Limit switch

A limit switch is a mechanical switch operated by physical contact. When a moving part pushes its lever or plunger, the internal contacts change over. Simple, robust and needing no power of its own, it is still everywhere: end positions, doors, lifts and cranes.

How it works

A limit switch has an actuator (roller lever, plunger, rod or wobble stick) connected to a snap-action contact block. Moving the actuator past its operating point flips the contacts; releasing it lets a spring return them. Most have one NO (normally open) and one NC (normally closed) contact.

Actuator types

Wiring to a PLC

A limit switch is just a contact. For a PNP-style (sinking) input: +24 V → switch terminal → other terminal → PLC input, with the input COM at 0 V. It needs no supply of its own.

NO or NC? Use the NC contact for anything safety-related or for over-travel limits: if the wire breaks, the PLC sees the same thing as "limit reached" and stops. Use NO for normal position signals.

Positive opening (safety)

Safety limit switches have positive (direct) opening NC contacts, marked with an arrow in a circle. The actuator forces the contacts apart mechanically, even if they are welded. Only such switches may be used on guard doors and in safety circuits (IEC 60947-5-1, Annex K).

Limit switch vs proximity sensor

Practise in the PLC Sim Lab Playground with the "cylinder with physical limit switches" example. See also the inductive proximity sensor and the cylinder reed switch.