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I have some needs

  • I'd like to ask my Google Home if my front door is locked.
  • I don't want my Google Home to be able to unlock my front door.
  • I don't even want my front door to be unlockable remotely.
  • IoT and unlocking my door should not mix.

Does a read only deadbolt exist, or are all smart locks motor equipped to allow them to be opened remotely?

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    Interesting question. Though I don't know if such a thing exists, it should be relatively simple to custom design something that would detect the presence of any old deadbolt either through a direct electric current flowing through it or by a metal detection in the area where the deadbolt would sit when it's locked. – anonymous2 Dec 4 '17 at 19:17
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  • If there's no automation of the locking mechanism desired, the device you seek is probably not a "smart lock" but rather a home security sensor. – Chris Stratton Dec 6 '17 at 3:56
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Unfortunately the usual smart locks on the market are all—to my knowledge—opening your door as well. As Aurora pointed out a lot of those are currently not allowing voice activated unlocking via Google Home or Amazon Alexa. Not every lock is that smart though.

However, there's good news too. Read-only deadbolts are nothing new. Every security system has those and you can buy even the parts quite easily, e.g. here. The tricky part will be to integrate it with Google Home. I guess in some way you'll need to use either a quite expensive security solution and make sure it has a proper API or build it yourself. If you build it yourself I'd suggest something along the lines of what I linked. You can decide if you use some sort wired solution (e.g. 1-Wire to a Raspberry for example. The wireless solution would probably along the lines of a small WiFi enabled controller (maybe this one).

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  • Perhaps worth noting also that many smart locks on the market don't allow unlocking via Alexa/Google Home (satisfying the first two bullet points). This is to avoid embarrassing incidents like this where intruders were able to unlock the door just by shouting a command to Siri, even though they were on the outside. – Aurora0001 Dec 5 '17 at 15:18
  • @Aurora0001 Somewhat amusing, but I'm more concerned about remote unlocking to be honest. Any fool near my house can get in by smashing a window -- If someone were to find they could unlock my door remotely with port scanning or a back door into my system (say I accidentally installed some weird software), not only can they get into my house, but I become an intriguing target because they can get in without to much fuss (look like they belong) and they know I am affluent enough to do things like spend $200 on a door lock. – Sidney Dec 5 '17 at 15:32
  • @Helmar Hey, that looks great! I hadn't thought to look into the enterprise security sector. Of course they are enterprise prices, but it gives me a start. Thank you! – Sidney Dec 5 '17 at 15:52
  • @Sidney maybe there are some more consumer-priced options out there. Admittedly I hadn't had the time to properly scout for those. – Helmar Dec 5 '17 at 15:55
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    @Helmar No problem, you pointed me in the right direction. If I find something I'll post back here for future users. :) – Sidney Dec 5 '17 at 15:57
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Building on Helmar's answer I would suggest a DIY solution: a miniaturized reed switch and a magnetized / magnet attached deadbolt.

Check if there is place for a miniaturized reed switch within the strike plate with the deadbolt fully extended. Those switches come in packages as small as 2 mm diamater and about 10..15 mm long. Since they actually come with a glass body you need to make sure the deadbolt does not actually pushes the switch. Depending on the design of the strike plate and door frame it might be possible to increase the available space (not compromising the mechanical strength of the strike plate though).

Check if the deadbolt can be magnetized, i.e. following the esoteric procedures as outlined here "Rubbing the Metal with a Strong Magnet". Test if the deadbolt actually actuates the switch if properly aligned. Alternative if magnetizing fails: superglue a magnet to the deadbolt's tip.

Fixate the reed switch someplace inside the strike plate and run two wires to an "IoT"-like microcontroller, e.g. an ESP8266 or similar.

Total cost < 5 $, but might well take an afternoon or two to build.

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  • Hey! Not bad! I might go with this (my search for inexpensive detection methods has yield little to no results). For your suggestion for a magnet, something that comes to mind would be to use a cnc or similar method to create a small groove (not enough to compromise the security of the deadbolt) at the end of the deadbolt to hold a magnet with superglue if there is not enough room for a to be attached directly to the end of the deadbolt. – Sidney Dec 5 '17 at 18:15
  • Would this z-wave "Dry Contact Sensor" be suitable for connecting a hypothetical reed switch to? – Sidney Dec 5 '17 at 18:28
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So after reading Helmar and Ghanima's excellent answers, I doubled down and did some research. After a little bit of work and research I came up with the following solution. Images are available here, in an Imgur album.

  1. Purchase a flush mount reed switch and a z-wave dry contact sensor from Amazon

  2. Drill a hole in the back of the deadbolt cavity to place the flush mount reed sensor

  3. Pull the wires through the side of the door casing

  4. Drill a small cavity in the deadbolt itself and place a magnet in it (I used a bucky ball)

  5. Wire the reed switch to the dry contact sensor

  6. Bang the door casing back together

  7. Use packing tape or glue to secure magnet in deadbolt.

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  • Great to see you got it working :) – Helmar Dec 18 '17 at 22:23
  • Wow! Thanks for sharing, glad the idea worked out that well. – Ghanima Jun 26 '18 at 20:31

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