Simple Circuit Reminds You To Lock The Door As You Rush Out Of The House


It seems that [pppd] is always rushing out of his apartment to catch the bus, and he finds himself frequently questioning whether or not he remembered to lock the door. He often doubles back to check, and while he has never actually forgotten to lock the door, he would rather not deal with the worry.

Since he finally had some free time on his hands, he decided to put together a simple device that would help end his worry once and for all. Using an ATtiny13, [pppd] designed a circuit that would detect when his door has been unlocked and opened, beeping every few seconds until the lock is reengaged. The circuit relies on a reed switch installed inside the door frame, which is tripped by the magnet he glued to his door’s deadbolt.

He says that the system works well so far, though he does have a few improvements in mind already.

24 thoughts on “Simple Circuit Reminds You To Lock The Door As You Rush Out Of The House

    1. Yes, because if I did not turn it off, it drops off in 2 hours on it’s own.

      Next one I get will be off the second it is done brewing when I get a stainless insulate carafe maker so the coffee is not ruined by sitting on heat.

      1. We have a maker that uses an insulated pot and eventho I am not a coffee drinker I can even tell it tastes better. The other advantage is even if it does cool off it tastes better warmed up in the microwave because it has not been cooking for hours.

        As for the original post, I think it is great. My sister suffers from anxiety and some thing like this would make her feel better.

    1. I’ve done this before. Luckily, though, my dog is a major bossy boots. If you don’t open the door within like 15 seconds of him knocking to come in he’ll back up 20 feet and bark in the direction of the upstairs window.

  1. how about something that you can connect to over the web (twitter maybe?) and _ask_ if the door was closed. Say when you’re standing at the bus stop. Obviously you’ll want to make sure it’s secure.

    1. That would be awesome to have something like this connected to the web. I’m sure there are quite a few people that would much prefer to just ask if your door is locked before they come over to rip off all your stuff.

    1. Yeah…then you go to the curb to check the mail, come back and realize you locked your keys in the house…or rather, your lock locked the keys in the house.

      I am glad he made this…not so much because I wanted instructions, but because I thought I was just getting paranoid.

      1. I dont know why this couldn’t be resolved with a piece of elastic that pulls the bolt to the lock position. However not hard enough that you cant use your key to open the door.

    2. I don’t know of any auto-locking deadbolts readily available here in the US, at least not at my Home Depot. Of course, I have no idea what they’ve got in Poland.

      But the important thing I think is he’s in an apartment, so if he were to change out the locks he’d have to do it at night when God and the landlord were sleeping.

      I’d like something like this for the garage door so I don’t drive off with it open — or have it re-open itself because a leaf blew by the optical sensor while it was closed. But that would take a heck of a buzzer — maybe a siren really, and I think the neighbors would probably hate me.

    3. If you can find a low-cost sensor that detects the flow of water in plumbing, you could make another circuit and attach the water-flow sensor to the bathroom sink using the lock-detect input. Connect the door circuit as is to the bathroom door. Change the programming a little and you’ve got yourself a “Didn’t Wash Hands” buzzer!

      Actually — with appropriate firmware — this little device could be useful (or irritating) in a number of domestic applications.

  2. I’m constantly checking the door too (even though I have an auto-locking door, I prefer to also engage the deadbolt), this is an awesome project.

    You could replicate this functionality with a simpler circuit though. Afaict, it beeps if the door is unlocked but closed, and not if the door is open or closed and locked.

    If you replace the door-open detector with a normally-open switch, and the door-locked detector with a normally-closed, then place these between a buzzer circuit and it’s powersupply, you would get the exact same functionality, but at 0 standby current. It also fixes the lock-then-unlock scenario he describes in the post.

    1. Read more carefully Jochem. The switch detects if the door is locked. It cannot be locked when it is open, so yes, it will beep when the door is open. A switch to monitor the door being open or closed is not needed.
      Besides, he can just look at the door and tell if it is open, you can’t look at the outside of a dead bolt and tell if it is locked.

      1. The normally-open switch I described to replace the door-closed sensor would cause the entire system to be turned off while the door is open.

        Something like this, sw1 being the door-open detector, sw2 being the lock detector.

        +5v –/[sw1, no]–/[sw2, nc]–[buzzer]– GND

        This way, power is only applied to the buzzer if the door is closed _and_ it’s unlocked.

    2. Thanks for the suggestions, I might do that in the next iteration. I had only normally open switches and that was just the easiest way to implement it for me. The buzzer beeps shortly when you open the door so you know if the battery is okay or not and then starts beeping again once the door is closed back. No beeping when the door is open, that would make both me and mailman angry :P

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