Key Switch
A key-operated switch is a maintained or momentary switch whose actuator is a removable mechanical key cylinder. Only an authorised key turns the rotor, and many designs trap the key in one or more positions to enforce a procedural lock-out.
A pin-tumbler or wafer-tumbler lock cylinder sits on the same shaft as a rotary switch wafer. Inserting the correct key aligns the tumblers along the shear line, freeing the rotor; turning the key rotates the wafer's wiper across the contact terminals. Key-trap designs use an internal ramp on the rotor that lets the key withdraw only at specific positions.
In plain terms
A door lock that closes a circuit instead of unlocking a deadbolt — turn the key, the cam rotates the contacts.
Why designers use it
- Ignition and master power on vehicles, forklifts, and industrial machines.
- Service-disable lockout on lifts, elevators, and gym equipment.
- Arming/disarming on alarm systems, safes, and ATMs.
Best for
- Vehicle ignition
- Service lockout
- Alarm panels
Key specifications
- Positions: 2 – 4
- Current rating: 0.5 – 10 A
- Voltage: 24 – 250 V
- Lifetime: 10k – 50k cycles
- Key withdrawal: 1 or multi-position
When not to use it
- On any product that needs cheap, mass-deployable user input — keylocks raise BOM cost dramatically.
- Where keys can be lost or duplicated easily and security matters — use electronic credentials instead.
- In high-vibration mobile equipment unless the lock is shock-rated.
Common mistakes
- Choosing a 2-position key when the application really wants 3 (off / standby / on).
- Forgetting to specify whether the key can be removed in any position vs. only the off position.
- Wiring the keyswitch to a logic input without any ESD or surge clamping for the panel-edge cabling.
Where you will find it
- An automotive ignition switch is a 4-position key switch (Lock / Off / Run / Start) where the key traps in Lock and Start is a momentary spring-return position so the starter motor cannot be left engaged.
- Hospital infusion pumps use a 2-position key switch for service mode — only a clinical engineer's key trips the cabinet open and re-enables firmware update.
- Elevator emergency stop on a service panel uses a key switch that only building maintenance can reset, preventing pranksters from disabling the car.
A short history
Key-operated switches grew out of locksmithing in the late 19th century. The combination of an off-the-shelf pin-tumbler lock cylinder (popularised by Yale in 1861) with a rotary switch wafer was standardised by industrial controls makers in the early 20th century. The key-trap mechanism, which forces operators to leave the key in a specific position before they can withdraw it, is a workplace-safety feature codified by safety standards like ISO 14118 (prevention of unexpected start-up).
Good to know
- Most automotive ignition switches use only a 5-pin tumbler — enough for ~100k unique keys, which is why dealer key codes wrap around within a model year.
- Push-to-start cars still contain a key-switch mechanically: it's hidden in the steering column for limp-home mode if the keyless RF chain fails.
- An OSHA-compliant lock-out/tag-out station uses a key-trap switch so the operator cannot leave the machine unlocked.