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Electrical DIY laser trip wire / counter / timer

  1. Dec 13, 2011 #1
    Hi people, please excuse me if this is the wrong forum to ask, I am new. I want to build a cheap DIY trip wire that records how many times it is set off. If I could use some sort of laser to be more discrete that would be a bonus (and just cooler). Also if it is possible to record the time that someone triggers it that would be awesome.

    The reason for this is I have a feeling my landlord comes into my house while I am away and I want to know for sure if she is because thats totally NOT in my contract. It would be best if it records the time so I could bust her sneaky @$$.

    I feel like a laser trip across the doorway, or some kind of simple circuit that registers when the door opens / closes would be best because a simple trip wire could slip out and fail by accident (not from her) where as the others would kinda show an entrance / exit pattern so I would know it was more likely a person.

    My last suggestion is that possibly I could attach an obnoxiously loud alarm to the system so the people who live upstairs would hear her and she would be totally busted with witnesses. But it has to still count ( and hopefully record time) and she can't be able to tamper with the counter.

    haha, anyways. I would love to hear suggestion or be directed to some kind of tutorial on this matter!
  2. jcsd
  3. Dec 13, 2011 #2


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    Welcome to Physics Forums!

    Imagine you are in a court of law and want to bust your landlady for illegal trespass. What evidence could you show the judge that she is guilty?

    Cops use videos for all kinds of convictions for crimes, so why not you also? Seems to me a hidden video camera (with time/date stamp) switched on by a motion sensor would show clear evidence of her trespass...enough to convict her.
    Last edited: Dec 13, 2011
  4. Dec 13, 2011 #3
    Or electronics aside, hide the camera and record while you are not home. It is your home, you have every right to record everything. It doesn't have to be mega quality.

    Cost of components for the circuit you want, as well as time and complexity overcomes the time and money you need to buy a cheap camera.
  5. Dec 13, 2011 #4
    I'm curious about this too. I've always wanted to build something like what the original post described to monitor activity in areas, so can anyone explain how this is done? My knowledge on lasers is pretty limited. Specifically, I'd like to know when my neighbor downstairs enters/exits their apartment room so I know when I can play my drums or loud music without disturbing them.

    But I have a feeling for your original problem that you could design a more passive detector though. Simply a reed switch or there's these bead switches that make contact when tilted that you could use to see if your door has been opened and how many times.
  6. Dec 13, 2011 #5
    I am a student so I really don't have money to spend on a new camera, I was hoping for some spare parts at radioshack. But what I did find is this video about a program "dorgem"


    I have an old HP laptop that I might leave on for the three weeks of christmas break that I wont be at my house. Providing it doesn't light on fire and burn down the house (which 3 weeks on without sleeping Im not sure haha....) But I could just point it at the door and leave it on a non flammable surface and it might provide what I need!

    Also, I leave in 2 days so I kinda left this very last minute, but I didnt get suspicious untill yesterday
  7. Dec 13, 2011 #6


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    I would hook something to a port on a PC and program something up to read the port and log the time and date.
  8. Dec 13, 2011 #7


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    It is really too late to make up anything electronic.

    If you have a wooden floor, sprinkle some flour in a thin layer as you leave. Then look for footprints when you get back.

    There is a good trick for triggering an alarm.
    You get a wooden clothes peg and mount two thumb tacks with their heads facing each other and touching if the peg is closed.
    Then you connect wires to the thumbtacks and place an insulator in the gap between them.
    Now connect the trip line to the insulator. If someone trips on the trip line the insulator pulls out and the alarm sounds.

    The owner is probably worried that you will trash her property or steal stuff before you leave, so you can understand it if she wants to protect her investment.
  9. Dec 13, 2011 #8


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    There is software you can run on a laptop with webcam. It detects motion and starts recording. Maybe that's what your youtube clip shows? (I didn't click on it.)
  10. Dec 13, 2011 #9


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    It's too late for this, but after you return maybe try it.

    To identify a trespasser, you could paint iodine solution on an internal door knob and leave the door closed. When the nosy parker opens the door with bare hands, they will be left with a harmless but disgusting-looking brown stain on their fingers (and clothes) for the next few days. Get iodine in alcohol solution from the local drug store for painting on cuts and abrasions. You might need to mix in a tiny bit of honey so when the solution dries it leaves a sticky and moist film on the door knob.

    Expect to have iodine left on items around your house as the trespasser scouts around and touches more things. :(

    It won't prove anything in court, but would give you peace of mind. You could leave an interesting-looking envelope open nearby, on which the person might obligingly leave their iodine-stained fingerprints. :)
  11. Dec 13, 2011 #10
    Yeah she is totally paranoid I donno why, I am quiet and pay my rent on time haha, its a **** house from early 1900s anyways.... I like the iodine idea, but I dont want it all over my stuff really, and the flour is a pretty cool, super simple idea, I have laminate floor so that would work.

    If all else fails I think I may just get one of those springs with a boxing glove on the end and crank it way up so if she comes in she'll just get knocked right out. If she has a black eye or is missing some teeth in January I would have a pretty good idea where it came from :D
  12. Dec 14, 2011 #11
    Sorry, I have strayed from anything remotely electrically inclined, but its down to two designs:

    I ruled out the computer because I'm honestly concerned that it will burn the house down. And I already have a bug problem so I don't really want to sprinkle anything edible on the floor.

    #1: trip line that spans doorway at knee level using dental floss. one end attached to the wall securely, the other end attached to a cardboard tray, with coins organized in a certain way that she could never guess, possibly with a note on it saying "try to put them back!" haha so she knows she is caught and humiliated because she can't beat such a simple design.

    #2. Rig up coat hangers attached to the inside doorknob such that when the door opens it knocks a container of coins onto the floor. However, there will already be a known ammount of coins on the floor. Its designed to look like an accident (no floss connecting anything) maybe put a jacket on the floor to make the random location of coat hangers more believable.
    The reason for doing this one would be she would just put all the coins back in the jar, and I would see the difference. And she would pretend it never happened and I would put her on the spot when I tell her.

    what to you guys think?

    #2 is harder to set up (have to set it up after I leave through the door) and more likely to fail if a coat hanger slips (especially because I cant see the final set up), and may be hard to actually look convincing / she might see the coat hanger and stop right when she begins to enter.

    #1 is fail proof, but she has time to prepare a legitimate excuse for entering

    #2.5 lean something against door as I leave (yoga mat) that when the door is opened it will fall on a ruler that will catapult the coin jar... less chance of failure and still might be able to pass off as an accident so I get the element of surprise haha.

    opinions please!!
    Last edited: Dec 14, 2011
  13. Dec 15, 2011 #12
    Well, if the design is not EE related anymore, I would try posting this in the general forum.
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