how elevators know that someone’s in the doorway
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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.
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