oldest and most reliable sensors in industrial automation. When its actuator — a roller lever, plunger, or whisker — is physically depressed by a moving machine part, a set of electrical contacts inside changes state. The normally-open (NO) contact closes; the normally-closed (NC) contact opens.

PLCs read this as a discrete input: 0 V or 24 V on the input terminal. Unlike electronic sensors, limit switches work independently of ambient light, metal content, or electromagnetic interference. They can also switch higher voltages directly (up to 250 VAC in some models) for simple relay-logic panels.

Actuator types include the roller lever (used when a cam or dog pushes against a lever arm), the plunger (used when a surface presses down directly), and the whisker or antenna (used in low-force or thin-material applications). Rotary and adjustable-head variants allow flexible mounting.

For an ordinary process indication, choose NO or NC to make the normal and failed states useful to the control logic. Safety-related guard functions need more than selecting an NC terminal: use a suitable safety-rated interlock with the required direct-opening or coded behavior, monitored wiring, and an architecture validated to the required PL or SIL. An NC circuit can make an open wire look like a stop demand, but it does not detect every dangerous failure and does not by itself make a circuit safety-rated.

Limit switches require periodic inspection. The mechanical contacts wear after millions of operations and can arc when switching inductive loads unless a snubber circuit is fitted.