Limit Switch
A limit switch is a mechanical position sensor: a spring-return lever, plunger, or whisker is depressed by a moving machine part to actuate snap-action contacts. The signal tells a controller that motion has reached a defined position so the controller can decelerate, reverse, or trip a safety stop.
A cam, roller, or lever rides on the moving member; reaching the limit pivots a plunger inside the housing. The plunger compresses a snap-action mechanism (almost always a Honeywell-style basic switch internally) that flips its contact bar across center, generating a clean, bounce-free transition that the PLC reads as a digital input.
In plain terms
A bumper that flips a switch when the moving part touches it; the machine knows it's at the limit and stops.
Why designers use it
- End-of-travel detection on actuators, gates, and conveyor systems.
- Door-closed sensing on industrial enclosures and elevators.
- Safety interlocks where a moving guard must be confirmed in place before motion is allowed.
Best for
- End-of-travel
- Door interlocks
- PLC inputs
Key specifications
- Operating force: 0.5 – 10 N
- Current rating: 5 – 15 A
- Voltage: 120 – 600 VAC
- Lifetime: 1M – 10M cycles
- IP rating: IP65 – IP67 oil-tight
When not to use it
- Where non-contact sensing is fine — an inductive proximity sensor lasts 100× longer with no wear.
- Where the actuator forces are very small — the snap-action requires real travel and force to trigger.
- In wash-down food/pharma environments unless the limit switch is properly IP69K-rated.
Common mistakes
- Wiring N.O. and N.C. backwards on a fail-safe interlock — the machine 'works' but is not actually safe.
- Mounting the switch where the lever can be over-travelled and bent by the moving member.
- Skipping the safety category — a single channel limit switch is not a Cat-3/Cat-4 safety device.
Where you will find it
- IEC 60947-5-1 covers specified electromechanical control-circuit devices, including position switches. Its scope is not a declaration that every limit switch satisfies a particular safety function.
A short history
The modern industrial limit switch grew out of the basic 'snap-action' switch invented by Peter McGall at the Burgess Battery Company in 1932 and commercialised as the Micro Switch. Honeywell acquired Micro Switch in 1950 and turned the product line into the de-facto industrial limit switch standard. By the 1960s, NEMA and IEC had standardised the heavy-duty oil-tight enclosure and the standard plug-in actuator heads (lever, roller, plunger, whisker, fork) that survive on factory floors today.
Good to know
- A switch's actuator shape does not by itself establish its electrical contact arrangement or safety function.
- Safety-related use requires the appropriate device, architecture and installation. Do not choose a protective switch because a broken mechanism is expected to sound obvious.