Thwart Robbers With An Old Smoke Alarm.

[Anders] tipped us off about his hack that re-purposes a smoke alarm as a burglar alarm. Unfortunately, he came home in the middle of a burglary but wanted to be ready the next time someone tries to break in. By cleverly patching into the test button on an old smoke detector he created a circuit-trip alarm. One side of each piece of wire is secured to the frame and jam of a window. A paper clip completes the circuit by pinching the two bare conductors. If the window is opened the connection is lost and the alarm sounds.

We see a few problems with this system. First off, never hack your ONLY smoke detector, you are putting lives at risk by doing so. But [Anders] says he’ll have a replacement detector and since these things need to be replace every ten years or so, chances are you can find an old one kicking around. We’d recommend disguising the case so that people aren’t confused about it being a smoke detector. Secondly, he’s mounted the alarm right in the window frame so most likely an invader will just smash the thing to bits.

Anyway, it’s still an interesting reuse of these ubiquitous, and life-saving, devices.

50 thoughts on “Thwart Robbers With An Old Smoke Alarm.

  1. “most likely an invader will just smash the thing to bits”
    Or switch it off, since the switch is right there. It might scare off really really dumb intruders, but a decent alarm system would be a better deterrent.

  2. Er… Don’t smoke detectors contain radioactive material? I don’t think I would want to take the chance of breaking the component by accident.

    On another note, what makes a decent alarm system? Is making noise enough? What other deterrents? Bad smell? Spray paint? What if a computer calls the police would they come to your house?

    I won’t retain the suggestion of ‘smearing neurotoxin on the window frame’, that’s pretty extreme and very probably illegal.

  3. Philippe: Most smoke detectors use americium dioxide (AmO2) for detection and, yes, americium is radioactive. However, the quantity used is so miniscule that there should be no health risk; the source I checked reports that even ingestion shouldn’t be an issue at this scale: the dioxide form is insoluble. Pure Am, on the other hand, would be a bigger deal.

  4. We used to have a professional alarm system. However, the one time it was set off it took the police 3 hours to arrive.

    I think just having the alarm stickers on your window and maybe something like this that just makes noise would be as good as anything else.

  5. Today, most smoke detectors are the optical type without americum (at least here in europe, but I think this is also true for the US), because of the issues connected with disposing the ones with americum.

  6. I agree with Lewis on this. Most people don’t want to get involved when someone is in trouble (girls are taught to scream “FIRE” instead of “HELP” or “RAPE”). Even people who want to stay out of things will call 911 if they think a neighbor’s house is on fire. For that reason, maybe an alarm that is the classic fire alarm sound is the best choice after all!

  7. Would be great to protect my bike while im in the store getting noms. we have smoke alarms in every room, and when one goes off, they all do. Ive never seen smoke detectors that werent in my house go off, but it would make a nice house-wide alert.

  8. actually your cellphone is pretty good.

    you must have a better cell phone or you are using an active cellphone or you are getting good signal (swedish law may say deactivated or out of range cellphones must disable or degrade their camera in light of peeping toms).

    i have an old camera cell phone and the quality is less and it happens to be deactivated and i wondered if us law required deactivated or out of range cell phones have to degrade the image.

  9. optical alarms are not as true they take actual smoke to set them off.

    the radio active warnings are placed on all all stuff containing radioactive stuff.

    and the nrc is pretty picky about it including filing an nrc report if a 50 cent light bulb burns out on the power plant control panel and the public alerted.

  10. I’m with @Coligny on this one- this would be great for a little boy’s fort or a young girl’s diary, but otherside it’s pretty lame. For one thing, forget smashing the alarm, if the burglar is going to come through the window, chances are he’ll smash the glass and leave the frame shut to avoid setting off an alarm.

    I think the thing that disturbs me the most is he’s already been broken into once, and this is his solution? I’d be on the phone to an alarm company discussing their free installed system with a monitoring contract. Even if it does take the cops 3 hours to get there, they call me first, and I’ll be there in 5 or 10 minutes. And I’m not bound by the same shoot/don’t shoot rules as the cops. ;-)

  11. In my home, all the smoke detectors are tied together.

    When you hold the test button for a few seconds, that detectors horn goes off. Hold the test button for another second or two, and they all go off. Bedrooms (4), hallways (2), other rooms (2), and one or two more all have detectors, and they are all (8+) blaring away.

    Have you ever been in a house full of detectors all blaring away? You can’t think beyond “I need to get away from the noise”.

    I’ve often thought I’d like to tap into that “network” and make it part of the security system.

  12. “First off, never hack your ONLY smoke detector, you are putting lives at risk by doing so.”

    Did this really need to be in the article? I’m reading a hardware hacking site, I’m not a fucking baby.

  13. Most smoke detectors are optical AFAIK and they contain nothing exotic at all and the most dangerous chemicals in it is the 9v battery.

    And I agree about the lame warning to not hack your only smokedetector but seeing this is indeed a kid-project I guess the warning fits the target-audience, young enthusiastic tinkering kids.

  14. reed switch and matching magnet on the door/window would probably do better, line the wire into the moulding if possible to help “hide” the detector. Run the wire to an inconspicuous area that the burger is unlikely to find. Daisy chain them to create more of a nuisance.

    Or take a 12volt 120db siren, the reed switch, and a small circuit to create a similar working circuit. Maybe wire it all to a pc to send an alert to you at work, or patch it into a cheap cell phone. Maybe wire up a couple web cams that kick on when the alarm is tripped too.

  15. From the article: “but I can’t seem to find my real camera.” Well, you were broken into. But, I digress; This seems like a rather… simple solution. Even if it isn’t the greatest deterrent I believe the idea is that it should attract attention. And no one wants _their_ house to burn down.

  16. this thing does have its merits, a buddy was looking for a door alarm for when his baby starts crawling. this would be a fun cheap little hack to know if the little tike opened the door.

  17. Ok, this is a simple hack. But calling it a kids project is a bit derogatory, don’t you think? Anyways, I would feel honored if I’ve managed to inspire a kid to start hacking away.

    Thanks for your otherwise encouraging and positive comments.

    And for those of you who were wondering, the smoke detector uses an optical sensor. And yes, I’ve taken some other security measures, so this isn’t my last line of defense.

  18. The flamethrower is a great idea Viadd, but since I suspect that my cats are quite easily ignited I don’t think it would be advisable in my particular case.

    I’ve considered many lethal options and they all have the advantage of leaving a pile of dead villains outside the window. There can hardly be any better deterrent.

  19. I like Viadd’s solution. The problem with lethal deterrents, however, is that if you set such a trap and it kills someone, you are now legally charged with first-degree murder. Also, you risk lighting your house on fire ;p Better to incap the invader and keep a baseball bat handy so you can make sure he never wakes up.

    And remember: the job of the police is to help you scrape the invader off the walls, not to save you from him. It typically takes hours for police to arrive(I’ve personally seen on-site university police take more than an hour to arrive when called in). It only takes seconds for a burglar to shoot you dead.

  20. “Now for a real house from a -supposedly- non brain damaged adult… that’s nearly as retarded as can be…”

    What you need is some decent critical thinking skills. This isn’t about you calmly sitting at your desk after reading this article wondering how to disable a buzzing smoke alarm, this is about some adrenaline-pumped dreg of society popping through a window late at night and hearing an alarm go off.

    What is the probability do you think that he (i) searches for the device (ii) identifies what it is (iii) discerns or is confident that it hasn’t triggered some other event (such as calling the police) and (iv) calmly continues about his burglary as if nothing has happened?

    If you open a window and an alarm goes off the thing to do is high-tail it and run. Stopping to figure out how to disarm should be reserved for people who don’t mind going to prison.

  21. @ anders
    What would be really cool would be to glue a tiny mirror about 1 inch square to each door or window. Then use a cheap laser pointer to shot a beam around the room into the optical detector of the alarm. Any mirror that moves in the room would set it off :)

  22. @cgmark: That won’t work. Smoke detectors work “the other way around”, they have a lightsource that does NOT shine on the sensor, but all around it. If light hits a smoke particle in front of the sensor, it is reflected and triggers the alarm.

  23. Perhaps it is different in other countries, but in the US almost all the alarms I see at the hardware store are still using Am. They have optical ones, but they are more expensive or part of “hybrid” detectors which use both optical and ionizing radiation, the theory being that you will get the advantages of both detection types in one device. I have been buying up the cheap ones ($4) to collect the Am out of them for awhile now…

    But as already stated, the risk from exposure is basically zero. You could literally eat it and not have anything bad happen to you in the form it is in.

  24. You can make something much better for probably a lot less with a single SCR, a few resistors, one of those personal safety alarms, and some wire. In fact, I did just that when I was about 14(12 years ago) and even used one of those RadioShack key-switches to arm/disarm.

    Why you need to hack a smoke alarm purely for the beeping is beyond me.

  25. I built a very similar modification to give myself notification of flood/water backup in my basement, even left the smoke detecting working just fine, ran wires to the test button, and then soldered to a bit of old tv remote circuit board.

    it even chirps when its time to replace the battery (was thinking of building my own alarm but i had a spare old detector laying about. And had no idea how to program a low bettery indicator)

  26. Isn’t the smoke alarm siren just a piezo buzzer? You could also claim to a smoke alarm as a resistor – it’s a bit of a waste though. Piezos are like $1, and a whole lot more inconspicuous than an entire smoke detector.

  27. My old roommate’s dog used to constantly tear apart the garbage in the bathroom, and run around the house with used kleenex, leaving them all over the place. For a while we lived with the garbage can on the counter, but that was annoying, and it often got moved due to lack of space. One day, I was highly irritated at this, so I rigged up a smoke detector, a paper cup lined with tinfoil, and a piece of wire with a weight on the end. When the apparatus tipped over, the smoke detector went off. Placed it in the bottom of the garbage can. A couple hours later, it went off – and apparently since that day, like 3 years later, the dog still won’t go near the garbage anymore.

    Effective for dogs, anyways.

  28. Shhee. If you happen to have a few old ones kicking around your junk box what the hell is wrong with re-applying one? In a world full of alarms a smoke alarm is about the only one people still respond to (around here, anyway).

    All the alarms I’ve seen in Australia are radioactive type – handy for more advanced hacking e.g. if you’re interested in measuring electric fields (heh heh).

    *Mostly* harmless. *Injesting* such a source is about the only really stupidly dangerous thing you could do with it – just think of the beta burn from it lodging in a fold of your gut for a while.

    Just moved house, boxes everywhere. Chilling out, then the flat battery “cheep” of an alarm buried somewhere. A couple of hours digging and waiting later…

    Woman friend told me of trying to silence an alarm with a low battery; still cheeping after smashing with hammer. Tough little bugger. Finally silenced it after several blows with an axe. Didn’t think to try removing the battery.

  29. @Dax
    It’s simply true that certain type of smoke detectors use/used radioactivity as a means, and so they contained radioactive materials, that’s not irrational but a historical truth.
    And yeah it can’t harm to avoid breaking such things open, might get away with it many times but you don’t want to be the one who draws the short straw do you?
    But modern and economical optical ones are not an issue, and I’m sure the older radioactive types have a warning label.

  30. Silver demister repair paint + contacts + some electronics = cheap smashed window detector.


    my current version is a spare mobile phone with a PIC micro that sends the appropriate codes to the keypad (Nokias are great for this) and sends an SMS to your mobile with the time and date.

    fwiw it is also possible to save data onto a serial E2PROM with a monochrome IR enhanced picture (simple matter of A-D + timing) for later analysis…

  31. When I was a kid, we moved to a new house and started painting the rooms right away. I guess my mom or dad painted over an old smoke detector and it started going off incessantly and wouldn’t stop. I guess (the old style smoke alarm units) are pretty much fried if they’re painted on (it says so on the labels of the old ones lol)so “They” threw it away, but it wasn’t long before I scooped it out of the trash and re-purposed it as a pencil box alarm.
    here’s a very rough schematic of what I did…
    and before you chide the simplicity of it, I was 9, bored, and infinitely mischievous at the time.

    I’m sure you could find an old, non functional smoke alarm, or buy a cheap one and just paint on it. lol prolly easier and safer than trying to hack the internals.

  32. The cool part here is that he will not only stop burglars, but slightly irradiate them in the process. Americium 135 is the relatively harmless radioactive element used to detect particulate matter in the air.



  33. Wow. You people are some arrogant, naive, probably overly affluent little brats. Burglar alarms that call the popo for you? Where I live, that would be a waste of time and money. The security company robs you blind, the police take an hour to arrive, and then they shoot you or your dog instead of the actual intruder. No thanks. I’m at home during the day, specifically to guard the house from outsiders, so I’d rather have a simple siren that alerts me to arm myself. This smoke alarm thing is a great idea, and I’m actually surprised “how to make a burglar alarm from a smoke alarm” turned up any search results. That said, I do agree, you should position this alarm much more securely. Otherwise, this seems like a lovely way to reuse scraps for home security.

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