Software Design

Subway Turnstile Access Control State Machine

State Diagrams · made in Skemio

State machine of a subway turnstile showing transitions between Locked, Verifying, Unlocked, Passing, and Alarm states.
State machine of a subway turnstile showing transitions between Locked, Verifying, Unlocked, Passing, and Alarm states.
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What it shows

  • Mechanical rotors lock by default to reject unauthorized rotation force
  • A valid tap unlocks the barrier and starts an optical passage countdown
  • Infrared sensors trigger an immediate relock to block following riders

The takeaway

Subway gates combine optical sensors and strict timers to guarantee exactly one entry per valid tap.

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