[SWLUG] Very simple input detection?

[Neil Jones]

Can anyone help with this simple piece of hardware design?

Ideally this works with a laptop. I simply want to detect a the absence 
of current flowing through a piece of circuitry.
In effect I want to know when a switch which is normally on becomes 
switched off.

I then want to use this to make the computer perform an action.

How would you approach it?

One idea that occurs to me is to put something like a memory stick in a 
usb connection where the breaking of the circuit is detected this way.
(.using Udev rules or something.)

Is there an easier and simpler way? I don't want to use complicated  
expensive circuitry for this.

Neil