A door alarm can help warn homeowners against intruders. It can also help parents monitor small children who may attempt to open a door that they shouldn’t. While there are many door alarms on the market today that range from the super simple to the very hi-tech, you can actually make your own door alarm.
One very simple way to make a door alarm is to connect a door contact to a smoke detector. Find a magnet door contact such as the ones used in alarm devices with an on and off switch. You also need to open the smoke detector to establish contact using a cable wire soldered to the contact areas of the smoke detector.
You can get full instructions with photos here for a detailed guide on how to make your own DIY door alarm.