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I am wondering how is it possible that there's no popular solution yet, for letting you know if you have forgotten to lock your exterior door.

I am not talking about a door left open - I am talking about door being left closed, yet unlocked.

I am often in a hurry and I am forgetful. This is why I need some sort of alarm or at least an indicator, about the door being left unlocked.

(Also, I do not any specific locking mechanism in mind, I am asking in general.)

Is there any simple low-tech solution for this? (High tech solutions are also welcome, though.)

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    You could improve the question by explaining how your door works. These things seem to be different over the world - for example in Germany outside doors usually always require a key to be opened from outside, so are "soft locked" by default; but you can use the key from the outside to "lock them even more" if you so wish. For such a door, your question would make little sense, so you likely have a different model. Which is it?
    – AnoE
    Dec 5, 2022 at 9:15
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    Sure, @TooTea, the focus of my comment is for OP to improve his question, not on the security of my personal door. ;)
    – AnoE
    Dec 5, 2022 at 11:55
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    @AnoE Sure, but I read your comment as claiming that the "question would make little sense" for the typical German door, which is IMHO not true (and could mislead people into thinking they can rely on that "soft locking", which is at best only marginally better than leaving the door ajar).
    – TooTea
    Dec 5, 2022 at 12:01
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    @TooTea, I guess we're going slightly off-topic now, but IMO every door/lock that is not specifically designed (and paid for) to be high security is little better than leaving it slightly ajar against a determined burglar, IMO. I personally treat locked doors as valid against snooping neighbours or misguided but harmless adolescents, at best. For everything else, the LockPingingLawyer wants to have a word. OP did not ask about the security of a locked door...
    – AnoE
    Dec 5, 2022 at 12:29
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    @dandavis Why isn't the answer to "Simple solution to let you know if you have not locked your closed door" simply "try opening it once outside" not sufficient? Dec 6, 2022 at 11:12

11 Answers 11

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OK, so you have a keyed entry type doorknob on the door. It has a slot for a key on one side, and a "push or twist" thingie on the other side.

You step outside and close the door, and now you don't know if it's locked, yes?

You didn't say whether the door also has a deadbolt, a common feature on many doors.

Use good practices. Always lock from the outside.

In my world, that's easy since all our doors have both a keyed-entry lock and a deadbolt (keyed the same). Our rule is we only use the deadbolt. We never lock the keyed-entry lock.

So if we step outside and close the door, is the house locked? Easy and sure answer: NO, because you didn't lock it!

So turn around and lock it and now you know it's locked.

Also, when this becomes habit/discipline, it becomes almost impossible to lock yourself out of the house lol.

If your house doesn't have a deadbolt it can probably be added. Any competent locksmith can key it to your existing house key. A few locksmiths have an actual retail store you can walk into. That's where I prefer to do business. It's incredibly cheap since no service call is required. I get lock-sets re-keyed for $20-30 and pay $50-60 for a whole deadbolt set. (quality class 2 stuff, not that rubbish from the box store).

You can also do the same trick with pure discipline. Simply stop doing that thing where you twist the inside knob as you step out. Never do that (except to lock the door while you are inside). Always lock it with the key.

Or, you can do the same trick with the twist-to-set keyed entry locks, simply by using discipline - never twist the twisty thing on the way out the door, only lock it with a key from the outside.

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    Hey @harp. You and I agree on most things but in this case I believe you are over thinking this. In my answer below is a very reliable, low tech solution, not expensive. Your suggestion of changing behavior is unrealistic and prone to failure. Dec 5, 2022 at 9:06
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    Once you get old enough, this simply Does Not Work. You get outside and ask yourself, "Did I lock the door?" and the answer is, "I haven't the foggiest clue". So you go and lock it, go a few steps and ask yourself, "Did I lock the door?" and the answer is, once again, "I have no idea, was that today or yesterday?" Rinse and repeat until you give up and leave.
    – Martha
    Dec 5, 2022 at 16:20
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    @manassehkatz-Moving2Codidact: totally agree with you about switched outlets. Every single one of the so-called advantages he cites would be achieved by having both a ceiling lamp and a table/floor lamp, each one controlled by its own proper switch.)
    – Martha
    Dec 5, 2022 at 21:48
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    I specifically disable the twist-to-lock tab on all my exterior doors to avoid this exact problem. No accidentally leaving it unlocked or locking yourself out.
    – bta
    Dec 5, 2022 at 23:59
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    @manassehkatz Everyone should be trained on correctly interacting with police. Innocent people most of all, because that's where the biggest injustices happen. It's like Damage Control training in a Navy ship - you hope you never need it, but if you do, it saves your bacon. Dec 6, 2022 at 3:53
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The problem with high-tech solutions is cost and reliability. Simply not necessary except when it comes to high-value situations (bank vaults, server rooms, etc.)

As far as low-tech solutions, you lose the remote capability. So what most people do is simply try to open the door after locking it. If it doesn't open, you locked it correctly.

What I find is the complication is people who use double locks - i.e., one lock that is a deadbolt (or equivalent) and one that is a simple lock.

When it isn't my decision, I lock the deadbolt first, test the door (turn knob and push or pull the door - if it doesn't open, it is locked) and then lock the other lock and test (usually just seeing if the doorknob will turn or not).

When it is my decision, I remove/replace/disable the non-deadbolt lock. It adds very little security but at the risk of (a) thinking you locked the deadbolt (by locking both without checking the deadbolt separately) and (b) locking yourself out (depending on lock design).

A simple solution might be to have something similar to the "Occupied"/"Unoccupied" signs on single-stall public bathrooms. But those are typically simple slide mechanisms, which might not be easy to adapt to a secure (e.g., deadbolt) lock. Plus that would advertise to anyone walking by that the door is unlocked if you forget to lock it.

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    When smart locks(or similar) are mentioned, always think of a video where a guy has voice control lock and comes back home in the pouring rain after having a root canal.
    – crip659
    Dec 4, 2022 at 19:13
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    It doesn't take too long to train yourself to always test the door after you leave. Closing the door, locking it, and testing it becomes a single fluid operation that quickly feels weird if you skip a step or get interrupted. You get to where you can do the whole thing on autopilot. Muscle memory is a strange beast.
    – bta
    Dec 6, 2022 at 0:03
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    @bta Agreed. Works really well for home - close door, lock door, check door, go to car. But at my office there have been many times where I set alarm, close door, lock door, check door, listen for alarm exit sound...go down elevator, out front door of office building...and think: did I set the alarm? did I lock the door? Back up the elevator...lather, rinse, repeat. Dec 6, 2022 at 0:09
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Some of the electronic locks have a timed-exit feature, which relocks the door N minutes after opening it. Then you don't need to check; you can just trust that it will be locked. Downside is that it may lock on you when you just dithered a bit too long, but you do have your keys with you, or a keypad on that side of the door, right? (Little is more embarrassing than a locksmith locked out of his own home.)

Fancier solutions use door-position monitoring rather than or in addition to the timeout, but use the same electric bolt mechanism to actually lock up. That avoids having the lock extend the bolt when you've deliberately left the door open (which tends to result in a thud when you forget that has happened and go to close the door).

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  • I use the auto-lock feature on my August smart lock and it's great. Dec 6, 2022 at 13:46
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    I hate it but it's what the OP is after, +1.
    – Mazura
    Dec 6, 2022 at 15:47
  • The alternative is a spring latch with an electric release strike. Still requires a keypad or other secure mechanism to activate it, and harder to install.
    – keshlam
    Dec 6, 2022 at 19:16
  • Or, even more expensively, a strike with a sensor.
    – keshlam
    Dec 9, 2022 at 1:49
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Low or no tech solution would be simple behavioral training.

Each time you exit try to open the door like you forgot something.

High tech solution would be a doorbell camera.

Then you can see yourself locking or not the door.

Additionally if you forget to lock the door, it will alert you of unfriendly visitors.

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    ...or friendly visitors!
    – gerrit
    Dec 5, 2022 at 11:41
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    I would think high tech solution would be a lock with wireless connectivity that will actually tell you if it's locked or not. Dec 6, 2022 at 13:45
  • @MichaelMior but it is not so much fun
    – Traveler
    Dec 6, 2022 at 17:24
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You can get locks, or in some cases set locks which have multiple modes, that are always locked when the door closes. Then you'll need to remember your keys...which should be easy after two or three lockouts and whatever that costs you in terms of calling someone holding your extra key, or a locksmith, etc...

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  • This is the exact reason why I am researching smart locks - I have children so the theoretical construction falls apart quickly. "Researching" because I want to build my own, for all kinds of good and bad reasons.
    – WoJ
    Dec 5, 2022 at 11:09
  • This locksmith's solution -- though I have a smart lock too -- is to simply leave a spare key with a trusted neighbor who is usually home. If I lock myself out, I walk two blocks to pick it up and return it to them later. Besides, their kid is my usual cat-sitter. If you really don't have anyone you can trust, or find asking for help painfully embarassing, or are just a gadget freak, that's when smart locks are worth considering.
    – keshlam
    Dec 6, 2022 at 19:19
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My son's new apartment has Smart Locks. They have a number pad and you input your number. You can program other numbers as well and get a report on your phone as well as alerts when the code is used to unlock the door. You can lock and unlock the door from your phone as well. I believe there is also a function where the door locks it's self when closed for a period of time. I have no idea of the cost, but it sure is a cool system.

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    Many of these are unfortunately quite flawed in design - either they're physically/mechanically badly designed or the electronics is actually easily fooled / bypassed.
    – John U
    Dec 6, 2022 at 15:56
  • This is definitely not one where you want to go with the cheapest option. Personally I use the August smart lock and I've been pretty happy with it. Mechanically, there are no vulnerabilities introduced by the lock since it simply replaces the deadbolt tailpiece. Dec 6, 2022 at 17:51
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I have often thought about solutions for this as I have a few doors that could benefit from it, using different mechanisms.

My thoughts so far have been that it would be possible in most cases to modify the door/lock & mechanism slightly and add a little indicator (a bit like on public lavatory doors with the red/green ENGAGED flag) or if you're feeling flashy, a low-power blinking LED and small coin cell with a simple microswitch or reed switch & magnet to blink to remind you.

Exact mechanics would vary with the door & lock type / mechanism and your available resources in terms of tools, skills, materials.

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Simple hi-tech solution: a camera.

Buy a cheap home security camera and point it at the door lock from inside, so that you can clearly see which direction the deadbolt knob is currently pointing from the video feed. You can add a cheap telephoto lens meant to clip/suction onto cell-phones to extend placement options. You might want it to have an IR-emitting ring of LEDs on it to see in the dark, which is a common option, or enhance the visibility of deadbolt handle using colored tape for best results.

While it doesn't immediately give you a digital value you can feed into your overall automation setup, though ML or careful digital image processing could provide that, it will give you a way to check from anywhere to see if the door is locked.

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I guess it depends on your definition of "high tech". I personally don't find bluetooth/wifi/smartphones high tech anymore, since they are so common. So I'll tell you what we have:

We have a Schlage Z-Wave keypad entry. It works with google home, amazon alexa, and Samsung Smartthings. With smartthings, we can program it so that if both people in the home leave (based on wifi connectivity to our home wifi), then the lock will automatically lock if it's not already locked. You can also program it so it will lock at a certain time of day, which we have it do at night just in case someone forgets to lock it.

It's a unit like this (the schlage is selling for $470 now, up from $160, holy inflation batman!)

https://www.amazon.com/Connected-Technology-Featuring-SmartKey-Security/dp/B09HR85FVP/ref=sr_1_4?crid=IM8U30GJI7U8&keywords=deadbolt+zwave&qid=1670359071&s=hi&sprefix=deadbolt+zwave%2Ctools%2C115&sr=1-4

As far a locking, it couldn't be easier, you leave the house, and you press the big button. You only need the code to enter.

I'm not sure what you're looking for with a "lo-tech" solution. Are you wanting to light a light or make a sound?

Update: To unlock, the model we have came with 2 preprogrammed 4-digit codes. You can also program temporary codes through the app (e.g. you have a dog walker coming by). It also have a physical key to lock and/or unlock. With the SmartThings, it will also record when the door was locked and unlocked, and which code was used.

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  • Just curious - does this setup let you unlock the door without a physical key?
    – Razzle
    Dec 7, 2022 at 13:34
  • Yes, it's a 4 digit code. I've updated my answer with additional details.
    – LarryBud
    Dec 7, 2022 at 20:14
  • "I'm not sure what you're looking for with a "lo-tech" solution. Are you wanting to light a light or make a sound?" - I don't have any specific expectation, just open to any reasonable idea.
    – drabsv
    Jan 19, 2023 at 12:48
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I'd get one of the locksets that is always locked, then hide a key outside someplace in case you forget to take your keys with you.

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    I would never hide a key outside. It's only marginally better than leaving the house unlocked. You think you have a unique hiding place, but burglars know them all. The real problem though is that, at least in the UK, insurers won't pay out if there is no sign of forced entry. It just isn't worth the risk.
    – Ian Goldby
    Dec 5, 2022 at 9:37
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    Leave a key with a trusted neighbour/local friend instead of hiding it
    – James
    Dec 5, 2022 at 10:09
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    If you don't have someone you know enough to trust them with a copy of the key, i have resorted to leaving a copy in your car, if you have one. Admittedly not as safe, but it works.
    – bracco23
    Dec 5, 2022 at 13:54
  • That's called a storage room lockset. Dec 5, 2022 at 22:08
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    CLearly this issue has generated quite a few comments and a lot of the opinions here are probably based on geographic location. I live in a rural area surrounded by trees. It would pretty safe for me to put a nail in a tree in the woods and hang a key from it. In a more suburban setting, yeah, you'll have to get more creative. The days of putting a key under the welcome mat are long gone. Maybe taped with duct tape to the side of a foundation vent? Apartments would be the hardest to find a hiding place for a key outdoors. I look forward to other comments and ideas. Dec 5, 2022 at 22:25
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Fit a spring to do door, so the door will not remain closed unless locked. Removal anything from the door that will keep it closed other then the lock.

Another option is to have a very loud alarm that sounds if the door is left unlocked for more then one minute. (Needing to call out an expensive company to reset the alarm before it will stop sounding would improve this)

Both of these have in common that they increase the cost of not locking the door so much, that forgetting to lock it is no longer an option.

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  • Most doors I know of that have a lock also have a simple turn knob/latch that, even if it is not lockable, will hold the door in place. Plus there may be times where you don't want the door locked but do want it closed - e.g., expecting guests but it is rainy/windy outside. Dec 6, 2022 at 14:59
  • @manassehkatz-Moving2Codidact I said to remove such turn knobs etc, and the spec of the problem does not including allowing people to let themselfs in.
    – Walker
    Dec 6, 2022 at 15:02
  • Question did not specify. So I don't think your answer is "wrong" (I did not, and won't, downvote it) but it doesn't allow for what I (and I'm not OP, so that may not matter) consider the typical situation. Dec 6, 2022 at 15:07
  • Actually any alarm which notifies me on my phone is a working solution., no need to wake up the neighbours at 3 am, in case I am going out at that time. Are such alarms available and is it easy to install one? (Btw, it is useful when you are home, but have forgotten to lock the door, too).
    – drabsv
    Jan 19, 2023 at 12:51

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