This Idea starts with a modified version of an ordinary deadbolt, which has a keyhole on the outside and a handle on the inside. It also has a little mechanical switch on the inside, and an internal gearing arrangement.
Suppose that from the outside, you turn the key clockwise to move the deadbolt from the locked to the unlocked position. This switch lets you decide whether or not it will be clockwise or anti-clockwise, that moves the deadbolt from the locked to the unlocked position. (In practice, you will probably need to move the deadbolt to the half-way-extended position, before being able to flip this switch.)
Now imagine 10 such deadbolts along the edge of the door. All take the same key, but some you set the deadbolt switches for clockwise locking, and others you set for anti-clockwise locking. The door has to have a security plate (see link) that covers the whole edge, so a thief cannot see which deadbolts are latched.
So, after setting the deadbolt switches, you go outside and lock, say, 5 of the deadbolts. If the burglar has a copy of your key, but doesn't know the code, then efforts to unlock the correct deadbolts will lead to locking other deadbolts. Eventually, the thief will give up. No burglar ever wants to be seen taking a long time to open a door.
After the thief has given up and gone away, and you return home, you might think at first you have a problem, because now the code of which deadbolts to unlock has changed.
But, the REAL code is the pattern of 10 switch-settings! You know which way to turn each of the 10 deadbolts to move them to the unlocked position. So you unlock the door, go inside, and you can now change the pattern using the deadbolt switches.-- Vernon, Mar 21 2014 Security plate http://www.thelocks...ca/images/donjo.JPGAs mentioned in the main text. This plate needs to be long enough to cover all the deadbolt latches, so the burglar can't see whether a particular deadbolt is locked or unlocked. [Vernon, Mar 21 2014] You could probably defeat this by simply pulling on the door as you turn the key in each lock. One deadbolt will bind on the door jamb, and when you find and successfully unlock it the door will move a tiny bit, and that deadbolt will resist being locked again (since it will now bind against the jamb). This is much the same way that tumbler locks are picked.-- ytk, Mar 21 2014 [ytk], yes, that is a good point. I suppose in the never-ending conflict between security makers and security breakers, the next step would be to have a sensor connected the door, to detect such a "pull", and to disable all the locks. Let the key rotate endlessly in any of them, with nothing happening. :)-- Vernon, Mar 21 2014 There's also the issue that for most deadbolts, lock and unlock feel and sound different.-- MechE, Mar 21 2014 So basically you turn your door into a combination lock with 2^10=1024 combinations?
I'm not convinced that's enough combinations to protect a house.-- Loris, Mar 21 2014 [MechE], in my experience that mostly has to do with how tight-fitting the deadbolt goes into the notch in the door-frame. We might prefer a loose fit here, relying on the door-handle-latch to be a tight fit.
[Loris], yes, it is not a huge number of combinations. However, trying each one takes time, and like I wrote, the thief doesn't want to spend a lot of time on unlocking the door. Note I assumed the thief had the help of a valid key -- most thieves don't have that. When you add the factor that you can easily change the combination, a thief that tries some now, and then some more later, and then some more later-still, could in theory be kept stymied, provided you actually do change the combination reasonably often (once a week?).-- Vernon, Mar 21 2014 Let it SMS you every time there's an error. If you accidentally make a mistake, you have to put up with the small annoyance of paying to be reminded of the fact by your phone; if not, maybe there's time enough to deal with the intrusion. (But then similar comments to the combination lock apply -- why not just get an alarm. Also simplicity is lost.)-- skoomphemph, Mar 21 2014 Maybe you could encode some kind of delay in between the trials in some fashion before another attempt can be made.-- RayfordSteele, Mar 21 2014 If this was an idea to use existing hardware in a clever new way to increase security or if it was a convienient or efficient way to provide two factor authentication, I might see the benefit. Unfortunately, this idea required a new reversible deadbolt design, and you end up with a system that is annoying to operate and is fairly vulnerable to being watched through a telescope. It may also be hard to ensure that there isn't some way to get a wire behind the security plate to feel out all the deadbolt positions.-- scad mientist, Mar 21 2014 This seems like a cumbersome way of making a combination lock with 1024 settings. It also solves a non-existent problem: very, very few burglaries are committed by picking locks, having copies of keys or whatever. It's generally faster and more reliable to gain entry another way.-- MaxwellBuchanan, Mar 21 2014 Are you-all forgetting this is the HalfBakery, where Ideas are sometimes only semi-sensible?-- Vernon, Mar 22 2014 Sometimes the point of increasing security isn't to deter break-ins but to cure paranoia. I think having ten switchable deadbolts does that quite well.
As [scad] points out though, the code can be stolen merely by watching the owner lock the door.
How about, instead of mounting the locks separately along the edge of the door, you mount them all in a stack behind the same keyhole. (Ten might be difficult, maybe three would suffice.) In this way, the code would be entered using one long-shafted key which has to be slid in and out to set each lock position. You would still need separate mortices to prevent feeling the solution.
In fact, why limit ourselves to two possible states for each lock? The locks could be continuously variable with only a certain key angle corresponding to an open lock. The key could sit only in the first mechanism, with the second mechanism engaged by a tab on the back of the first, and so on, so that entering the code would be like twisting a combination padlock, a series of specific angle alternating direction turns. This would probably baffle even those in possession of both the key and the code.-- mitxela, Mar 22 2014 I notice that nobody else has mentioned the obvious flaw in this plan, namely that it can be defeated by a 12-lb sledgehammer and thirty seconds without adult supervision. This makes it no more secure than any deadbolt.
I mention this not because I am a luddite moron who has missed the point (I ain't and I hasn't), but because the overwhelming majority of professional burglars do not bother picking locks. In locales where 'possession of burglar's tools' is a felony add-on, the basic definition of said usually reads 'a crowbar and a short-handled hammer in any sort of bag or sack'. The only people who bother with picking locks are locksmiths and people who have legally obtained a finely-crafted set of lock picks and taught themselves how to pick locks simply because they enjoy knowing that they know how to pick locks.-- Alterother, Mar 22 2014 I never found out how the visitors next door got in. They must've broken in very quietly, because there was no time to run or phone. I wonder if our professional robbers now pick locks instead of the old fashioned lo-tech brick through a window, then hide in the vicinity to see if anyone noticed. One thing I do know is that waiting for people to leave the house is now very "old school". When the very latest in cutting edge crime technique reaches you (and it's only a matter of time, I'm afraid), you'll find that the true pro waits till you're *at* home, not away from it.-- skoomphemph, Mar 22 2014 I may have mentioned in the past that I dislike the attitude of "don't bother, burglars don't pick locks."
For the same reason that some people enjoy learning to pick locks, others enjoy trying to make them unpickable.-- mitxela, Mar 22 2014 Good point. I'm always forgetting to check my pragmatism at the door when I come here.-- Alterother, Mar 22 2014 random, halfbakery