Hey guys, I haven't been here for awhile, but this forum is always the best source for original and unconventional home and shop solutions, metalworking-related or not!
I need to replace the flimsy pine front door of my condo unit. Instead of laying out like two thousand bucks for a proper prefab security door (I live in a very expensive country), I'm thinking of building one myself from good dense plywood, and installing a basic lock, but for fun and erudition, reinforcing it with a home built electromagnetic system, with electromagnets in the doorframe, iron plates embedded in the door, and a hidden magnetic reed switch in the frame. I'd hang small magnet on my keychain, and even if someone watched me go in, they wouldn't ever guess what I was doing!
I know from my high school electricity class that wound-for-the- purpose electromagnets can be super strong but still have minimal current drain. I want to power it with a with like a motorcycle battery which would be kept up with a trickle charge from the mains.
I'm thinking that there must be industrial uses for electromagnets suited to my purpose (high turn # and resistance) and that I might get some surplus or even new for cheap.
Can anyone suggest what kind of applications would use such magnets or where to get them? Some kind of solenoid? Even if I bought new, I bet they could be gotten cheaply if I could find the right source.
Note: For the sceptics who think I'm making work; I've been doing major renovations, so the tools are out, and I live in a country with a 19% national sales tax!
Magnetic doorlocks are off the shelf components, but not small enough to realistically make concealed. There are some odd ball specialty oens that can be concealed, but its a pretty involved installation. They range from
300-1200 lbs of holding force. They usually integrate with a controller and power supply that uses an alarm style gell battery for backup.
I'm note sure what you were hoping to do with the magnet on your keychain. Operate a magnetic reed switch to deactivate your maglock? Perhaps you were thinking of a magnetically activated hidden mechanical dead bolt? Some high end jewlery cabinets are made this way, but realistically It isn't much of a deterrant.
I suppose if you could clarify the gain you hope to achieve I could point you towards some components that might work. So far everything you have mentioend is all off the shelf stuff from Securitron, or any number of other access control hardware manufacturers.
I work with this sort of stuff everyday. Its not super cheap, but expensive is relative. Some clients throw a hissy fit about paying for a service call and others think several thousand dollars for a camera is pretty cheap if it fits their specific needs and/or helps solve a problem.
The basic maglocks start around a couple hundred dollars by themselves. Add a controller, power supply, battery, unlocking / releasing mechanisms, labor, wire, etc.
Realistically, a metal security door with a hard screen and a deadbolt may have more real benefit for you. Its a visible deterrant as well as a physical barrier.
A high-grade of mechanical lock is worth looking into. So long as the door, windows, at al, and strong enough to defeat prybars or kicking with heavy boots.
Power door lock motors out of a car. 12 volt, designed for long life, high power for the size of the units. Direct pull units or cable pull are both easy to find in many makes. Cheap in a salvage yard or even new they are not really expensive.
As Joe stated a good door/window is ONLY as good as how it is installed. Of course the only thing a good lock stops is a casual thief or an honest person. ALL locks can be defeated with time. If they don't want to waste the time they can just cut through the walls! Hit the local box store or tool place, buy a cordless drill/saws-all kit.
What I'm talking about is way simpler than some of you presume. I simply want electromagnets in the doorframe which hold to metal plates in the door when on. No moving parts! The magnetic reed switch would be used to swich the circuit off. don't need super high security, but something a bit better than what I have now. Since they would be on whenever I'm out, I would want very low current drain. I've found some suitable units, but I would prefer to pay less.
Industrial fire doors have magnetic locks like you describe, that fail open. For a residence you might want one that fails locked, with an alternate means of entry.
This is simpler and cheaper, just a solenoid that opens a standard door and doesn't waste electricity or require large backup batteries. <
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The keying scheme is up to you, maybe a remote car starter or such. A magnet in your pocket could erase your ATM card.
--Sound better than another method I saw at a dorkbot.org meeting; the guy had surgically inserted an RFID chip into his hand so simply waving at his door would unlock it. Handy, he said, when he was too drunk to deal with keys!
Yeah, my design would would of course fail open. I found an example. A small rectangular 160 lb. magnet that draws .83 amps at 12 volts and is about the size of a guitar pickup. If I used 4 it would draw 39 watts. I bet there is something out there that gives even more force with less power, but this seems workable. If I had it on a continously charging 50 amp-hour battery it would still run for 15 hours if the power went out. They cost $64 each new, so if that's where I have to get them I'll probably just go with a traditional lock. If I can find some somewhere in a bulk bin for under 20 bucks, I think I'll go ahead with this idea!
The magnetic hold-backs for commerical fire doors are deliberately weak, meant to be easily overridden by a person with a pull. Because you arern't going to turn opff the smoke detector power to close the door, just pull it closed.
The magnetic door locks meant to keep people out of the room or to keep a vehicle gate closed are a LOT stronger than that. With the much larger power draw that goes with it.
You can use magnetic strikes with a backup battery so they stay locked a while when the power fails - then you are at the mercy of battery capacity.
But the electric release strike is the far more practical option. It is fail locked, but you always have the regular doorknob for emergency access or egress.
And you can rig your proximity switch to an electric release strike so you walk up to the door and it's magically unlocked as you push through - but I'd still strongly suggest some kind of a proximity card reader to trigger the door lock as you walk up to it, instead of a simple hidden reed switch. Too easy for a bad guy to figure it out.
And like JSW says, waving around strong magnets at random is not a good thing. Most proximity card systems are non magnetic, or use tiny ones that won't cause data erasures.
No its not. An electromagnet with enough holding force will be a couple hundred bucks. More for a hidden spring bolt design one. Like I said I do this. Making your own mag-lock is doable, but will take a great deal of time. Unless you have a good source for fine wire, and the ability to wind it perfectly it will be extremely bulky.
I know what you want. Like I said I deal with this kind of stuff everyday. For one that works properly there will not be "no moving parts." A concealed one will draw locking bolts into place with the mag force, and a surface mount one will have a spring loaded plate on a swivel mount so it will engage fully each time.
That's what I thought, but I asked to be clear. I don't not think it would hold up unless you used it to operate a relay.
Shop ebay then. You may find some guys may be clearing out old inventory eventually.
Mag door holders are totally inappropriate. Way to weak.
Yup!
"Relatively higher." The expanding field takes a fair amount of current, but much less to sustain it. Also, the collapsing field will create a surge back which can cause an arc welding contacts closed or carbon score them preventing contact. We protect relay contacts with a diode across the contact to prevent this. Most magnetic contacts (as the OP suggests) will not handle it at all.
Yeah, but its basically just as weak as a regular door strike. In many cases weaker because you have to remove so much material to install it. They are ok for low to medium security in steel framed steel doors, but otherwise they are strictly for traffic control. It does not nothing to protect it from kick in, or from picking or jimmying the lock. Also many do not even properly engage the secondary bolt on the lever or knob set which effectively reduces the security of the door directly.
Its pretty long, and in all high security applications we use multiple locking mechanisms, and monitor conditions and status of all devices with an alarm system, but with the OP's emphasis on cheap I doubt this is likely for his application. Batteries can last quite a while, but again with the emphasis on cheap I suspect in 3-5 years he won't have much battery capacity left.
The term is fail secure. A lot of strikes are configurable as fail safe or fail secure. Its easier to install, but its not much cheaper than a mag-lock. The only real advantage is that it does not require a lot of extra equipment for a proper installation.
Remember the OP's emphasis on cheap? With a reader you also need a controller. Although most controllers will incorporate a power supply and backup battery.
A "professional" bad guy will defeat anything. The key is to slow him down and make sure he can not get inside information about the configuration. A tweaker will either bust in or go elsewhere.
It does not take a super strong magnet to operate a magnetic reed switch.
Most modern proximity cards use a coil that is energized by the electromagnetic field of the reader, and then it transmits its code back to the reader. (over generalization) No magnet at all in the card, but the reader continuously generates an EM field. I have several applications where users hold their purse or wallet up to the reader. I have not heard of any particularly problem from any of them with erasing their credit cards. The EM field is pretty weak. Also, the drop in field strength with distance to the reader is exponential. Same with mag locks by the way. A
1200lb hold force lock at the very top of a door will not have enough lines of force in a downward direction to disturb media more than a foot or two below it. I have a tech paper around somewhere on this from Securitron. Its been pretty well because media is one of the prime things covered in "secured" areas.
There are still some mag stripe cards around as well. Things like credit card for instance. If you take a magnet and put it in your wallet you will erase them, but that is the extreme case.
I supervised construction of nav-aid shelters and the client was in love with those to the point where they never changed the factory set combination since all the techs found it so easy to remember. Problem was that the client insisted on adjusting the door weather seal so tight that when they tried to open the door, they would shear off the shear pin which resulted in a $250.00 invoice from a locksmith. Gerry :-)} London, Canada
Uncle Sam used Locknetics {?sp} or similar mag locks for the ballistic-resistant doors I knew too well. They weighed 400-600# and were a mite hard to push open. They have manual dogs to back up the maglock.
There's a backup to that backup; a Marine with sidearm and 870 shotgun...
How about relying upon self to take care of self, as opposed to a cheap and effective solution identified to you by others -- and expressing disappointment when that didn't happen.
Holding a door shut with an electromagnet rather than a simple bolt seems absurd to me, but nevermind that. If you wanna electromagnet and can't find one, then find a source of surplus magnet wire and wind one.
It's not absurd at all. Previously, there were electromagnetic deadbolts in the top header, but they then needed a reed switch to keep the bolt retracted until the door was closed. Further the door could not lock if the door was even slightly warped. Worse; if the deadbolt failed closed; the door was truely locked and YBF'ed. You needed the door open to access the lock....
The maglocks were fully troublefree. The Von Duprin panic bars on the doors, however.... Designed for normal weight doors, they'd last only a few months in heavy traffic areas.
I can see how an EM door-holder could make sense if remote control and/or fire safety are issues. In the case of fire safety, fail-safe would be fail-open. Lousy security, but security isn't always the highest priority.
If the issue is simply and only portal security, bolts have been in use for centuries and still can be at least as strong as the portal.
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