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.