How would you go about to sense if a door is locked or not?

Every night my wife makes me go downstairs to double check that all the doors are locked and the lights are off. I’ve been toying with the idea of building something to check these things with electronic sensors and wireless transmitters and receivers.

Checking that the lights are off and doors are closed is simple enough with LDR’s and magnetic reed switches, but I’m looking for some ideas of how to sense if the doors are actually locked or not.

There are probably many ways to do this, but it would be interesting to hear what lateral thinking ideas other people may have. Presume each sensor will be powered by a small micoprocessor like mbuino, so anything is posssible.

Most doors have a narrow slit where wires or some or other kind of small sensor with thin wires can be sqeezed in but you generally do not have more than 1 or 2 mm free play.

How would you do it? Best suggestion gets all the browny points.

To test if a dead bolt is extended and seated in the receiver, put a micro switch in the receiver so that the bolt presses on it when fully extended.

You can instrument the cylinder or bolt in the door, but checking the receiver is probably the best way to be sure the door is closed AND the bolt extended. Other non-dead-bolt locking systems are so insecure, it probably doesn’t matter if they are locked. :wink:

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but I’m looking for some ideas of how to sense if the doors are actually locked or not.
It is ALWAYS better to do what the wife asks in the first place… But I guess you know that already.

My first thought was to turn each door knob and see if you can open the door…

But maybe sensing the dead bolt position may be the easiest. Fancy sensors are ok but they usually require extra power from the battery so I would use a mechanical or magnetic switch. If power is not a problem then a led indicator on the door frame could show a visible indication.
Hall effect sensors and a magnet work really well.

Not sure what this exactly means - Most doors have a narrow slit where wires or some or other kind of small sensor with thin wires can be squeezed in but you generally do not have more than 1 or 2 mm free play.

If it applies to running wires between the door and frame I would forget that. Doors swell and shrink and you may loose your connection.

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You could talk to one or more security experts / companies what they have to say / advice about such an undertaking …

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Wow, I did not expect replies so quickly. So far my own idea was to use two thin fiber optic cables to sense reflection agains the striker using infra red rx and tx LED’s. I would like the sensor to be battery powered, but it is probably not necessary to pulse the LED’s more than say once a minute.

I thought another idea that might work is using magnetometer to detect a change in the earths magnetic waves due to the lock striker proximity. Those things are quite sensitve to metal proximity and it might just work. I have one on a gadgeteer board and could do a bit of experimentation. The only disadvantage would be that I would probably have to fine tune the sensor on each door individually.

I would be surprized if there is not alreay some commercial solution for this available as I can well imagine a need for something like this.

I use a Z-Wave enabled lock. I can control it and see the lock status.remotely . I can also see the user codes being entered and enable\disable the codes based on day\time\or whatever.

Not a true DIY options, but it works. I also have a device for the garage door, as that was another entry that has bothered me and I always had to check it.

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@ KiwiSaner - You could use inductive sensing to detect the lock striker. You can get their eval kit and it will have everything you need.

I was watching this video and when it got to 1:24 I thought of your post.
Check out the door lock there, I dont know if that is real or a camera trick.

Watched the video - This is obviously something near and dear to my heart, but I think it will fall flat in its current incarnation. Here’s why: I don’t see any evidence of them using an array mic. Without an array mic, you can’t do beam-forming and multi-path correction that’s important for doing open-space speech recognition. I also don’t see any evidence of them using alternate audio paths to help with cancellation of media streams (important for recognizing speech over the background sound of tv’s and stereos). Amazon Alexa and XBox both use these techniques to get even passable performance at speech recognition. A basic mic will only work under very optimal conditions. (Try getting Cortana to work with a webcam mic)

To be honest, all the sensor-actuator, service discovery, and comms stuff is straightforward plumbing with 10 different ways to solve - no mysteries there, just engineering and workmanship. But speech reco in unconstrained environments and without close-talk mics is still terribly hard to pull off, and there doesn’t seem to be much recognition of that built into their project.

@ skeller - I like this! I was also thinking of inductance sensing but did not want to design it all from scratch. However this chip looks very interesting a probably a very good way to go. Thanks for the info.