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.

When wiring to a PLC, most engineers use the NC contact for safety-critical functions (loss of continuity reveals a problem) and the NO contact for process signals. E-stop circuits and guard interlocks nearly always use NC wiring so that a broken cable registers as a fault rather than a safe state.

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.