Super Simple Door Alarm

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Introduction: 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.

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.

Step 2: Open the Smoke Alarm

Now the fun starts. My smoke alarm had 4 plastic pieces that kept the top locked to the bottom. IT was very easy to spot once I turned the alarm up and down. I had to bend the plastic inward (small srew driver) and of it came. Now remove the PCB from the bottom plastic shell.

Of particular interest is now the little metal strip next to the LED. That is the test button. On the picture not visible is a small metal below. The "contact area".

Turn the PCB upside down and find the contacts for the switch. You'll need one wire approx. 10cm long. (Take a part of the cable, dismantle it and use the wire.) Solder the wire to one of the "test button" contacts. Dismantle about ten centimeters from the cable. Solder one wire to the other "test button" contact.

Step 3: On/off It Goes

Now you should have two loose wire ends. These are to be connected to the switch.

Drill a hole in the smoke alarm lock. Please make sure the switch will fit in when you close the lock. There is not much space in the device. So really check it out first. Mine had some free space directly besides the battery. Though I had to remove a small plastic bit.

Now fasten the switch to the lock.




Step 4: Close It

Put the PCB back in place. Put the the lock back on.

Step 5: Install It

Now connect the lose end of the cable to the door contact.

Put the switch to "on".

Open and close your door contact. With some delay your alarm should go off / silent.

Install on a door.

In case of a childs room the on/off switch is very handy. At night you can check on your kids without waking them up. At day they can have all the fun with a "real" burglar alarm.

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    45 Comments

    What can you use instead of a magnet contact

    how does magnet helps in disconnecting the circuit ?

    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.

    1 reply

    You could use the magnetic switch to trigger a 555 timer in mono-stable mode. The timer would then give a high pulse to the smoke alarm for a set time. You would have to build another circuit however.

    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.

    1 reply

    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

    How does it work?

    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.

    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

    2 replies

    youd need a lot more cans

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

    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!

    1 reply

    OCE14LUPA: Check out Home depot or any hardware store!

    see that battery the 9v that is hackable i hacked it 6 battery only that brand and some other brands

    1 reply

    Energizer brand batteries are hackable. you take it apart and u get 6 AAA batteries

    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).

    1 reply

    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

    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?)