How do those automatic doors at stores work?

Is that an infra-red sensor in the little black box above the door? Ultrasonic? How do they work?

They are probably either infra-red or microwave. See here for details on each.

They set off my car’s radar detector, so it must involve radio waves (K band mostly).

There used to be a pad in front of the door you had to walk on, but that’s the old-fashioned kind. You can still find them some places, though.

Some sniff the air for DNA that is human or not. If you see someone hanging around the door, hoping to “tailgate” in on your DNA signature, turn around and go to a different store. :wink:

The real question is, How do the doors on the Enterprise no to open when you want to walk through or stay closed when someone gets thrown against them in a fight? Those were some amazingly intelligent doors. Sometimes they open and sometimes they don’t.

Hey, at least they didn’t have Genuine People Personalities.

When I was a kid my Mom told me that there were little elves that hid above the doors and opened them whenever someone came near.

I always imagined a line up of elves with time cards checking out as the grocery store closed. :stuck_out_tongue:
But I have a funny feeling that is not what you are looking for.

My dad owned and operated an automatic door business for about 40 years, and while my friends would all be off enjoying their school breaks, I’d be working for dad, or suffering through a sales convention (unless it was being held in some place cool like Hawaii, then I didn’t get to go, and dad wonders why I didn’t want to follow in his footsteps :rolleyes: ). How they work depends upon who made them and where they’re installed. Many of them use a K-band microwave based motion detector, others, in areas where you don’t want electronic interference, or because the manufacturer doesn’t offer microwave, use IR detectors. Except Stanley, they still (AFAIK) use the mats. The mats are basically two pieces of spring steel curved so that there’s a gap between the plates, when enough weight is put on them, the plates touch, completing the circuit and the door opens (or is prohibited from opening, depending upon which side of the mat one is on and the type of door).

Many units have an IR unit in the doorjam to detect if someone’s standing in the doorway so that the door won’t close on them, other units have (though the IR ones may have this as well) a pressure sensative mechanism so that if the door starts to close on something, it will detect the increased resistance and reverse direction.

Most of the electronics and motors are located in the sill above the door, except for Stanley’s, which tend to use hydraulics instead of motors and have their’s located in the floor (meaning someone’s got to jack hammer a bunch of concrete to install the door).

John DiFool, if you pay attention to the readout on your detector, and then inspect the door, you’ll find that every door manufacturer’s units trip your detector differently. One of the guys who worked for my dad would notate who had doors where, so that my dad could have an idea of how the competition was doing.

More than you want to know about motion sensors .

Yeah, and when someone’s about to leave at the end of a scene, then turns around and delivers a Deep Thoughtful Line, the doors rarely open until he actually leaves.

That’s the Starfleet Drama Detectors at work.