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
- 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
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
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 5: Install it
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.