You are twenty minutes down the road and the thought arrives: did I turn the bolt, or did I just pull the door shut behind me. Your phone can tell you that the door closed at 08:14. That is not the question. The question is what the lock looks like right now, and almost nothing in a normal house is able to answer it.
An event is a change at an instant. The door opened. Motion started. The bell rang. Events are what cheap sensors are good at, because a change is easy to measure and easy to send.
A state is a condition that is true right now. The door is unlocked. The gate is standing open. The keys are in the outside cylinder. States are what people actually lie awake about, and hardly anything reports them.
The usual workaround is to rebuild the state from the events: remember the last thing that happened and reason forwards. That works until one event goes missing, and then it is quietly wrong for hours. A dead battery, a dropped packet, a hub reboot, a door pushed shut by wind rather than by a person, and the little model in your app now believes something that is not true. It will not tell you it is unsure, because it does not know it is unsure.
A contact sensor cannot see a lock. It is two magnets that know whether the leaf is next to the frame. Shut with the bolt fully retracted looks exactly like shut with the bolt thrown. Both are "closed", all night, and only one of them is what you meant.
A vision model is reading a photograph rather than diffing two frames, so it can answer questions about how things stand rather than about what moved. On an ordinary inside view of a front door, that covers most of what people worry about:
AI Vision Monitor turns a phone you already own into the eye. There is no live feed and no recording. A frame is captured on your interval, a vision model answers the question you typed in your own words, and you get a sentence back describing what it saw.
Because the answer is a sentence rather than a bit, you can tell the difference between "the bolt is thrown and the chain is on" and "the door is shut but the bolt is back", from the car, without opening an app that shows you a picture you then have to interpret yourself.
| What actually happened | What a door sensor reports | What a described check reports |
|---|---|---|
| Door pulled shut, deadbolt never turned | Closed. Correct and useless. | The thumb turn is horizontal, so the door is on the latch only |
| Door not pushed home, resting on the latch | Usually still closed, magnets are close enough | The door is not flush with the frame, there is a gap on the hinge side |
| Keys left in the outside cylinder | Nothing. There is no sensor for this. | There is a key in the lock with a fob hanging from it |
| Sensor battery died in March | Silence, which reads exactly like everything is fine | Checks stop arriving, and a check that stops arriving is visible |
The mistake worth avoiding. "Tell me if the door is unlocked" is too loose, because the model has to guess what unlocked looks like on your particular lock. Describe the picture instead: which way the turn points when it is locked, what the chain looks like when it is on. You are not writing a command, you are describing a photograph to someone who has never been in your hallway.
There is no subscription. You pay per check, and only for frames that are actually analysed.
A check is half a credit on the cheap model, and the starter pack is 2000 credits for 2.99 euro. Two checks a day, one after you leave and one at bedtime, is one credit a day: about fifty cents a year. An hourly check across an eight hour night is four credits, well under one cent a night. Even a check every 30 minutes around the clock is 24 credits a day, roughly one euro a month.
New accounts get 50 free credits with no card required, so you can find out whether the model reads your particular lock before spending anything. Push notifications are free, email notifications cost one credit each, and every camera has a daily credit fuse so a mistyped interval cannot run away with your balance.
Why does my door sensor not tell me whether the door is locked?
Because it is two magnets measuring whether the leaf is next to the frame. Shut with the bolt back and shut with the bolt thrown are the same reading. Closed is not locked, and no amount of automation on top of that sensor can recover the difference.
Can a camera tell if a deadbolt is locked?
If the lock is in frame and its state shows from the inside, yes. A thumb turn pointing one way and the other way are two different pictures, and a vision model can say which one it sees. A lock whose state is invisible from where the phone stands is out of scope.
What is the difference between an event and a state?
An event is a change at an instant, a state is a condition that is true now. Sensors report events because events are cheap to measure. People worry about states. Rebuilding a state from stored events works until one event goes missing, and then it is confidently wrong.
How often should it check?
Less often than you think. One check after you leave, one at bedtime, or hourly overnight. A bolt that has not moved since eight in the evening does not need looking at every ten seconds.
What does it cost?
Half a credit per check. Two checks a day is about fifty cents a year. Every 30 minutes around the clock is about one euro a month. Fifty free credits to start, no card, no subscription.
Does it lock the door for me?
No. It tells you, or it triggers something else that can act. Knowing beats not knowing, but it is still only knowing.
Stand a spare phone where it can see your front door lock, type one sentence describing what locked looks like, and set it to check ten minutes after you normally leave. Most days you will never hear from it, and the day you do is the day it was worth setting up.
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