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.