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warpling's avatar

How are hotel-style door handle/keypads powered?

Asked by warpling (849points) October 14th, 2009

I live in on a college campus in an apartment where to get in you must first insert a smart card then punch in a pin and the door will unlock. The unlocking mechanism is not very audible and there are LEDs on the lock. I mention these things because the device I assume draws very little power. I cannot find a power source through the door hinge, or anywhere else. It must be battery powered right?

I share this apartment with 3 other students who all probably come and go at least a dozen times a day. With all that power being drawn to “unlock” the door in a secure fashion, how is this thing being powered? Do they maintain them with new batteries? Are they powered by the action of opening the door and turning the handle? Ideas?

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13 Answers

se_ven's avatar

Not sure, but wouldn’t it be cool if they were powered by the turning of the handle?

grumpyfish's avatar

Most of them have power running in the door near the hinge.

It’s possible (although unlikely) that it’s powered by an inductive link in the door frame. It’s probably more likely they have a fairly sizable lead-acid battery in the door that only needs to be changed perhaps yearly.

Assuming a 12Ah battery, and 30mA for three seconds to open the door, that’d be around 480,000 openings to drain the battery. Assuming 36 openings a day, that’d be around 30 years on a 12Ah battery. If you want to have to replace the battery yearly (assuming my 30mA for 3 seconds is right), you could easily use a couple of AA batteries.

grumpyfish's avatar

Of course, if I actually looked it up:

(4) AA batteries:
last 18 months under normal hotel use. I’d guess 9 months for you.

warpling's avatar

Thanks! I’d wish I could understand what the power is used for to unlock the door too… another question perhaps…

grumpyfish's avatar

Usually there’s a solenoid or something holding the latch shut, when the door unlocks, that solenoid is pulled out of place using electromagnets, and is generally closed again with a spring.

There’s typically some sort of mechanism on the inside handle that pulls the solenoid out when you turn the handle, so even if the power is dead, it’ll still open the door.

warpling's avatar

@grumpyfish that’s what I figured, but can a solenoid be powerful enough to both hold the lock securely and use minimal energy?

grumpyfish's avatar

Doesn’t need to hold the lock securely, only needs to hold it when it’s open.

When the door is locked (the normal state), there’s a pin behind the latch keeping it from being slid back (by the turning of the handle), when the door is unlocked, this pin slides out of the way, permitting the handle to open the latch.

Holding the pin in place doesn’t require any energy (it’s normally done by a spring), only removing the pin requires energy.

charhalCDW's avatar

For safety reasons all these doors, especially in a dorm, will be linked back to a master control panel near the entrance. If there’s ever a fire alarm pulled, this is the panel that the FD will open to reset the building.

In a modern dorm these doors all have to “fail safe”, which means that if the power goes out the door has to open from the inside. In order to orchestrate something like this each door must be individually powered. In order to garauntee this a battery is not possible. If they were battery powered, then each door in the building would be a point of failure. You’d have techs going around, 24/7/365, making sure that students doors were working, because if a battery died you could have a chance (however remote) of burning alive in your room.

Assume then that each door is linked back to that master control panel, through a proprietary interface, comprised of electrical conduit and plenum (fire resistant) cables. This allows a technician or FD personal to, at a glance, see if the doors in the building are operating properly. They may be linked with standard power cables, or cat5 cables like in a datacenter. If they are cat5 cables, also assume that the building has an “intelligence” and monitors who goes where. This intelligence would keep a record of all access to any room in the building for a period of time, say 7 years.

The entire system may also be backed up with a diesel generator, either located near your building or centrally on campus.

warpling's avatar

Thanks for the great answers and insight guys.
@grumpyfish That would make sense but that spring must be pretty strong, I wish I could find a diagram on how the solenoid pulls the pin back.
@charhalCDW Very interesting idea, and quite scary, but the doors are always open from the inside. Turning the handle down results in the door opening, turning it up results in it locking, but turning it down while locked unlocks it and opens the door, so I don’t think the fire scenario makes sense.

I’ve checked out in the door jam and there are no connections and the hinge side seems totally normal unless there’s a hidden mercury connection in one of those. :P The thing at the top of the image is the automatic door closing thing, no connections there either…

grumpyfish's avatar

@charhalCDW Would be easiest to make that be a mechanical fail-safe, rather than an electronic fail-safe? E.g., when you push the handle down inside the room, it lifts the aforementioned pin to allow you to open the door?

grumpyfish's avatar

@warpling Doesn’t need to be very strong (and could just be gravity—but that would be slightly more prone to failure)

charhalCDW's avatar

Good point grumpyfish.

grumpyfish's avatar

It reminded me of the sheer terror I felt when I got into my dad’s car to drive it and realized that the emergency/parking brake was an electronic switch, not a handle or pedal connected to a linkage that pulled brake calipers.

I tend to like mechanical backups over some kind of electronic redundancy. =)

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