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The same way a cat can set off infrared burglar alarms. All it takes is a temporary breaking of the beam for it to dispense. It could be anything floating across the path of the beam near the dispenser; dust, loose paper; even a shadow cast by another object, broken by your movement.
It's the same process used in most automatic garage door open/closers. Some use resistance against the chain, but most just shoot a beam across the opening. If anything breaks the beam, the door will reverse direction.
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"I'm typing on a computer of science, which is being sent by science wires to a little science server where you can access it. I'm not typing on a computer of philosophy or religion or whatever other thing you think can be used to understand the universe because they're a poor substitute in the role of understanding the universe which exists independent from ourselves." - Willravel
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