Personal Alarm System
Managed to do this when I was still about 11 or 12.

Take a couple of bits of cardboard about 60cm x 30, some foil, selotape, thin wire, some foam (but not from your mums sofa) a battery and a buzzer.

Wrap the foil round the cardboard and selotape down.
Trim the foil to size and make the thickness about 1cm. Cut holes in the foam, leaving just the edges and a bar across the middle (ie should look like a filled in H). Lay the foam between the two sheets on cardboard foil making sure the foil does not touch the other 'plate'.

Attach one wire to one plate and one to the other, attach buzzer and battery in series.

After making this bit of kit in my beadroom I sneaked downstairs equiped with trusty screwdriver and stanley knife, undid the carpet holder by the hall door, lifted said carpet and cut out the right size of underlay and replaced with foil plate. Reattach the carpet and hide the wires up the side of the stairs.

When all fixed up I could tell when anyone was about to come up the stairs and disturb my next cunning mission of world domination.ie they stepped on the plate, completing the circuit which made the buzzer go off.

Later additions included lights and further plates so I could tell exactly where other people were. I even rigged up the catflap so I knew when the vicious bastard was in.
(Suck it and Sea, Fri 21 Aug 2009, 15:00, 2 replies)

cunning mission of world domination...
Were you thinking about girls with no clothes on?
(sabre-tooth monkey, Fri 21 Aug 2009, 15:01, closed)

I made a similar device.
I think I got the idea from a 'Spy' book or something.
(theremin put the lotion in the basket on, Sat 22 Aug 2009, 8:14, closed)