Time delay door alarm

Ever accidentally left your front door ajar and had a pet escape? [BlackCow] came up with a simple solution to this problem. The circuit is fairly rudimentary but a great example of using the basics to get the job done. Now, instead of having an alarm that sounds as soon as the door is open, he has a 30 second delay. This helps avoid the “boy who cried wolf” effect also known as the “vista security warning” effect of being bothered too many times for a non issue.  We also have to say that we like his taste in blog layouts.

[via Hacked Gadgets]


  1. sM10sM20 says:

    “also known as the vista security warning”

    Proud Vista user but I got a good chuckle out of that, thank you.

  2. me says:

    Shouldn’t that be a PNP transistor?

  3. me says:

    Uh… I mean’t shouldn’t the emitter of the NPN be connected to ground?

  4. macegr says:

    “We also have to say that we like his taste in blog layouts.” You mean a fairly vanilla WordPress blog? ZING! :)

  5. cyrozap says:

    “We also have to say that we like his taste in blog layouts.”

    You mean his Blogger blog that has a coloring scheme similar to HaD’s WordPress theme? I get the joke, but it’s a bit of a stretch.

  6. MacroHard says:

    Nice hack. Need to build me one of these. Only in my case it’s not the cat leaving the front door open, but the kids.

  7. pretorious says:

    Also good for unwanted solicitors. if they don’t get the hint within 30 seconds;

    “öops, gotta go, ovens done”

  8. D- says:

    I guess I need that. The rear door of my place will mysterious will open. Why I don’t know. In addition to loosing expensive heat or coolth. There’s a possibility of small wild of wildlife coming inside to visit.

  9. Daniel says:

    Simple solution?

    With 555s instead of a simple RC time delay circuit?

  10. googfan says:

    That transistor doesn’t look right…

  11. Patrick says:

    I just hired a band of ninjas to stand guard at my door 24/7. They’re non-union, so it was actually pretty cheap.

  12. pwrx says:

    I actually can’t stand the HAD color scheme. Sure I know we used to have nice monochrome monitors, but nowadays in the GUI world when I switch away from HAD I’m temporarily blinded for a few seconds to adjust to the normal black on white scheme.

  13. marty says:

    In Reply to below… 555’s cost a few pence each and have no other use than timing… why waste a relatively expensive and versatile arduino on a simple alarm……

    “No Arduino used for that?

    Posted at 1:51 am on Mar 8th, 2010 by liebesiche”

  14. liebesiche says:

    I fully agree with your statement. It was meant to be a joke (http://en.wikipedia.org/wiki/Joke)

  15. BlackCow says:

    Haha, going though the posts I was thinking to myself, “gee, it sure would be cool to have one of my projects on this site” and what do you know, there was my project. Thanks for making my day ^_^

    And yes, my blog layout was inspired by hackaday… and my classic green on black terminal fetish.

  16. Shaz says:

    Great Project! I plan on making two for an industrial fridge and freezer to remind employees to close the door. How loud is it? And is there any way to make it wait 1 min. before going off?

    thanks in advance for your answers

  17. Stephen G says:

    There’s a market for this. Someone build one for sale, please!

  18. Stephen G says:
  19. BlackCow says:

    It’s much cheaper to build your own XD

    But seriously, how can someone patent such an idea as simple as a time delay alarm?

  20. Amy says:

    If I could build my own I would, for me it’s easier just to buy one :-)

  21. V says:

    Could you make it much smaller?
    Thinking of an anti pickpocket device, you pin a magnet into your pocket and have a 2nd that you use to disable the alarm.

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