how elevators know that someone’s in the doorway

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how elevators know that someone’s in the doorway

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Newer ones have a sensor. Older ones just start to close until they hit something/sense resistance, then open back up. You still don’t want to risk putting your hand in between the doors though in case it’s one of the even older ones that doesn’t do either, unless you want to lose the hand.

Image a invisible line that runs from one side of the door to the other. If this line is broken then the elevator opens the doors as it believes someone is attempting to enter.

There usually is and infrared laser near the bottom part of the door frame.
If the laser is interrupted the door won’t close.
Look out for a dark piece of plastic/glass on both sides of the frame, roughly 30cm from the ground.

They have things called “light curtains”.

There’s a number (usually more then 48) of infrared LEDs(light emitting diode, basically a light bulb) that make a invisible pattern between the two doors. If one of the beams is broken, then the signal is sent to the lift controller and that tells the doors to open again.

There are models of the sensor that project outwards as well to make sure anyone approaching the lift gets picked up as well.