Instructables

Super simple door alarm

My seven year old son wanted to build a door alarm. Obviously it had to be very simple. But of course he wanted it to look "professional" not like a DIY or toy. I came up with a simple design and some easy steps he could follow. Maybe not do it himself, but at least understand.

The basic idea is to take a door contact usually used in alarm devices and connect it to a smoke detector. For convenience (of the parents) an on/off switch should be included.
 
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Step 1: Collect all parts

You'll need

- a magnet contact (reed contect), type=NO normaly open
- a smoke detector
- an on/off switch SPST
- cable (1m, 2 wires 0.7mm)
- 9V battery for the smoke detector

Please be sure to get the right type of reed contact. Alarm devices usually use NC (normaly close). For this project we need NO (normaly open). When I first bought one, it was a NC although the label said NO. False packaging. Be sure yours work as expected.

The smoke alarm should have a test button. I've never seen one without, but who knows. The button is exactly where we are going to tap into the device, so it is kind of a central requirement.

The switch shouldn't be to big, it has to fit into the smoke alarm and there isn't that much space left inside the device.

My smoke detector uses a 9V battery. Check what your device needs.
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user474 months ago

Hello, I love this concept. I was thinking about using this as an alert to remind folks in my household to not stand at the front door with it open letting the hot air out, or in depending on season. To do this I would need to swap out the NO for a NC, but wondering how I could incorporate a delay timer? This is my first attempt to put something like this together since I took an electronics class many, many years ago in high school. Any guidance or suggestions you could give would be sincerely appreciated.

tyler96134 years ago
Ok, so I'm looking at making a panic button alarm system with multiple alarms. This looks like a very good idea.

Instead of the contact switch, I'd just  use a pushbutton switch. Would it work ok if more than 1 wire was connected to each terminal of the switch so that you could have multiple alarms per switch?

Just an idea, please give feedback.
you probably had your anser by now, but yes, you just connect the buttons in parallel, all that counts is that one or the switches closes the circuit to set off the beeper
Roboto122 years ago
How does it work?

Tommy W4 years ago
wich smoke alarm should i use?
There are many kinds of detectors,like smoke detectors ,gas detectors,heat detectors and fire detectors.addressable detectors and stand alone detectors ,it depends on where you use.If at home the stand alone and smoke detector is OK.
Colonel884 years ago
OMFG RADIATION IN A CAN!!! Lolz the little can has radiation. Just collect like 9000 little cans and make a nuke with em... Nice 'ible by the way
youd need a lot more cans

Smoke detectors use americium 42 (radioactive) to detect smoke ! thats why its got a radiation sticker
Thats americium 241 i mean

oce14lupa5 years ago
Where can I Get the reed contact? I've looked at Walmart, Radio Shack, The Source, Future Shop, Costco And Canadian Tire and haven't found one yet. plz help!
OCE14LUPA: Check out Home depot or any hardware store!
hhhh19975 years ago
see that battery the 9v that is hackable i hacked it 6 battery only that brand and some other brands
Energizer brand batteries are hackable. you take it apart and u get 6 AAA batteries
kelseymh5 years ago
Very nice project. Except for the soldering, it is really quite suitable for a youngster -- screwdriver, positioning parts to fit, testing the final product. Seeing what's inside the smoke detector can itself be a good learning experience!

You could even (if you can stand the noise!) do some experimenting while it's open. The metal enclosure on the board (as other people have posted) is the actual smoke detector itself. A very small (< 1 uCi) quantity of americium-241, which emits ionizing radiation, is inside along with a couple of metal foils kept at voltage. Normally, the radiation (a mix of alphas and betas) ionizes the air inside, which leads to a continuous current between the foils.

When there are particles ("smoke") in the air passing through the side vents, those particles pick up the ions (and the electrons) instead, "quenching" the current flow. That drop in current is what triggers the circuit to set off the loud buzzer.

With the box opened up, you and your son could experiment and see just how small, or large, particles will act like smoke. Use a small suction bulb and blow a bit of talcum powder near (not into!) the detector with everything turned on. Does the alarm go off? How about with corn starch? Regular baking flower?

You will only need very small quantities to see an effect or not, and you will definitely want to clean off the circuit board when you're done experimenting.

If you want a bit more adventure, you can demonstrate that a "smoke" detector is not a fire detector. Hold the detector 30-50 cm over a small candle flame (e.g., with a tea light) and it should go off immediately. Now try a small alcohol flame, using a clean glass bowl without a wick. Hold the detector in the air above the flame (at least 10-20 cm).
When I activate the test switch on my (and others I've seen) smoke detector there is a second or 2 delay before the alarm sounds. Is there a way to stop this delay and have the alarm go off instantly when the switch contacts touch. Thanks for any help
pnorder5 years ago
Can this be made into a SILENT intrusion alert system, activating a concealed, flashing LED? I have a problem with someone sneaking into my office when I step out... I would also possibly like to connect a small camcorder to the contact, activating when someone walks in...
a word of warning: when doing this, don't tamper with the little round metal thing - it contains radioactive substances and could make you seriously sick. (probably not enough to kill you, but you don't like being sick now do you?)
due, i was fooling around with americaniaum 121, and i didnt get sick at all. its only mildly radioactive.
it contains a radioactive material americium, i think any way i messed with it and it did nothing to me... but i also dont recomend messing with it.
that's actually not the radioactive sign, that's the fallout sign...
Well your body is able to fight back and recover from radiation, and some people are more tolerant of it than others. It's still damaging, but its probably not long term damage.
RGM La Paz6 years ago
check my alarm out its kind of similar
RGM La Paz6 years ago
hey where can i buy door contacts at and how much do they cost
RGM La Paz6 years ago
DOES THE SMOKE ALARM GO ON AFTER A FIRE ALSO
wisser (author)  RGM La Paz6 years ago
Yes, the original function as smoke alarm still works.
thanks thats cool
wisser (author)  Yerboogieman7 years ago
Yes, I guess so. The transition between the alarm on/off states won't be as smooth, but the alarm should definitely work.
Thanks for the very useful instructable. I will definitely make this one. BTW, where did you get your contact?
I found a great site www.allelectronics.com that has the switches as well as a ton of other things both used and new. What do ya'll think about a "Favorite Sources" section? It could have voting/popularity tools so the most used sources for things would peculate to the top of the list.
Wow, thanks for that link. It's great
I've seen those switches available at radio shack in the states (But they're probably going to discontinue them. if they haven't already) ... they're in the home security type sections .... also they have 'Reed Relays' which are about the same, just not in the nifty plastic case. This is a great instructable, I would've never thought about hacking a smoke detector, I would've just got one of those "Piezo Buzzers" a battery & the switch.
wisser (author)  Technical Renaissance7 years ago
Here in Sweden the most obvious choice is Clas Ohlson, but your mileage may vary depending on where you live. ;)
Mag-Sec7 years ago
Hmmm, it would be fun to remove the contact and place a timer instead. So that you can leave it in school or something. Any ideas ?
It is almost April Fools day!
lemonie7 years ago
This will still function as a smoke alarm? L
All he did was solder wires onto the "test" button contacts, so it should still work as a smoke detector.
Thought so. The 'radioactive' label was removed for one of the pictures, Americium 241...
evy-wevy7 years ago
Wow that's great, and when He is a teenager, you will know if he is token in his room. :D
PetervG7 years ago
Not super simple..
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