How to make a door alarm?

Best fit a microswitch which closes when the door opens. if you must stay on the paper-clip level then you need a very small swing at the top of the door which carries a contact to a stationary arm. The swing is pushed out of contact when the door closes and falls back into contact as it opens.

You can check http://www.eeweb.com/blog/extreme_circuits/fridge-...

This one only made for the Refrigerator's door, but can be also applicable in any door like your home door. On that link, is an instruction on how to make it.

They provided a Circuit Diagram for your references, Parts and PCB layout. It warns when the door of the refrigerator or freezer is left open for longer than a preset time period. It is great for indicating when someone is standing with the door open for too long and a real asset in warning when the door looks shut but is still partially ajar.

Cheers,

Jeremy

Attach one paper clip to door frame and one clip to door. These are the two terminals to make the buzzer ringing while door is closed ( clips in contact ). Open door opens the clips contact and stop ringing.