Step 1: Tools and stuff, you will need...
Thin copper plate
Thin card/ plastic
String/ invisible thread
Device you want power
Pen knife or some other kind of cutting implement.
Step 2: Constructing the switch.
Step 3: The Trip gizmo.
Poke a hole through one end and tie the tripwire through it.
Step 4: Finally there.
You can either wedge it under something , but I put a string through the center of the spring to anchor it to something solid.
Simply put, the card acts as an insulator an gets pulled out when the tripwire is.. well, tripped.
The copper makes contact and the whole thing acts as a switch. Done.
Works well with my Quick and Dirty Squib instructable (but only when used with caution). Check it out, NOW.