Saturday, February 27, 2010

Smoke Detector and Burglar Alarm in One

Update: I'm proud to announce that this project has made it to Hack-a-day, one of my all time favorite blogs.

Three weeks ago some burglars broke in to our ground floor flat in central Stockholm. I was coming home from work and surprised the vile wantons going through our stuff. I can ensure you it was quite an unpleasant experience. Anyway, I managed to chase the despicable degenerates out of the apartment.

Since then I've been pondering various ways to improve my home security. I looked at a 127dB motor siren to hook up to a window alarm; that would surely deafen, and quite possibly kill, anyone trying to break in. For reference, hearing damage will occur around 120dB and the human pain threshold is ~130dB. The problem with this solution, aside from my neighbors discomfort while trying it out, is that I have cats; and I don't want them to get hurt in any way. This pretty much rules out my other ideas, like smearing neurotoxin on the window frame.

But I still felt I had to do something. How about hacking a smoke detector and using it as a window burglar alarm? Those things are loud, but not loud enough to cause damage to the cats. Just loud enough to catch the attention of my neighbors and anyone passing by on the street outside.

So, today I started hacking away. It would be great if the new burglar alarm functionality didn't interfere with the original smoke detecting function; that way I would get another smoke detector as well and you can't have to many of those.

This smoke detector has a test button that, when pushed, will cause the device to emit a loud hi-pitched sound and flash a red LED. I figured that this switch could be piggypiggybacked for the alarm functionality. I couldn't use the switch directly since it closed when pushed and you normally want a signal when the alarm "switch" opens. Besides, I still want a test switch for the detector. I decided that I also want a bypass switch to turn the alarm part of the detector of (if you for instance want to open the window yourself). You can see the very basic old and new circuits in the image.

In the next step, a pull-up was soldered across the old, now obsolete, test switch. A ground and signal wire was also soldered in place. In the next image you can see the new test and bypass switches mounted and the white wires to the alarm "switch" sticking out.

I'm using a paper clip as the alarm breaker. This is classic. I love the MacGyvery look of this solution.

In the last two images you can see how it looks when it's mounted. The paper clip is attached to the window and the wires are pinned to the window frame, causing the circuit to break if anyone attempts to open the window.

I love tinkering with microprocessors and computers, but it feels good to do something really simple and quite useful for a change. There you have it, a smoke detector and burglar alarm in one!

And yes, I know the image quality is awful, but I can't seem to find my real camera so I had to make due with my cellphone. Sorry about that.


  1. What are the values for the pullup circuit resistor and gate?

  2. Maybe you should have made it earlier, so your camera wasn't lifted now. ;)

  3. C: The pullup is a 100k resistor; this usually works with CMOS logic and consumes less power than your typical 10k pullup.

    Dennis: I've found my camera :) The burglars didn't manage to lift it. Phew.

  4. It would be great if it was more in depth. Where do you solder the 100k resistor? Right across the test switch? And further and most importantly, where do you solder the normally closed switches? Damned hard to see.

  5. That project is cool, Anders! It's good to hear that you are able to protect your home from burglars by using a simple circuit that you made. Anyway, even though this project is really useful, I wish that it remained intact to where you installed it. But most of all, I hope that it will not used its main purpose because that only means that you're not safe at all.

    Alexander Landrum

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